Biomonitoring of Environmental Status and Trends (BEST) 

Program: Environmental Contaminants and their Effects on 

Fish in the Rio Grande Basin

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004—5108

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Front cover. The U.S. map shows the Rio Grande Basin (green) and stations sampled in this study (orange). Shown in gray are major

river basins and stations in the conterminous U.S. sampled during other Biomonitoring of Environmental Status and Trends Program

(BEST) investigations.
Biomonitoring of Environmental Status and

Trends (BEST) Program: Environmental

Contaminants and their Effects on Fish in

the Rio Grande Basin

By Christopher J. Schmitt, Gail M. Dethloff, Jo Ellen Hinck, Timothy M. Bartish,

Vicki S. Blazer, James J. Coyle, Nancy D. Denslow, and Donald E. Tillitt

Scientific Investigations Report 2004—5108

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Schmitt, C.J., Dethloff, G.M., Hinck, J.E., Bartish, T. M., Blazer, V.S., Coyle, J.J., Denslow, N.D. and Tillitt, D.E., 2004, 

Biomonitoring of Environmental Status and Trends (BEST) Program: Environmental Contaminants and their Effects on 

Fish in the Rio Grande Basin: U.S. Geological Survey, Columbia Environmental Research Center, Columbia, Missouri, 

Scientific Investigations Report 2004—5108, 117 p.
Preface iii 

Preface 

The study described in this report was conducted as part of the Biomonitoring of Environmental Status and Trends (BEST) program of the U.S. Geological Survey (USGS). BEST evolved 

from two earlier Federal monitoring programs: the National Pesticide Monitoring Program 

(NPMP) of the 1960s and the National Contaminant Biomonitoring Program (NCBP) of the 

1970s and 1980s (Schmitt and Bunck, 1995). The U.S. Fish and Wildlife Service (USFWS) 

participated in the NPMP and maintained the NCBP by monitoring concentrations of persistent contaminants in freshwater fish and avian wildlife through 1986. BEST was initiated in 

the 1990s to build on information produced by these earlier programs and to provide more 

biologically relevant information regarding potential contaminant effects on lands and species 

under FWS management. The program was transferred to the National Biological Survey in 

1993, and ultimately to USGS in 1996; its primary goal is to measure and assess contaminants 

and their effects on selected U.S. species and habitats. One component of BEST continues 

to monitor contaminants and their effects on fish in large rivers. The 1997 Rio Grande Basin 

(RGB) study, which was implemented together with a companion investigation of the Columbia River Basin, was part of this effort. The 1997 investigations represented continuations of a 

pilot study conducted in the Mississippi River Basin during 1995 (Schmitt, 2002a). Together 

with ongoing investigations in other basins, these studies comprise an expanded continuation of 

the NPMP/NCBP freshwater fish monitoring networks; four NCBP stations in the RGB were 

sampled as part of the 1997 study. 

Acknowledgements 

This study was conducted jointly by USGS, through the Columbia Environmental Research 

Center (CERC) in Columbia, Missouri and the National Fish Health Laboratory of the Leetown 

Science Center (LSC) in Kearneysville, West Virginia; USFWS, through its Ecological Services Field Offices in Albuquerque, New Mexico and Corpus Christi, Texas; and by cooperating scientists at the University of Florida, Gainesville (UF). Many individuals representing 

USGS, USFWS, UF, and other organizations contributed substantially during the conduct of 

the investigation. C. Bunck (USGS) managed the BEST program and S. Finger coordinated 

most of the work at CERC. M. Wilson (USFWS-Albuquerque) and C. Lee (USFWS-Corpus 

Christi) supervised field portions of the study. Chemical analyses were conducted at laboratories operated by Mississippi State University and the Research Triangle Institute through 

contracts managed by the Patuxent Analytical Control Facility (PACF) of the USFWS; J. 

Moore and P. McDonald of PACF facilitated this part of the study. Laboratory analyses for 

biomarkers were conducted by D. Nicks and S. Birke (CERC); D. Bowling, K. Spring, and 

E. Frankenberry (LSC); and M. Chow and K. Kroll (UF). A. Donahue (CERC), J. Smith 

(USFWS, Albuquerque), D. Buckmeier (Texas Parks and Wildlife, Ingram, Texas) and E. 

Buckner (Missouri Department of Conservation, Columbia, Missouri) aged the fish. The study 

was partly funded through a Research Work Order with the USGS-Florida Cooperative Fish 

and Wildlife Research Unit at UF; F. Percival, Unit Leader, and T. Gross of the USGS FloridaIntegrated Science Center facilitated this agreement. A. Donahue also managed the data and 

prepared report graphics. P. Anderson assisted with the preparation of the maps and obtained 

data and information from extant sources. N. Bauch and C. Crawford (USGS-Colorado and 

Indiana Water Resources District Offices, respectively) obtained and summarized the NASQAN 

data as part of an ongoing project. B. Wright (Oregon Department of Fish and Game, Corvallis, Oregon) and M. Ellersieck (University of Missouri-Columbia) assisted with statistical 

analyses. D. Buckler (USGS-CERC), M. Mora (USGS-CERC Field Research Station, Brazos, 

Texas), J. Whyte (AScI, c/o USGS-CERC), J. Lusk (USFWS-Albuquerque), J. Moring (USGSAustin, Texas) and C. Caldwell (USGS-New Mexico Cooperative Fish and Wildlife Research 

Unit, New Mexico State University) reviewed all or parts of earlier versions of the report and 

provided additional information. R. Lipkin (CERC) managed the bibliographic database and 

prepared the report for publication. 
iv Environmental Contaminants and their Effects on Fish 
Contents v 

Contents


Preface .......................................................................................................................................................... iii


Acknowledgements..................................................................................................................................... iii


Abstract .......................................................................................................................................................... 1


Introduction ................................................................................................................................................... 2


Rio Grande Basin Overview ............................................................................................................... 4


Hydrology and Infrastructure ................................................................................................... 4

Ecoregions and Climate ............................................................................................................. 5


U.S. Dept. of the Interior Resources at Risk from Contaminants in the Rio Grande ........ 5


Extant Sources of Information on Contaminants in the RGB........................................................ 6


Major Sources of Contaminants to the Rio Grande ....................................................................... 9


Agriculture ................................................................................................................................... 9


Municipal Wastes ..................................................................................................................... 10


Mining and Energy Extraction ................................................................................................ 10


Industry....................................................................................................................................... 11


Materials and Methods ............................................................................................................................. 12


Collection Sites and Sampling Dates ............................................................................................. 12


Target Species and Sampling Strategy .......................................................................................... 12


Monitoring Methods Overview........................................................................................................ 12


Field Procedures ................................................................................................................................ 16


Fish Collection ........................................................................................................................... 16


Sample Processing................................................................................................................... 16


Laboratory Analyses ......................................................................................................................... 17


Composite Sample Preparation.............................................................................................. 17


Elemental Contaminants and Moisture Content .................................................................. 17


Organochlorine Chemical Residues and Lipid Content ...................................................... 18


H4IIE Rat Hepatoma Cell Bioassay ........................................................................................ 18


EROD Activity............................................................................................................................. 20


Fish Health Indicators ....................................................................................................................... 20


General Histopathological Analyses ..................................................................................... 20


Macrophage Aggregates ........................................................................................................ 20


Quantitative Organism-Level Indicators ............................................................................... 20


Reproductive Indicators ................................................................................................................... 21


Gonadal Histopathology .......................................................................................................... 21


Vitellogenin ................................................................................................................................ 21


Data Set Composition and Statistical Analyses ........................................................................... 21


Results and Discussion.............................................................................................................................. 23


Geographic Distribution and Demographic Characteristics of the Fish Samples .................. 23


Accumulative Contaminants, H4IIE Bioassay, and EROD Activity ............................................. 24


Elemental Contaminants ................................................................................................................... 24


Arsenic ....................................................................................................................................... 24


Selenium..................................................................................................................................... 29


Mercury ...................................................................................................................................... 32


Lead..... ........................................................................................................................................ 35


Cadmium..................................................................................................................................... 36


Zinc...... ........................................................................................................................................ 37

vi Environmental Contaminants and their Effects on Fish 

Copper. ........................................................................................................................................ 38


Chromium and Nickel ............................................................................................................... 38


Organochlorine Chemical Residues ............................................................................................... 39 


DDT and its Primary Metabolites .................................................................................................... 39


Cyclodiene Insecticides ................................................................................................................... 43


Chlordane and Heptachlor ...................................................................................................... 43


Dieldrin ....................................................................................................................................... 46


Endrin... ....................................................................................................................................... 47


Other Organochlorine Pesticides.................................................................................................... 47


Toxaphene .................................................................................................................................. 47


Mirex.... ....................................................................................................................................... 48


Hexachlorocyclohexanes (HCH) ............................................................................................ 48


Hexachlorobenzene (HCB) ...................................................................................................... 49


Organochlorine Pesticides not Measured ........................................................................... 49


Total PCBs, H4IIE-Derived Dioxin Equivalents, and EROD Activity ............................................ 49


Total PCBs .................................................................................................................................. 49


H4IIE Bioassay .......................................................................................................................... 50


Ethoxyresorufin O-Deethylase (EROD) Activity ................................................................... 50


EROD in carp..................................................................................................................... 51


EROD in bass .................................................................................................................... 51


EROD in other fishes........................................................................................................ 54


Accumulative Contaminants, H4IIE, and EROD: Summary ......................................................... 55


Fish Health Indicators ....................................................................................................................... 58


Organism-Level Indicators ...................................................................................................... 58 


External Gross Lesions ................................................................................................... 58


Health Assessment Index .............................................................................................. 58


Condition and Organosomatic Indices .................................................................................. 60


Condition Factor ............................................................................................................... 60 


Hepatosomatic Index (HSI) ............................................................................................ 60


Splenosomatic Index (SSI) ............................................................................................. 61


Cellular and Histopathological Indicators ............................................................................ 63


Macrophage Aggregates ............................................................................................... 63


MAMM ..................................................................................................................... 63


MEANAREA ............................................................................................................. 64


TISOCC...................................................................................................................... 66


Fish Health Indicators: Summary .................................................................................................... 67


Reproductive Biomarkers................................................................................................................. 69


Gonadal Histopathology .......................................................................................................... 69


Gonadal Stage .................................................................................................................. 69 


Atresia .............................................................................................................................. 73


Atresia in Carp......................................................................................................... 73


Atresia in Bass ....................................................................................................... 74


Ceroid Pigment ................................................................................................................ 75 


Ovotestis in Male Bass .................................................................................................. 75


Gonadosomatic Index (GSI) .................................................................................................... 76


GSI in Carp ....................................................................................................................... 76


GSI in Bass........................................................................................................................ 77


Vitellogenin (vtg) ....................................................................................................................... 78

Contents vii 

Vtg in Female Carp .......................................................................................................... 78


Vtg in Male Carp .............................................................................................................. 79


Vtg in Female Bass .......................................................................................................... 79


Vtg in Male Bass .............................................................................................................. 79


Reproductive Biomarkers: Summary .............................................................................................. 80


Spatial Patterns in Contaminant Concentrations and Biomarker Responses ......................... 81


Upper Rio Grande (Stations 64, 63, and 516) ........................................................................ 81


Middle Rio Grande and Pecos River (Stations 65, 515, and 514) ....................................... 83


Lower Rio Grande and Arroyo Colorado (Stations 16, 511, 512, and 513) ........................ 85


Correlations Between Contaminant Concentrations and Biological Endpoints ............ 86


Summary and Conclusions ........................................................................................................................ 88


References ................................................................................................................................................... 89


Appendix 1. ................................................................................................................................................ 101


Figures 

1. Map of the Rio Grande Basin illustrating waterways and impoundments, federal and Native 


American lands and facilities, major metropolitan areas, state boundaries, and loca­


tions sampled. .............................................................................................................................. 3


2. Concentrations of arsenic (As) and selenium (Se) in composite samples of whole fish ........ 26


3. Concentrations of mercury in composite samples of whole fish ................................................ 32


4. Concentrations of lead (Pb) and cadmium (Cd) in composite samples of whole fish. ............. 36


5. 	 Concentrations of zinc (Zn), copper (Cu), chromium (Cr), and nickel (Ni) in composite 


samples of whole fish ............................................................................................................... 37


6. 	 Geometric mean concentrations of p,p’-DDT, DDE, and DDD in composite samples of whole 


fish. ............................................................................................................................................... 41


7. 	 Concentrations of p,p’-DDE, toxaphene, total chlordanes (sum of cis- and trans-chlordanes 


and nonachlors, heptachlor epoxide, and oxychlordane), and dieldrin in composite 


samples of whole fish ............................................................................................................... 42


8.	 Geometric mean concentrations of chlordane-related compounds (cis- and trans-chlor-


danes and nonachlors, heptachlor epoxide, and oxychlordane) in composite samples 


of whole fish ............................................................................................................................... 46


9. Concentrations of H4IIE bioassay-derived TCDD-EQ in composite samples of whole fish .... 50


10. Hepatic EROD activity in male and female carp and bass (Micropterus sp.) ............................ 55


11. Histologic appearance of external gross lesions observed in carp ........................................... 59


12. Health Assessment Index scores of male and female carp and bass (Micropterus sp.). ....... 60


13. Condition factor (CF) of male and female carp and bass (Micropterus sp.) .............................. 62


14. Hepatosomatic index (HSI) values of male and female bass (Micropterus sp.) ....................... 62


15. Splenosomatic index (SSI) values of male and female carp and bass (Micropterus sp.) ....... 63


16. 	 Splenic macrophage aggregate density (MAMM) in male and female carp and in bass 


(Micropterus sp.) ....................................................................................................................... 65


17. Splenic macrophage aggregate area (MEANAREA) in male and female carp and in bass 


(Micropterus sp.) ....................................................................................................................... 65


18. 	 Percentage of splenic tissue occupied by macrophage aggregates (TISSOC) in male and 


female carp and in bass (Micropterus sp.) ........................................................................... 66


19. Microscopic pathology observed in carp from the Rio Grande Basin. ...................................... 69


20. Thyroid follicles within the kidneys of carp from the Rio Grande Basin. ................................... 70


21. Gonadal stage of male and female carp and bass ......................................................................... 71


22. Gonadal stages in female carp and bass ovaries .......................................................................... 72


23. Ovarian section of a carp from Station 64 ....................................................................................... 73

viii Environmental Contaminants and their Effects on Fish 

24. Gonadal stages in male carp and bass testes ................................................................................ 74


25. Percentage of atretic oocytes in female carp and bass (Micropterus sp.) ............................... 75


26. Ovarian section of a largemouth bass from Station 514 ............................................................... 76


27. Pigmented cell accumulations in the gonads of carp from the Rio Grande Basin ................... 77


28. Gonadosomatic index (GSI) values of male and female carp and bass (Micropterus sp.) ..... 78


29. Plasma vitellogenin (Vtg) concentrations in female carp and bass (Micropterus sp.)............ 79


30. 	Maximum concentrations of total DDT (p,p’-homologs), toxaphene, and total chlordanes in 


composite samples of whole fish............................................................................................ 81


31. 	Maximum concentrations of mercury (Hg), arsenic (As), and selenium (Se) in composite 


samples of whole fish ............................................................................................................... 82


32. 	Maximum concentrations of TCDD-EQ in composite samples of whole fish and mean hepatic 


Ethoxyresorufin O-deethylase activity in carp and bass (Micropterus sp.)..... ............... 84


33. 	Percentages of male bass from the indicated stations in which ovotestis was detected 


through histopathological examination of the gonads........................................................ 85


Tables 

1. 	 Location, program of origin, and collection dates of sampling stations in the Rio Grande 


Basin................................................ .............................................................................................. 8


2. 	 Methods used to assess contaminant exposure and effects in fish from the Rio 


Grande Basin .............................................................................................................................. 13


3. 	 Organochlorine chemical and elemental contaminants measured in composite 


fish samples. ............................................................................................................................... 14


4. 	 Monitoring and assessment strategy for polycyclic aromatic and polyhalogenated 


hydrocarbons (PAH and PHH). ................................................................................................ 16


5. 	 Results of quality assurance (Q/A) analyses and limits-of-detection (LODs) for elemental 


contaminants.............................................................................................................................. 18


6. 	 Results of quality assurance (Q/A) analyses and nominal limits of detection (LODs) for 


organochlorine chemical residues......................................................................................... 19


7. Fishes collected in the Rio Grande Basin ........................................................................................ 23


8. 	 Total lengths, weights, and ages of common carp collected in the Rio Grande Basin, by sta­


tion and gender .......................................................................................................................... 24


9. 	 Total lengths, weights, and ages of bass (Micropterus sp.) collected in the Rio Grande 


Basin, by station and gender ................................................................................................... 25


10. 	Occurrence (percentages of samples and stations) and censoring levels for elemental con­


taminants in composite samples of whole fish from the Rio Grande Basin .................... 26


11. 	Geometric mean, minimum, and maximum concentrations (μg/g, wet-weight) of elemental 


contaminants in fish from stations in the Rio Grande Basin .............................................. 27


12. Least-squares mean concentrations of p,p’-DDE, mercury (Hg), length-adjusted Hg (HgL), 

weight-adjusted Hg (Hg), arsenic (As), cadmium (Cd), lead (Pb), zinc (Zn), and 


selenium (Se) .............................................................................................................................. 28


13. 	Least-squares mean concentrations of p,p’-DDE, mercury (Hg), length-adjusted Hg (HgL), 


weight-adjusted Hg (HgW), arsenic (As), cadmium (Cd), lead (Pb), zinc (Zn), and sele­


nium (Se) ..................................................................................................................................... 31


14. 	Geometric mean concentrations of mercury (Hg) in bass a from Stations 16 and 63, 1970­

1997 .............................................................................................................................................. 34


15. 	Occurrence of organochlorine chemical residues in composite samples of whole fish from 


the Rio Grande Basin ................................................................................................................ 40

Contents ix 

16. 	Geometric mean, minimum, and maximum concentrations of organochlorine chemical 

residues in fish from stations in the Rio Grande Basin ....................................................... 44 

17. 	Results of preliminary analysis-of-variance investigating the effects of various factors on 

biomarker a responses in carp and bass (Micropterus sp.) .............................................. 52 

18. Geometric means and ranges of EROD activities, by species, station and gender ................. 56


19. 	Percentages of carp, bass (Micropterus sp.), and all fish from the Rio Grande Basin with 

external lesions, by station. Also shown are the numbers of fish with lesions and of 

fish in each category examined .............................................................................................. 58 

20. 	Distribution of Health Assessment Index (HAI) scores in carp and bass (Micropterus sp.) 

from the Rio Grande Basin, by station .................................................................................... 61 

21. 	Age-adjusted station means for splenic macrophage aggregate (MA) parameters in carp 

and bass (Micropterus sp.) ...................................................................................................... 64 

22. Summary of chemical and biological indicator results, by sub-basin and station ................... 83


23. 	Statistically significant correlations a between biological and chemical endpoints in bass 

(Micropterus sp.), carp, and catfish (Ictalurus sp.) from the Rio Grande Basin ............. 87 

Conversion Factors 

Multiply By To obtain 

Length 

centimeter (cm) 0.3937 inch (in) 

millimeter (mm) 0.03937 inch (in) 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi) 

Area 

square kilometer (km2

) 0.3861 square mile (mi) 

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 

Volume 

milliliter (mL) 0.03381 fluid ounce (fl . oz) 

liter (L) 1.057 quart (qt) 

liter (L) 0.2642 gallon (gal) 

cubic meter (cm3

) 0.0002642 million gallons (Mgal) 

Mass 

kilogram (kg) 2.205 pound, avoirdupois (lb) 

Concentration 

microgram per gram (μg/g) = part per million (ppm; 106

) 

nanogram per gram (ng/g) = part per billion (ppb; 109

) 

picogram per gram (pg/g) = part per trillion (pptr; 1012) 

milligram per millimeter (mg/mL) = part per thousand (ppt; 103

) 

milligram per liter (mg/L) = part per million (ppm; 106

) 

microgram per liter (μg/L) = part per billion (ppb; 109

) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8 
x Environmental Contaminants and their Effects on Fish 
Biomonitoring of Environmental Status and Trends (BEST) 

Program: Environmental Contaminants and their Effects 

on Fish in the Rio Grande Basin 

By Christopher J. Schmitt1, Gail M. Dethloff2, Jo Ellen Hinck1, Timothy M. Bartish3, Vicki S. Blazer4, James J. 

Coyle3

, Nancy D. Denslow5, and Donald E. Tillitt1 

Abstract 

We collected, examined, and analyzed 368 fish of seven 

species from 10 sites in the Rio Grande Basin (RGB) during late 1997 and early 1998. Four sites were National 

Contaminant Biomonitoring Program (NCBP) stations 

where organochlorine and elemental contaminants in fish 

had been monitored from 1969 through 1986. The other six 

were USGS-National Stream Quantity Accounting Network 

(NASQAN) stations where water quality is monitored. The 

objectives were to document temporal and geographic trends 

in the concentrations of accumulative organic and inorganic 

contaminants in RGB fish and the effects of contaminants on 

the fish; to continue testing the feasibility of incorporating 

biomarkers (that is, biochemical, histopathological, and other 

biological indicators of contaminant exposure or effects) into 

a monitoring program for large U.S. rivers; and to evaluate the 

compatibility of monitoring methods based on the analysis of 

fish with those used to monitor water by NASQAN. Common 

carp (Cyprinus carpio; carp) and black basses (Micropterus 

sp.; bass) were the targeted species; together, they represented 

77% of the fish collected. Each fish was examined in the field 

for externally and internally visible gross lesions, selected 

organs were weighed to compute various ponderal and 

organosomatic indices, and samples of tissues and fluids were 

obtained and preserved for analysis of fish health and reproductive biomarkers. Composite samples of whole fish from 

1

U.S. Geological Survey, Columbia Environmental Research Center 

(CERC), 4200 New Haven Road, Columbia, MO 65201. 2

AScI, c/o U.S. Geological Survey, Columbia Environmental Research 

Center (CERC), 4200 New Haven Road, Columbia, MO 65201. 3

U.S. Geological Survey, Office of the Chief Science-Biologist, 12201 

Sunrise Valley Drive, MS-300, Reston, VA 20192. 4

U.S. Geological Survey, Leetown Science Center, 1700 Leetown Road, 

Kearneysville, WV 24530. 5

Protein Chemistry and Molecular Biomarkers Laboratory, P.O. Box 100156 

Health Center, University of Florida, Gainesville, FL 32610. 

each station were grouped by species and gender and analyzed 

by instrumental methods for persistent organic and inorganic 

contaminants and for dioxin-like activity (TCDD-EQ) using 

the H4IIE rat hepatoma cell bioassay. 

Overall, fish from stations in the lower RGB contained 

greater concentrations of some contaminants and appeared to 

be less healthy than those from sites in the central and upper 

parts of the basin, as indicated by general gradient of pesticide 

concentrations and biomarker responses from upstream to 

downstream. In the upper RGB, a minimal number of altered 

biomarkers and few or no elevated contaminant concentrations 

were noted. The exception was elevated concentrations [up to 

0.46 μg/g wet-weight (ww)] of total mercury (Hg) in predatory species from Station 63 (Rio Grande at Elephant Butte 

Reservoir, NM), a condition noted in the past. 

Elemental contaminants were most evident in fish from 

sites in the central RGB. Fish from Station 514 (Rio Grande 

at Amistad International Reservoir) contained slightly elevated 

concentrations (1.1-1.5 μg/g ww) of selenium (Se) and were 

also characterized by comparatively high organosomatic 

indices, proportions of fish with external lesions, and health 

assessment index (HAI) scores. Bass from Station 514 also 

contained elevated concentrations of total Hg (up to 0.4 μg/g 

ww). Se concentrations were also elevated (>1.0 μg/g ww) 

in carp from Station 515 (Rio Grande at Langtry, TX), and 

arsenic (As) concentrations were comparatively high (>0.3 

μg/g ww) in some samples from Stations 65 (Pecos River at 

Red Bluff Lake, TX) and 514. A comparatively large percentage of the carp from Station 515 also had high HAI scores 

caused primarily by the presence of parasite-induced external 

lesions. Concentrations of As and Se in fish from Station 65 

were lower than in the past, but Se concentrations at this site 

as well as others in the central RGB were sufficiently high to 

represent a threat to fish and wildlife. 

In the lower RGB, organochlorine pesticide residues 

(DDT metabolites, chlordane-related compounds, dieldrin, and 

toxaphene) were evident in fish from most sites, and concentrations exceeded toxicity thresholds for fish and wildlife at 
2 Environmental Contaminants and their Effects on Fish 

Stations 16 (Rio Grande at Mission, TX), 511 (Arroyo Colorado at Harlingen, TX), and 512 (Rio Grande at Brownsville, 

TX). Arsenic concentrations were also comparatively high 

(about 0.3 μg/g ww) in bass from Station 512. Channel catfish 

(Ictalurus punctatus) from Station 511 contained especially 

high (by contemporary standards) concentrations of p,p’-DDE 

(1.3-1.6 μg/g ww), chlordane-related residues (0.13-0.21 μg/g 

ww), toxaphene (1.4-2.4 μg/g ww), and dieldrin (0.03-0.05 μg/ 

g ww). These concentrations nevertheless represent a substantial decline relative to those reported historically. In addition, 

little or no (<0.01 μg/g ww) p,p’-DDT was detected at any 

site; nearly all DDT-derived residues were p,p’-DDE. Declining overall residue concentrations and increasing proportions 

of p,p’-DDE in 1997 samples relative to the past collectively 

indicate the weathering and re-distribution of old DDT rather 

than the influx of new pesticides to the lower RGB. Conversely, proportionally high concentrations of cis-chlordane 

relative to trans-nonachlor in fish from Station 511 indicates 

more recent pesticide inputs. 

Concentrations of total PCBs (<0.05 μg/g ww) and 

TCDD-EQ (≤6 pg/g ww) were comparatively low in all RGB 

samples, but rates of ethoxyresorufin O-deethylase (EROD) 

activity in carp, bass, channel catfish, or multiple species 

were elevated above basal rates at all sites. Collectively, these 

findings indicate that the fish had been exposed to polycyclic aromatic hydrocarbons (PAH). Environmental PAH can 

originate from petroleum, the incomplete combustion of coal, 

and from industrial activities such as smelting and metal fabrication; the former is consistent with the widespread energy 

extraction and transportation activities that occur throughout 

much of the RGB. Although elevated relative to basal activity 

in these species, EROD activity in RGB fish was generally low 

relative to levels reported in fish of the same species collected 

by others from heavily contaminated locations. In addition to 

EROD, slightly elevated HAI scores characterized carp from 

Station 511 and a comparatively high frequency of external 

lesions was noted on bass at Station 512. Histopathological 

examination revealed that the latter were parasite-induced. 

Comparatively high individual HAI values were noted at 

Stations 16, 512, and 513, and one or more macrophage 

aggregate parameters were elevated in carp, bass, or both at 

all lower RGB stations. Reproductive biomarkers were also 

consistent with chronic contaminant exposure at the four 

lower RGB sites; comparatively large percentages of intersex 

male bass, relatively low gonadosomatic index scores, and 

elevated vitellogenin concentrations in male fish were noted 

at three of the four stations, and large percentages of atretic 

eggs were observed in the ovaries of female carp from Station 

512. Although there are other causes for many of the conditions noted, the biomarker data for the lower RGB stations are 

nevertheless consistent with subtle responses to contaminants, 

an interpretation supported by the chemical data of this and 

other recent investigations. 

Introduction 

The Rio Grande (RG), known as the Rio Bravo del Norte 

in Mexico, represents a vital source of water to the growing 

human population of the Southwestern U.S. and northern 

Mexico and to the fish and wildlife resources of the region. 

Together with the Rio Conchos, a northward-flowing tributary 

that originates in Mexico, the RG provides irrigation water for 

agriculture from its headwaters in the San Juan Mountains of 

Colorado to the Texas Gulf Coast (Fig. 1). Although irrigation accounts for more than 80% of water withdrawals, RG 

water is also important for human consumption and supports 

water-based recreation. It also sustains riparian and estuarine 

habitats and the organisms that depend on them. U.S. Department of the Interior (USDI) agencies manage large tracts 

within the RGB in Colorado and New Mexico as well as much 

of its hydrologic infrastructure and many conservation and 

recreation facilities associated with the river and its tributaries (Fig. 1). The U.S. Fish and Wildlife Service (USFWS), a 

USDI agency, is also responsible for the many migratory birds 

and jeopardy-status species inhabiting these lands and waters. 

Heavy and increasing demands are placed on RG waters 

to meet growing water supply and waste disposal needs of 

population centers in the U.S. and Mexico, which have also 

affected water and habitat quality. Natural, agricultural, 

industrial, and urban erosional processes contribute to high 

sediment loads, and dams and diversions have dramatically 

altered flow regimes and water quality. Although the RG 

was historically characterized by elevated concentrations of 

dissolved solids, salinity has been increased by water removal, 

irrigation return flows, and structural alterations, especially 

in lower reaches (Mora and Wainwright, 1997). Elevated 

concentrations of organochlorine pesticides such as DDT and 

its metabolites have been found historically in sediments and 

river-dependent organisms, as have elevated concentrations of 

metals and metalloids in water, sediment, and biota (Davis and 

others, 1995; Levings and others, 1998; Mora and Wainwright, 

1997; Mora and Wainwright, 1998; Schmitt and others, 

1999b; Texas Natural Resources Conservation Commission 

(TNRCC), 1994a; 1997). Accordingly, many previous studies 

have characterized the biota in parts of the RGB as being at 

risk from contaminants. 

The RG and the Pecos River (PR), the largest U.S. tributary of the RG, were studied by the USGS in late 1997 and 

early 1998. As part of the USGS Biomonitoring of Environmental Status and Trends (BEST) program, the primary 

objectives of the study were to document the occurrence and 

distribution of contaminants and their effects on fish in the 

largest rivers of the RGB; and to evaluate the potential risk 

represented by these contaminants to other biota. Secondary 

objectives were to compare biomonitoring results from the 

RGB to other major river systems in North America, and to 

further define benchmarks for the quantification of long-term 

trends and interpretation of biomarker results. These latter 

objectives were achieved by building on the results of a similar 
Introduction 3 

Figure 1. Map of the Rio Grande Basin illustrating waterways and impoundments, Federal and Native American lands and facilities, major metropolitan areas, state boundaries, and locations sampled. Shown are Native American Tribal lands (Tribes) and lands 

managed by the U.S. Forest Service (USFS), Fish and Wildlife Service (USFWS), National Park Service (USNPS), and Bureau of Land 

Management (USBLM). See Table 1 for station descriptions and collection dates. 
4 Environmental Contaminants and their Effects on Fish 

investigation conducted in the Mississippi River Basin (MRB) 

in 1995 (Schmitt, 2002a). The RGB study was conducted in 

conjunction with a similar 1997 investigation of the Columbia 

River Basin. Together, the 1997 projects were also designed 

to evaluate the compatibility of the BEST large rivers component with the USGS National Stream Quantity Accounting 

Network (NASQAN) program, which monitors concentrations 

of dissolved pesticides and other constituents in the waters 

of the largest U.S. rivers (Hooper and others, 2001). Collection of contemporary information on contaminants in fish 

also provides linkages to and continuity with other historical 

and contemporaneous data sets (Carter and Anderholm, 1997; 

Gamble and others, 1988; Levings and others, 1998; Schmitt 

and others, 1999b; TNRCC, 1994a; 1997; U.S. Environmental 

Protection Agency (USEPA),1992a; 1992b). The availability 

of a regional contaminant assessment (Mora and Wainwright, 

1997), and associated database (Mora, 2001), provided an 

informational framework to support interpretation of the 1997 

results. 

Findings of the RGB study are reported here. The data 

have also been incorporated into a national database (<http:// 

www.cerc.usgs.gov/data/data.htm>). Results from this study, 

together with those from similar investigations conducted in 

other river basins, will help resource managers and scientists 

assess contaminant impacts on fish and on wildlife and human 

consumers of those fish, and to identify areas that warrant 

further investigation of contaminant threats. 

Rio Grande Basin Overview 

Hydrology and Infrastructure 

The RG originates in the southern Colorado Rocky 

Mountains within the Rio Grande National Forest. It flows 

generally southward into and through New Mexico, entering 

Texas northwest of El Paso. Below El Paso, the river tracks 

southeasterly before reaching the Big Bend area, roughly 90 

mi (145 km) downstream from Presidio (Fig. 1). The RG then 

changes course at the apex of the Big Bend, flowing northeasterly for another 90 mi (145 km) before bearing east and then 

southeasterly again towards its terminus in the Gulf of Mexico. 

At approximately 1,901 mi (3,059 km) in length, the 

RG is the second longest U.S. river (Fig. 1) (TNRCC, 1997). 

It represents the international boundary between the United 

States and Mexico from El Paso to the Gulf of Mexico, a 

distance of about 1,276 mi (2,053 km) (TNRCC, 1997). The 

RGB is also large in area, encompassing about 335,500 mi2 

(924,300 km2). Of this total, about 88,968 mi2 (231,317 km2) 

in the U.S. (in Colorado, New Mexico, and Texas) and 87,968 

mi2 (227,149 km2) in Mexico (in Chihuahua, Coahuila, Nuevo 

Leon, Tamaulipas, and Durango) contribute flow to the RG 

(TNRCC, 1997). The remaining areas within the RGB comprise endorheic (closed) basins. The U.S. portion of the RGB 

below El Paso comprises 48,300 mi2

 (125,580 km2), of which 

38,800 mi2 (100,880 km2) contribute flow (TNRCC, 1997). 

More than 20 Native American nations are located within the 

U.S. parts of the RGB (Fig. 1). 

Snowmelt from the Rocky Mountains is the primary 

source of surface water to the RG. Flow is augmented by 

diversions from the Colorado River Basin via the San JuanChama Project and by groundwater and available surface 

water pumped out of the 2,940 mi2

 (7,617 km2) San Luis Valley closed basin. The Chama River and Rio Puerco represent 

major upper basin tributaries. Between El Paso and the Big 

Bend of Texas the Rio Conchos, which flows northward from 

the Sierra Madre Mountains in Chihuahua, Mexico, joins the 

RG near Presidio and contributes most of the flow. Below the 

Big Bend the major tributaries are the PR, which drains parts 

of eastern New Mexico and western Texas and joins the RG 

upstream of Amistad International Reservoir; and the Devil’s 

River, which drains parts of south-central Texas and joins the 

RG within Amistad International Reservoir (Fig. 1). 

A complex network of flow-control structures has been 

constructed on the RG and many of its tributaries to provide 

water storage and flood/sediment control and to meet interstate and international water delivery obligations. Much of 

the hydrologic infrastructure in Colorado and New Mexico 

is operated by the USDI Bureau of Reclamation (USBR). 

Cochiti Reservoir, located roughly 50 mi (80 km) north 

of Albuquerque, New Mexico is the northernmost major 

impoundment on the RG mainstem (Fig. 1). Below Cochiti 

Reservoir, the Angostura Dam diverts water to supply parts 

of the Middle Rio Grande Conservancy District. Waters from 

the Jemez Canyon Reservoir enter the RG below Angostura 

Dam, upstream of Albuquerque. Below Albuquerque, water is 

diverted again at the Isleta Diversion Dam. To reduce excessive evaporative losses below San Acacia, New Mexico, a 

70-mi (113-km) canal known as the Low Flow Conveyance 

Channel (LFCC) was constructed parallel to the RG in the 

1950s; however, subsequent sedimentation of the structure 

necessitated its closure as a conveyance. Currently, the LFCC 

collects agricultural return flows from irrigated areas near the 

river and provides water to the Bosque del Apache National 

Wildlife Refuge (NWR; Fig. 1). Downstream of the refuge, 

the entire flow of the RG is captured by Elephant Butte Reservoir (USBR), then again by Caballo Reservoir (USBR; Fig. 1). 

Exiting Caballo Reservoir, the RG flows southwesterly along 

the western edge of Las Cruces, New Mexico and enters Texas 

northwest of El Paso. Above El Paso, at the American Dam 

(USBR), nearly all the remaining water is diverted into the 

American Canal (U.S.) and the Acequia Madre (Mexico) for 

agricultural and municipal uses. 

The waters of the RG and its hydrologic infrastructure 

downstream of Fort Quitman, Texas (east of El Paso) are 

managed by the International Boundary and Water Commission (IBWC) of the U.S. Department of State according to the 

terms of several treaties with Mexico (TNRCC, 2002). For 

about 250 mi (402 km) below El Paso to the confluence of the 

Rio Conchos near Presidio, Texas, flows in the RG are inter­
5 

mittent and comprise storm runoff, irrigation return waters, 

and municipal waste effluents (Mora and Wainwright, 1997). 

From Presidio downstream to Amistad International Reservoir and the confluence of the PR most of the water in the RG 

originates in Mexico and is contributed by the Rio Conchos 

(Fig. 1). 

The PR originates in the Sangre de Cristo Mountains 

east of Santa Fe, New Mexico and flows some 926 mi (1,480 

km) southward and eastward across New Mexico and Texas 

to its confluence with the RG near Langtry, Texas near the 

upstream end of Amistad International Reservoir (Fig. 1). The 

PR drains some 38,300 mi2

 (99,228 km2). Over its length 

the PR is controlled by a series of impoundments and other 

water management structures from which water is diverted for 

agriculture and other uses. Impoundments on the PR in New 

Mexico include Brantley Reservoir (formerly Lake McMillan), 

Lake Sumner (formerly Alamogordo Reservoir), Santa Rosa 

Lake, and Avalon Lake. Of these, all but Santa Rosa Lake 

are operated by the USBR. Impoundments in the PR Basin 

in Texas include Red Bluff Lake, Lake Toyah, and Imperial 

Reservoir (Fig. 1). 

Upstream of Del Rio/Ciudad Acunua the waters of 

the RG, along with those of the Pecos and Devil’s Rivers, 

are impounded by Amistad International Reservoir (Fig. 

1). Below Amistad Dam the river is free-flowing for 273 mi 

(440 km) until it is again impounded in Falcon International 

Reservoir (Fig. 1). From Falcon Dam to the Gulf of Mexico, 

a distance of 298 mi (480 km), the RG is again generally freeflowing; however, much of the flow is diverted into the Arroyo 

Colorado (AC) at the Anzanaldus Dam, north of Hidalgo, 

Texas (Fig. 1). The AC, a distributary, is managed for flood 

control via a series of floodways as well as for irrigation. It 

ultimately flows through the Laguna Atascosa NWR and 

then into the Lower Laguna Madre. The remainder of the 

RG continues southeasterly, passing through the outskirts of 

Brownsville, Texas before discharging into the Gulf of Mexico 

(Fig. 1). 

Water resources development has also been extensive in 

Mexico, and additional projects are being planned. In addition to Amistad and Falcon International Reservoirs, there are 

currently 13 storage reservoirs on RG tributaries in Mexico. 

Of these, seven [San Gabriel, Boquilla, F.I. Madero, Pico Del 

Aguila, Chihuahau, El Rejon, and El Granero (Luis Leon)] 

are in the Rio Conchos basin; two are in each of the Rio San 

Diego (Centenario and San Miguel) and Rio San Juan (El 

Cuchillo and Marte Gomez) basins; and there is one in each of 

the Rio San Rodrigo (La Fragua) and Rio Saldado (Venustiano 

Arranza) basins (TNRCC, 2002). 

Ecoregions and Climate 

The RGB lies entirely within the dry domain ecoregion 

as defined by Bailey (1995). Most of the Colorado parts of the 

basin are classified as temperate steppe. Annual rates of precipitation in Colorado are heterogeneous, however; they range 

Introduction 

from as much as 44 inches (112 cm) in the western alpine 

areas (mostly as snowfall) to as little as eight inches (20 cm) in 

the central portions. These precipitation patterns and ecosystem designations extend into the central portion of the basin in 

New Mexico. Most of the RGB in New Mexico receives less 

than 14 inches (36 cm) of precipitation annually, and much of 

the upper basin ranges from tropical/subtropical steppe and 

mountain to temperate steppe and mountain. The central part 

of the basin within New Mexico is primarily tropical/subtropical mountain surrounded on both sides by tropical/subtropical 

desert. From El Paso through the Big Bend area, the RGB is 

classified as tropical/subtropical desert except for the extreme 

eastern edge of the basin, which is tropical/subtropical steppe 

(Bailey, 1995), this area generally receives less than 14 inches 

(36 cm) of precipitation annually. Precipitation increases with 

distance below the Big Bend; much of the lower RGB receives 

20 inches (51 cm) annually, and the areas nearest the Gulf 

Coast receive as much as 28 inches (71 cm). The lower, narrow portion of the RGB extending from Eagle Pass to the Gulf 

is therefore classified as tropical/subtropical steppe (Bailey, 

1995). 

U.S. Department of the Interior (USDI) Resources 

at Risk from Contaminants in the Rio Grande 

Large tracts within the RGB in Colorado and New 

Mexico are managed by the U.S. Forest Service (Department 

of Agriculture) and the USDI Bureau of Land Management 

(BLM; Fig. 1). The USDI lands support multiple uses, including wildlife habitat and water-based recreation such as sport 

fishing. Because demands for water have greatly reduced 

the amount and quality of riparian and wetland habitat in the 

RGB, much of the remaining habitat and many of the organisms it supports are under USDI stewardship. Refuges associated with the RG and its major tributaries are managed by the 

USFWS. These include Alamosa and Monte Vista NWRs in 

Colorado; Bosque del Apache, Sevellita, Las Vegas, and Bitter 

Lake NWRs in New Mexico; and Lower Rio Grande Valley, Santa Ana, and Laguna Atascosa NWRs in Texas. Many 

of the refuges in the RGB were identified as being at risk of 

injury from pesticides, selenium (Se), salt seepage, and other 

contaminants emanating from agriculture and oil and gas 

production (U.S. Fish and Wildlife Service (USFWS), 1986). 

Laguna Atascosa NWR is situated on the landward side of the 

Laguna Madre at the confluence of the AC (Fig. 1). The AC, 

which flows through and supplies fresh water to the refuge, 

is heavily contaminated by agricultural chemicals and is the 

primary reason that Laguna Atascosa is among the RGB refuges identified as being at risk due to contaminants (Gamble 

and others, 1988; USFWS, 1986). The Mercedes Unit of Rio 

Grande Valley NWR, along with state and private refuges, are 

situated on the north shore of the AC at Llano Grande Lake, 

upstream of Harlingen, Texas (MacWhorter, 2004). Llano 

Grande Lake has a long history of contamination by organo­
6 Environmental Contaminants and their Effects on Fish 

chlorine pesticides and it remains under a fish consumption 

advisory [unpublished Texas Deparment of Health (TDH) data 

summarized by Mora (2001; Mahler and Van Metre, 2002; 

White and others, 1983). 

Federal recreational resources associated with the RG 

are also significant. The 68-mi (109 km) segment extending 

from the Colorado-New Mexico boundary downstream to Taos 

County, along with the lower 4 mi (6.4 km) of the Red River, 

have been designated as Wild and Scenic Rivers (WSRs) by 

the U.S. National Park Service (USNPS) and BLM of the 

USDI; this segment hosts the Wild Rivers and Orilla Verde 

National Recreation areas. Wild and Scenic status has also 

been proposed for Rio Embudo, another tributary. Elevated 

concentrations of metals from historical mining have been 

reported in fish and sediments of the Red River (Carter and 

Anderholm, 1997; Levings and others, 1998; Roy and others, 

1992). In addition, these waters may all be affected by a proposed copper mine. Farther downstream in New Mexico, the 

RG represents the southeast boundary of Bandelier National 

Monument (USNPS) in White Rock Canyon. Segments of the 

East Fork of the Jemez River, the PR, and the Rio Chama have 

also been designated as WSRs; the Rio Chama is jointly managed by the U.S. Forest Service and the BLM. 

In Texas, the RG represents the 69 mi (109 km) south 

boundary of Big Bend National Park (BBNP). This 69-mi 

reach begins upstream of Mariscal Canyon in BBNP at the 

Chihuahua/Coahuila border; it and an additional 118 mi (190 

km) downstream to the Val Verde/Terrell County line also 

comprise the Rio Grande Wild and Scenic River (Fig. 1). 

Amistad National Recreation Area is situated on the north 

shore of Amistad Reservoir and extends upstream along the 

Rio Grande about 83 mi (134 km), along the Devil’s River 

about 25 mi (40 km), and along the Pecos River about 14 

mi (23 km). These areas are managed by the USNPS. A 

cinnabar mine from which mercury (Hg) was extracted and 

smelted until 1943 is located within BBNP, and other derelict mines and smelters of the Terlingua Mining District are 

situated nearby (Sharpe, 1980). The seaward boundary of 

the Laguna Madre is formed by Padre Island, the northern 

part of which comprises the Padre Island National Seashore 

(USNPS), a nationally significant recreational and wildlife 

area that includes nesting habitat for the endangered Kemp’s 

ridely sea turtle (Lepidochelys kempii) and other species of 

wildlife. Additional recreational and wildlife areas managed 

by the states are situated along the RG and its tributaries and 

impoundments; these are also important to migratory species 

(USDI, 1995). 

The cumulative impacts of physical and chemical 

changes together with the introduction of exotic species such 

as salt cedar (Tamarix aphylla) have profoundly affected fishes 

and other river-dependent biota in the RGB (Bogan, 1998). 

Consequently, some taxa have been extirpated, some have 

received Federal protection, and others have been proposed 

for Federal listing (USDI, 1995). The shovelnose sturgeon 

(Scaphirhynchus platorhynchus), American eel (Anguilla 

rostrata), phantom shiner (Notropis orca), and the Rio Grande 

bluntnose shiner (Notropis simus simus) have been extirpated 

from portions of the RG, and the latter is federally listed 

as threatened (Propst, 1999). The Rio Grande silvery minnow (Hybognathus amarus), Big Bend gambusia (Gambusia 

gaigei), and Pecos gambusia (G. nobilis) are endangered, and 

the Pecos pupfish (Cyprinodon pecosensis) has been proposed 

for Federal listing; the latter two species now exist only at 

Bitter Lake NWR, where elevated concentrations of selenium 

(Se) have been reported (unpublished data, USFWS, Albuquerque, NM). Selenium and pesticides from agriculture and 

Hg from historical cinnabar mining in the Terlingua District 

have been implicated in the reproductive failure of the TransPecos population of peregrine falcons (Falco peregrinus), a 

threatened species, in areas adjacent to the RG (Mora and others, 2002). The Southwestern willow flycatcher (Empidonax 

traillii extimus) is endangered primarily because its riparian 

habitat along the RG and PR has been eliminated by the operation of water projects (El-Hage and Moulton, 1998). Small 

populations of the endangered interior least tern (Sterna antillarum athalassos) nest at Amistad and Falcon reservoirs and at 

Bitter Lake NWR, and ocelots (Leopardus pardalis) and jaguarundi (Herpailurus yaguarondi), both endangered, frequent 

riparian brushlands adjacent to the RG and other waterways 

and inhabit Laguna Atascosa NWR (USDI, 1995; University 

of Texas-Pan American (UTPA), 1995). An experimental population of the endangered whooping crane (Grus americana) 

was established at Bosque del Apache NWR (Roy and others, 

1992), which winter at the refuge along with other migratory 

species. Wetlands in the western Texas parts of PR Basin also 

host breeding populations of the threatened white-faced ibis 

(Phlegadis chihi) (El-Hage and Moulton, 1998). Bald eagles 

(Haliaetus leucocephalus) nest at Elephant Butte and Caballo 

reservoirs in New Mexico and over-winter along much of the 

RG, and brown pelicans (Pelecanus occidentalis), whooping 

cranes, and Eskimo curlews (Numenius borealis) frequent 

the Laguna Madre (UTPA, 1995). American white pelicans 

(Pelecanus erythrorhynchos), which were proposed for federal 

listing, also reside at Falcon Reservoir. Other federally listed 

plants and animals occur in the RG corridor but are not exclusively associated with riparian habitats (UTPA, 1995). 

Extant Sources of Information on Contaminants 

in the RGB 

Water quality in the RG and its major tributaries has been 

studied intensively over the last decade in response to dwindling water supplies caused by rapid population growth and 

economic expansion and prolonged drought. In addition to 

focused studies, many national investigations and monitoring 

programs have incorporated RG sites. Relevant findings of 

these studies were summarized and tabulated for comparison 

with 1997 results (Appendix Table 1). Information on local 

sources of contaminants, such as permitted discharges, fish 

consumption advisories, and hazardous waste sites, was also 

available from the web-sites of the New Mexico Environment 
7 

Department and the TDH. Additional unpublished TDH data 

on organochlorine and elemental contaminant concentrations 

in RGB fish from 1970-97 were available from the database 

compiled by Mora (2001). Databases of the TNRCC; the 

Toxic Release Inventory (TRI), Permit Compliance System 

(PCS), and Comprehensive Environmental Response, Compensation, and Liability Information System (CERCLIS) 

databases of the USEPA; and the USGS Minerals Availability 

System (MAS) were accessed and queried for information 

on documented permits, releases, and facilities that might be 

influencing water quality in the RGB (Appendix Table 1). 

Such information was only available from U.S. facilities and 

sites, however. 

The RGB has been the focus of many national and 

regional investigations over the last three decades. Fish 

samples collected at four RGB sites (Stations 16, 63, 64, and 

65; Table 1) from 1970 through 1986 under the auspices of the 

NPMP/NCBP (Lowe and others, 1985; Schmitt and Brumbaugh, 1990; Schmitt and others, 1985; 1990; 1999b) were 

analyzed for organochlorine chemical residues and elemental 

contaminants. The NASQAN program periodically collected 

water samples, which were analyzed for a large number of 

dissolved and suspended constituents including pesticides, 

nutrients, and trace elements, at six RGB sites sampled in our 

study (Stations 511-516; Table 1) beginning in 1996 (Wells 

and Reutter, 1996). In the early 1990s, the NAWQA program studied the upper RGB from its headwaters to El Paso, 

Texas; the study included the measurement of waterborne 

pesticides and metals as well as contaminant concentrations 

in sediments, fish livers (inorganic contaminants), whole fish 

(organochlorine chemicals) (Carter and Anderholm, 1997; 

Levings and others, 1998), and bryophytes (Carter and Porter, 

1997). White and others (1983) investigated toxaphene and 

p,p’-DDE concentrations in RGB fish and avian wildlife in 

the late 1970s; this study included collection sites at Alamosa, 

Colorado (Station 64), Falcon and Amistad International Reservoirs (Stations 513 and 514), and the AC at Harlingen, Texas 

(Station 511). The USFWS and USGS studied elemental and 

organochlorine chemicals in the New Mexico waters of the 

RG and its tributaries during the late 1980s (Ong and others, 

1991; Roy and others, 1992). 

The lower RGB has been the subject of especially 

intense study due to the region’s rapid population and economic growth. The USFWS conducted an extensive study 

of contaminants in lower RGB and Laguna Madre sediments 

and biota during the mid-1980s (Gamble and others, 1988). 

A multiphase, interagency assessment titled the “Binational 

Study Regarding the Presence of Toxic Substance in the Rio 

Grand/Rio Bravo and its Tributaries Along the Boundary 

Portion Between the United States and Mexico” (hereafter 

the Binational Study) was initiated in 1992 under the overall guidance of the IBWC. Participating agencies included 

USEPA, TNRCC, Texas Parks and Wildlife Department, and 

TDH of the United States; and the National Water Commission and Secretaria de Desarrollo Social of Mexico. In Phase 

1 of the Binational Study, 19 mainstem and 26 tributary sites 

Introduction 

(13 in each nation) from El Paso to Brownsville, Texas were 

assessed using traditional water quality measures; chemical 

analysis of water, sediment, and fish tissue; toxicity tests on 

water and sediment; and benthic community indices (TNRCC, 

1994b). In early 1994, Davis and others (1995) sampled eight 

sites in the lower RGB that were not included in Phase 1 of the 

Binational Study, including the AC (sampled in 1997; Station 

511—see Table 1). Phase 2 of the Binational Study was initiated in May 1995; 27 mainstem and 19 tributary were assessed 

(TNRCC, 1997). In Phase 2, sites that showed little or no 

impacts in Phase 1 were eliminated for further assessment and 

16 new sites were added to provide information on areas not 

previously assessed; the latter included four sites in Amistad 

and Falcon International Reservoirs, which we also sampled 

(Table 1). In addition to chemical measurements, the Binational Study incorporated sediment toxicity and macrobenthic 

community investigations at some sites, and included extensive reviews of contaminant issues in the RGB. The USEPA 

included two 1997 RGB sites (Stations 516, RG at El Paso and 

511, AC at Harlingen; see Table 1) and a reference site on the 

Mora River in northeastern New Mexico in its “National Study 

of Chemical Residues in Fish”, which was conducted during 

the late 1980s (Bahnick and others, 1994; Kuehl and others, 

1994; USEPA, 1992a; 1992b). Moring (1999) measured concentrations of waterborne organic chemical residues at six RG 

sites between Presidio and Brownsville using semipermeable 

membrane devices (SPMDs) during July-August, 1997. One 

of these sites was at Del Rio, downstream of our Station 514 

(Amistad Reservoir tail water) and one was below Brownsville, near Station 512. Van Metre and others (1997) studied 

temporal trends in contaminant concentrations based on sediment cores from Elephant Butte Reservoir (our Station 63), 

Amistad International Reservoir (immediately upstream from 

Station 514) and Falcon International Reservoir (upstream of 

Station 513). Mahler and Van Metre (2002) reported on 1989­

2001 temporal trends in sediment cores from Llano Grande 

Lake, on the AC upstream of Harlingen, Texas. 

Several reviews of contaminants in the lower RG have 

also been completed recently. An overall assessment of contaminants and other water quality issues in Texas parts of the 

RGB was released the TNRCC in 1994 (TNRCC, 1994a). The 

results of all lower RG studies through Phase 1 of the Binational Study (TNRCC, 1994b) and Davis and others (1995) 

were incorporated into a database (Wainwright and others, 

2001) and summarized by Mora and Wainwright (1997; 1998); 

and a review of contaminant discharges from the RG and AC 

to the Laguna Madre was conducted in 1994 by the University 

of Texas-Pan American (UTPA, 1995). 

Many focused investigations of contaminants and their 

effects on RG biota have also been conducted. Contaminants in fish from resacas (oxbow lakes) along the lower RG 

in Texas and Mexico were investigated by Mora and others 

(2001); this study also incorporated biomarkers. Mora and 

others (2002) investigated contaminants in peregrine falcons 

and their prey in Trans-Pecos Texas, and Caldwell and others 

(1999) reported on Hg in fish and wildlife from Caballo and 
8 

Table 1. Location, program of origin, and collection dates of sampling stations in the Rio Grande Basin (listed upstream to downstream). 

Station Sub-basin and River Location Program (NASQAN Collection date(s) Latitude, longitude number station ID) 

Upper Rio Grande 

Rio Grande 64 Rio Grande 63 Rio Grande 516 

Middle RioGrande/Pecos

Pecos 65 Rio Grande 515 Rio Grande 514 

Lower Rio Grande

Rio Grande 513 Rio Grande 16 Arroyo Colorado 511 

Rio Grande 512 

Alamosa, CO

Elephant Butte Reservoir, 

NM

El Paso, TX

Red Bluff Lake, TX 

Foster Ranch, Langtry, TX 

Below Amistad Reservoir, 

TX

Below Falcon Dam, TX 

Mission, TX

Harlingen, TX 

Brownsville, TX 

NCBP

NCBP

NASQAN (08364000)

NCBP

NCBP

NASQAN (08450900)

NASQAN (08461300)

NCBP

NASQAN (08480400) 

NASQAN (08475000) 

09/23/97-10/21/97

10/22/97-10/24/97

10/30/97-12/3/97 

1/21/98-1/22/98

11/6/97

11/4/97-11/5/97

11/18/97-11/19/97

12/2/97-12/3/97

9/30/97-10/2/97

10/28/97-11/25/97 

37°25’06.42”N, 37°23’38.57”W 

33°12’48.55”N, 107°13’27.26”W 

31°47’55.00”N, 31°48’15.00”W 

32°00’00.00”N, 104°58’30.00”W 

29°46’40.91”N, 101°45’13.22”W 

29°26’49.06”N, 101°03’10.58”W 

20°08’06.66”N, 99°08’06.42”W 

26°09’28.74”N, 98°20’02.82”W 

26°11’44.28”N, 97°36’20.52”W

25°52’12.96”N, 97°27’06.30”W 

Environmental Contaminants and their Effects on Fish 
9 

Elephant Butte Reservoirs, NM; the latter was sampled in 

1997 (Station 63; Table 1). Ong and others (1991) reported on 

contaminants in water, sediment and biota in the RG upstream 

of Elephant Butte Reservoir collected during the 1980s. 

Wainwright and others (2001) analyzed fish and fish-eating 

birds collected in 1997 from resacas and settling basins in the 

lower RG valley for organochlorine chemical residues and 

indicators of reproductive condition (reproductive biomarkers). Additional unpublished contaminants data for NWRs 

were available from the USFWS (Albuquerque, New Mexico 

Field Office). 

Beyond the RGB the primary basis of comparison for 

biomarkers and contemporary concentrations of contaminants 

in whole fish is the 1995 BEST-MRB study (Schmitt, 2002a) 

and the references cited therein. The 1995 study, which was 

the pilot for the investigation reported here, spanned 34 NCBP 

stations and 13 NAWQA sites distributed throughout the 

central U.S. and was focused on the same fish species targeted 

by the RGB study. Additional relevant NAWQA investigations outside of the RGB were conducted in the Red River of 

the North (RRN) Basin during the early 1990s (Goldstein and 

others, 1996; Goldstein and DeWeese, 1999). The RRN study 

is important because it is among the only NAWQA investigations in which whole fish, in addition to liver samples, were 

analyzed for elemental contaminants and because it represents 

fish of a species collected in the RGB from an area of the U.S. 

that is largely unaffected by urban and industrial activities 

(Goldstein and others, 1996; Goldstein and DeWeese, 1999). 

The frequency distributions of organochlorine chemical residue concentrations in composite samples of whole bottomdwelling fish collected from 20 NAWQA Study Units in the 

conterminous United States have been characterized (Wong 

and others, 2000); these represent useful benchmarks against 

which to compare results. Mercury in fish has been the topic 

of recent investigations by NAWQA (Brumbaugh and others, 

2001) and the USEPA (Peterson and others, 2002; Yeardley 

and others, 1998), and was the only inorganic contaminant 

included in the USEPA National Study of Chemical Residues 

in Fish (Bahnick and others, 1994; USEPA, 1992a). 

Major Sources of Contaminants to the Rio 

Grande 

Much of the RGB is highly mineralized and arid, and 

parts of the basin are underlain by petroleum-rich geologic 

formations. As a result, salts, oil, and potentially toxic trace 

elements such as arsenic (As), Se, and heavy metals are potentially available for release to the environment. Some areas of 

the basin are also naturally rich in As and Se. The rates of the 

processes controlling the release and distribution of these naturally occurring constituents are accelerated by human activities 

such as irrigation, mining, and oil and gas extraction (Schmitt, 

1998), all of which are extensive in the RGB. Accordingly, 

Introduction 

elevated contaminant concentrations in the large rivers of the 

RGB emanate primarily from mining, agriculture, and energyrelated activities and from the population centers they support 

(Appendix Table 1). In addition, the complex systems of dams 

and diversions has profoundly affected the flux of water and 

sediments and their associated contaminants throughout the 

RGB. Irrigation leaches potential toxins from soils, and reservoirs act as sinks in which they may accumulate. 

Agriculture 

In the United States, irrigation accounts for more than 

80% of water withdrawals from the RG (TNRCC, 1994b). 

Irrigated agriculture is concentrated near rivers and where 

groundwater is available (Levings and others, 1998); consequently, agricultural activities are among the most significant 

sources of contaminants to the RG and its biota (Appendix 

Table 1). Pasture, range, and livestock production are distributed throughout the RGB (Metz, 2002; Texas Center for Policy 

Studies (TCPS), 2002); however, crops grown in various parts 

of the RGB, and corresponding pesticide uses, vary seasonally 

in response to economic factors and water availability. In Colorado and New Mexico, the primary crops grown in the RGB 

are potatoes, barley, wheat, and forage, and substantial logging 

occurs in the uplands. Crops grown in the central RG valley 

and PR Basin of New Mexico and western Texas include cotton, peppers, onions, and pecans (El-Hage and Moulton, 1998; 

Roy and others, 1992). In the Rio Conchos Basin the primary 

crops are similar; they include pecans, alfalfa, chilies, and peanuts, along with lesser quantities of onions, tomatoes, melons, 

wheat, cotton, sorghum, and dairy products. In the lower RG 

valley, where more than 75% of the geographic area is devoted 

to agriculture (Fipps, 1991), major crops include cotton, grain 

sorghum, sugar cane, vegetables, peanuts, and citrus. 

Despite the intensity of the irrigated agriculture in the 

RG valley, relatively few currently used pesticides have been 

detected in mainstem RG waters by recent investigations. 

None were detected in the transboundary RG by either Phase 

1 or Phase 2 of the Binational Study (TNRCC, 1994b; 1997), 

and only a few compounds [atrazine, carbaryl, chlorpyrifos, 

Dacthal® (DCPA), diazinon, malathion, metolachlor, and simazine] were detected by NASQAN prior to the collection of fish 

for our study in late 1997. Of these, only three compounds 

(the insecticides chlorpyrifos and malathion and the herbicide 

atrazine) exceeded current water criteria—all three in the AC 

at Harlingen, and chlorpyrifos in the RG below Falcon Dam 

(Appendix Table 1). The maximum atrazine concentration 

at Harlingen during this period was 8.4 times greater than 

the current criterion (Appendix Table 1). In the Upper RGB, 

NAWQA reported the presence of as many as 27 compounds 

in surface water at some sites located on tributaries, but few 

were detected at mainstem sites (Levings and others, 1998). 

USEPA (1992b) reported traces of trifluralin and chlorpyrifos 

in fish from the RG at El Paso and the AC at Harlingen, and 

Davis and others (1995) detected chlorpyrifos in two of 23 
10 Environmental Contaminants and their Effects on Fish 

lower RG fish samples analyzed. Both of these latter compounds are currently registered and heavily used, and were 

detected in water samples from the lower RGB by NASQAN. 

Moring (1999) also detected six contemporary pesticides 

(trifluralin, metribuzin, pendimethalin, diazinon, Dacthal®, 

and chlorpyrifos) as well as seven organochlorine pesticide 

residues [p,p’-DDD, p,p’-DDE, dieldrin, trans-nonachlor, 

trans-chlordane, γ-hexacholorocyclohexane (HCH), and pentachloroanisole] in 1997 SPMD samples from the transboundary 

portions of the RG. Arsenic-containing herbicides and defoliants are used on cotton in several parts of the RGB, and residues of historically used pesticides such as DDT, chlordane, 

and toxaphene remain elevated in the lower basin. In addition, 

naturally high salinity and As and Se concentrations have been 

further elevated by irrigation and other water use activities in 

some areas (Appendix Table 1). 

Municipal Wastes 

Heavy and rapidly increasing demands are being placed 

on the RG to meet municipal supply and wastewater disposal 

needs of population centers in the U.S. and Mexico. Although 

the exact human population of the RGB is unknown and 

growing rapidly, it was conservatively estimated at nearly 10 

million in 1992, of which only about 2 million resided in the 

United States (TNRCC, 1994a). Most of the RGB is sparsely 

populated; human inhabitants are generally concentrated in 

population centers located in the river corridors. In the RG 

corridor these include Taos, Santa Fe, Albuquerque, Truth or 

Consequences, and Las Cruces, New Mexico; El Paso, Presidio, Del Rio, Eagle Pass Laredo, Zapata, McAllen, Mission, 

Harlingen, and Brownsville, Texas; and the Mexican cities of 

Ciudad Juarez, Ojinaga, Ciudad Acuna, Piedras Negras, Nuevo 

Laredo, Reynosa, Rio Bravo, and Matamoros. Population centers in the PR Basin include Las Vegas, Roswell, Artesia, and 

Carlsbad, New Mexico; and Pecos, Texas. Flows downstream 

of El Paso are dominated by irrigation return water, storm 

water, and effluents from treatment plants. Texas has therefore designated the RG from below Riverside Diversion Dam 

in El Paso to the confluence of the Rio Conchos as impaired, 

primarily due to elevated chloride and total dissolved solids 

concentrations. Much of the rapidly growing portion of the 

human population inhabiting the lower RG valley resides in 

areas with inadequate sewage treatment (Chapman and others, 1998; TNRCC, 2002). Segments of the lower RG have 

therefore been designated as impaired due to the presence of 

pathogens such as fecal coli form bacteria (Appendix Table 1). 

In addition, sewage effluents represent significant sources of 

As in some sections of the RGB where this element is naturally enriched in groundwater (Wilcox, 1997). 

Mining and Energy Extraction 

Mining and the extraction of oil and natural gas have profoundly affected biota throughout the RGB. Most mining for 

metals occurs in the mineral-rich mountains of the upper RGB, 

in Colorado and New Mexico. In contrast, and although oil 

and gas deposits are widespread in the RGB, most oil and gas 

extraction occurs in the lower RG and PR basins, in southeastern New Mexico and Texas. In Colorado and New Mexico, 

both historical and active mining sites are situated on USDOI 

lands. In Texas, oil and gas are extracted from NWRs and 

mining occurred historically on lands managed by the NPS. 

Oil shales and tar sands underlie much of the central parts 

of the RG and PR basins, and exploitable petroleum resources 

are widely distributed throughout much of the RGB. Exploitation has been most heavily focused in two areas, the Permian 

Basin and the Rio Grande Embayment. The Permian Basin is 

a series of geologic formations beneath the PR valley in southeastern New Mexico and west-central Texas. This area hosts 

some of the most productive U.S. oil and gas fields; in 1992 

the Texas counties comprising the Permian Basin produced 

over 217 million barrels of oil, and total production for the 

region through 1992 exceeded 14.9 billion barrels (Vertrees, 

2002). In South Texas the lower RG valley from Laredo to 

the Gulf of Mexico is underlain by components of the Rio 

Grande Embayment, another series of geologic formations 

hosting petroleum deposits. Numerous oil and gas wells and 

associated storage and transportation infrastructure are located 

in and around the floodplain from Zapata downstream to 

Brownsville. Pipelines and storage facilities are also distributed throughout the region. In addition to petroleum hydrocarbons, produced waters (that is, the water accompanying 

petroleum in geologic formations) containing high concentrations of salts, elemental contaminants, and radionuclides are 

discharged to surface waters from oil and gas production and 

distribution facilities, and natural gas compression and metering stations represent sources of PCBs and Hg (Schmitt, 1998; 

Wren and Farrell, 1995). 

Coals of varying quality are also present in the RGB. 

These are located in central New Mexico and along the lower 

RG near Eagle Pass and Laredo, Texas, where lignite deposits 

are mined (Henderson and Kleiner, 2002). Active coal mines 

in Colorado and New Mexico are located outside the RGB 

(Pfeil and others, 2002); however, coal was mined intensively 

in the mineral-rich Ortiz Mountains near Madrid, New Mexico 

until the 1950s (Hoffman, 1999). 

Mining for metals is largely concentrated in the upper 

basin; deposits of copper (Cu), silver, gold, manganese, 

molybdenum, beryllium, nobium, tin, tungsten, lead (Pb), zinc 

(Zn), and vanadium have been exploited, and additional mines 

are being considered. A Cu mine was also planned for a 

deposit near Dixon, in Taos County, New Mexico; had it been 

developed it would have drained into tributaries of the Rio 

Grande Wild and Scenic River. 

A search of the USEPA-TRI indicated that most mining 

sites near the RG are presently inactive, and that many of the 
11 

active sites involve sand, gravel, and similar materials (Appendix Table 1). Several historical mining sites have affected 

water quality, however. In Colorado, historical silver mining 

and related activities in the Creede Mining District polluted 

two RG tributaries, Willow Creek and the Alamosa River. 

Willow Creek has been contaminated by a variety of elemental 

contaminants (Carter and Porter, 1997; Levings and others, 

1998) and has been listed as an impaired waterway due to low 

pH (Colorado Department of Public Health and Environment 

(CDPHE), Water Quality Control Division, 2002). The RG 

from its confluence with Willow Creek to Wagon Gap [about 

8 mi (13 km )] is also listed as impaired due to elevated Zn, 

and from Willow Creek to the City of Del Norte [about 30 mi 

(48 km)] it is considered impaired due to elevated cadmium 

(Cd). Historically, fish kills occurred in this reach of the RG. 

Contamination from historical (late 1800s) and recent (1984­

1992) mining operations in the vicinity of the Summitville 

Superfund site in southwest Colorado has also affected nearly 

the entire length of the Alamosa River below its confluence 

with Alum Creek, including Terrace Reservoir. Parts of the 

Alamosa River are classified as impaired due to depressed pH, 

elevated waterborne metals concentrations (aluminum, Cu, 

iron and Zn), or both (CDPHE, 2002). The significance of this 

contamination to the RG is tempered by the fact that due to 

topography and irrigation diversions in its lower reaches, the 

Alamosa River rarely flows into the RG (Personal communication: C. Cotton, Colorado State Division of Water Resources, 

Alamosa, CO 3/14/03). As a result, the RG is not designated 

as impaired in the vicinity of Station 64 (RG at Alamosa; 

Appendix Table 1). The Red River, which joins the RG near 

Questa in Northern New Mexico and is part of the Rio Grande 

Wild and Scenic River (Fig. 1), has been contaminated by 

molybdenum mine tailings. Elevated concentrations of Cd, 

Pb, and Cu in sediments and Se, Cd, As, and Zn in fish and 

other biota in the Red River have been documented (Carter 

and Porter, 1997; Roy and others, 1992). Some gold and 

silver mining also occurred historically in Trans-Pecos Texas. 

In addition, and as noted earlier, cinnabar deposits in the 

Trans-Pecos were exploited historically (Sharpe, 1980), which 

resulted in significant Hg contamination of biota in this region 

(Mora and others, 2002). 

Significant deposits of non-metallic minerals with the 

potential to affect water quality, such as salt, gypsum, potash, 

barite, talc, fluorspars, and sulfur, are also widely distributed 

in the RGB and have been exploited to varying degrees. Naturally occurring saline seeps as well as brines pumped from 

oil wells create water quality problems in many parts of the 

basin. Uranium mining is also significant. Heavily exploited 

uranium deposits are located around Grants, in the Rio San 

Jose-Rio Puerco watershed of west-central New Mexico, 

where radionuclides and potentially toxic trace elements have 

been washed into streams and reservoirs through the erosion of 

natural deposits, production wastes, and tailing piles (Ellis and 

others, 1993). In Texas, uranium deposits are located in the 

Trans-Pecos and along the South Texas coastal plain, the latter 

to within a few kilometers of the RG in Starr County (Eargle 

Introduction 

and others, 2002). In addition to radionuclides, exploitation 

of the South Texas uranium deposits has released As, Se, and 

other trace elements to surface and ground waters (Herbert and 

others, 2001). 

Industry 

There is little industrial development and correspondingly few industrial discharges to the RG or its tributaries in 

Colorado and northern New Mexico; most permitted point 

sources are municipal wastewater treatment facilities (Appendix Table 1). In contrast, Albuquerque is home to a variety of 

manufacturing facilities that produce electronic components, 

plastics, fertilizer, optics, aircraft engines, petroleum products, 

and paint. Most of New Mexico’s Superfund sites, the bulk of 

which involve groundwater contamination by organic solvents, 

are located in or near Albuquerque (Appendix Table 1). 

No major industrial sources discharge to the RG downstream of Albuquerque until it passes into Texas. In the El 

Paso vicinity, discharges originate from a variety of industries 

including paint, corrugated cardboard, petroleum refining, 

plastics and resins, metal (primarily copper) and wire production, electronics, and the manufacture of toys and medical 

supplies (Appendix Table 1). A Pb and Cu smelter situated on 

the east bank of the RG at El Paso has been in operation since 

1887 (Kohout, 2004). Water quality concerns in this area are 

therefore directed towards Cu and pollutants associated with 

municipal waste effluents and urban runoff; waters from this 

reach were found to be toxic in short-term tests (Appendix 

Table 1). Many small manufacturing facilities are located in 

Laredo and Nuevo Laredo, and large volumes of hazardous 

materials are transported between the United States and Mexico in this region and are stored there (TNRCC, 2002). The 

remaining centers of industrial activity in the Texas portions 

of the RG valley are concentrated primarily in McAllen and 

Brownsville. Permitted discharges in McAllen originate from 

plating and polishing facilities and manufacturers of electronic 

components, aircraft parts, steel springs, and medical supplies. 

In addition, cyanide was identified as a concern in this area by 

the Binational Study (TNRCC, 1994b) (Appendix Table 1). 

In Brownsville, industrial facilities with permitted discharges 

manufacture ceramics, non-ferrous wire, and electrical equipment (Appendix Table 1). In Mexico, textile and electronics manufacturing are important and growing rapidly, as are 

assembly plants for a wide variety of consumer goods (Chapman and others, 1998); however, the extent of contamination 

emanating from these sources is largely unknown (Chapman 

and others, 1998; Mora and Wainwright, 1997). Illegal dumping has been identified as a significant problem throughout the 

border region (TNRCC, 2002). 

Superfund sites in Texas also cluster in urban/industrial centers, and four are located in counties adjacent to the 

RG. Two sites are located in El Paso and are associated with 

metal plating; both involve soil contaminated with a variety of 

metals including chromium (Cr), nickel (Ni), As, Cd, and Zn 
12 Environmental Contaminants and their Effects on Fish 

(Appendix Table 1). The other two sites are located near the 

lower end of the RG and involve the storage (Mission, TX) or 

manufacture (Harlingen, TX) of pesticides. Soil and groundwater contamination are the primary concerns at these sites, 

but neither is located near the RG and both have undergone 

remediation (Appendix Table 1). 

Elevated concentrations of Hg in fish from Elephant 

Butte Reservoir in New Mexico have been reported for more 

than two decades (Kidd and others, 1974; Schmitt and others, 

1999b), and the reservoir remains under a fish consumption 

advisory (Appendix Table 1). Recent studies have also documented elevated Hg concentrations in piscivorous birds from 

this reservoir and Caballo Reservoir, which is just downstream 

(Caldwell and others, 1999). A combination of atmospheric 

transport, fires, and natural biogeochemical processes are 

suspected of maintaining elevated concentrations (Canavan 

and others, 2000). 

Military facilities are also present in the RGB; however, only one is located near the river (Appendix Table 1). 

Groundwater contamination by fuels, lubricants, and solvents is often associated with military sites. In New Mexico, 

radionuclides from weapons development at the Los Alamos 

National Laboratory have been deposited in the RG (Graf, 

1994). 

Materials and Methods 

Collection Sites and Sampling Dates 

Fish were collected at ten sites in the RGB (Fig. 1; Table 

1). Eight of the 10 sites were located on the RG and one each 

was located on the PR and the AC. Eight sites were located in 

Texas, one site was in Colorado, and one was in New Mexico. 

The greatest density of stations was in the lower RGB, downstream of Big Bend (Fig. 1). To ensure spatial and temporal 

continuity with historical data, four sites were NCBP stations 

(Schmitt and others, 1999b). The other six were NASQAN 

stations (Hooper and others, 2001). In keeping with NCBP 

protocol and other studies (Schmitt, 2002a), fish collection 

was scheduled for autumn to avoid the spawning season. Most 

fish were collected between late September and early December 1997, but Station 65 was not sampled until January 1998 

(Table 1). At all sites except Stations 64 and 512, which had 

to be visited twice, sampling was completed during one visit 

spanning 2-3 d. Station 64 was sampled in late September 

and again in late October, and Station 512 was sampled in late 

October and late November (Table 1). 

Target Species and Sampling Strategy 

This study was designed to retain comparability with 

NCBP data (Schmitt and others, 1999b) and other RGB 

investigations incorporating composite samples of whole fish 

(Carter and Anderholm, 1997; Davis and others, 1995; Levings and others, 1998; TNRCC, 1994b; 1997; USEPA, 1992a) 

while also accommodating biological measurements (Table 2) 

(Schmitt and Dethloff, 2000). Many of the latter are genderspecific and require live or freshly killed individual fish. To 

standardize fish health and biomarker results, the same species were sought at each site. In previous NCBP collections 

(Schmitt and others, 1999b), the most prevalent bottom-dwelling species was the common carp (Cyprinius carpio, henceforth carp), and the most prevalent predator species was the 

largemouth bass (Micropterus salmoides). These were also 

the species targeted in other BEST projects (Schmitt, 2002a). 

In addition to their widespread distribution and the abundance 

of extant contaminant data available for them, the biological 

endpoints had been most thoroughly tested in these species. 

Accordingly, carp and largemouth bass were the preferred taxa 

at all RGB sites. If these taxa could not be obtained, alternate 

species were permitted. Preferred alternate species included 

suckers (Catostomidae) as alternate benthivores, and another 

black bass (Micropterus sp., henceforth bass), or catfish (Ictaluridae) as piscivores. The collection target at each site was 

10 (each) adult male and female of each taxon—40 fish per 

site. Collectors were instructed to obtain adult carp and bass 

or alternate species of a size representative of those believed 

to be present at the sites based on extant information, and to 

avoid extremely large or small fish. More than two species 

were collected at sites where complete quotas for the target or 

preferred alternate taxa could not be obtained. 

Monitoring Methods Overview 

A suite of chemical and biological methods was 

employed to characterize the exposure of fish to contaminants 

and the effects of exposure (Schmitt and Dethloff, 2000). 

The suite included reproductive biomarkers, measures of 

cytochrome P450 enzyme induction (to assess response to 

planar halogenated hydrocarbons (PHH) and PAH, fish health 

assessments, and chemical analyses of fish carcasses (Table 

2). Additional information on these methods is available 

elsewhere [see (Schmitt, 2002a; Schmitt and Dethloff, 2000; 

Whyte and others, 2000; 2004) and references cited therein]. 

Concurrent determination of tissue residue concentrations 

along with the suite of fish health, immune response, and 

reproductive assessments supports the interpretation of relationships between chemical exposure and biological responses. 

The organochlorine and elemental contaminants analyzed 

in the composite samples (Table 3) were selected to provide 
Materials and Methods 13 

Table 2. Methods used to assess contaminant exposure and effects in fish from the Rio Grande Basin. 

Method Description Tissue(s) 

examined Sensitivity Primary 

reference(s) 

Histopathology Microscopic examination for 

the presence of lesions; can 

provide early indication of 

chemical exposure 

Liver, gill, 

gonads, spleen, 

and kidney 

Overall organism 

health and 

contaminants 

Hinton and others (1992); 

Hinton (1993); Goodbred 

and others (1997) 

Ethoxyresorufin Odeethylase (EROD) 

activity 

Enzyme induction by planar 

hydrocarbons 

Liver PCBs; 

chlorinated , 

dioxins, and 

furans; PAHs 

Pohl and Fouts (1980); 

Kennedy and Jones (1994); 

Whyte and others (2000) 

Macrophage 

aggregate analysis 

Macrophages are important in 

the immune system, serving as 

a first line of defense for the 

organism and as an antigen 

processing cell 

Spleen Multiple 

contaminants 

including PAHs 

and metals 

Blazer and others (1994); 

Blazer (1997) 

H4IIE bioassay A screening tool to determine 

the presence of certain classes 

of planar halogenated 

compounds 

Whole fish 

(composite 

samples) 

PCBs; 

chlorinated 

dioxins and 

furans 

Tillitt and others (1991); 

Whyte and others (2004) 

Vitellogenin A precursor of egg yolk, 

normally synthesized in the 

liver of female fish 

Blood plasma Endocrinemodulating 

substances 

Denslow and others (1999) 

Chemical analyses Organochlorine chemical 

residues and elemental 

contaminants 

Whole fish 

(composite 

samples) 

Specific analytes Schmitt and others 

(1999) 

Somatic indices The relative mass of some 

organs is often indicative of 

chemical exposure 

Gonads, spleen, 

liver 

Overall organism 

health 

Grady and others (1992) 

Necropsy-based fish 

health 

assessment 

Visual assessment of 

external/internal anomalies (for 

example, lesions, parasites, 

tumors), which may indicate 

contaminant-related stress 

All Overall organism 

health 

Goede (1988; 1996); 

Adams and others (1993); 

Adams (1990) 

the maximum amount of information on accumulative contaminants of potential concern at minimal cost, and to maintain continuity with the NCBP and BEST databases (Schmitt, 

2002a; Schmitt and others, 1999b). Extracts of the composite 

samples were screened with the H4IIE rat hepatoma cell 

bioassay (Whyte and others, 2004), which responds to planar 

PHHs (Table 4). In addition, the livers of the individual fish 

were assayed for ethoxyresorufin O-deethylase (EROD) activity, which indicates recent exposure to exogenous AhR ligands 

including PHH and PAH (Whyte and others, 2000). Together, 

these assays and analyses allow estimation of the relative 

concentrations and potential biological effects of PHH and 

PAH (Table 4) without the expense of instrumental analyses 

for these compounds (Schmitt and others, 2002c). 

Measures of fish health at various levels of biological 

organization, immune system responses, and reproductive status were included in the suite of indicators to address potential 

impacts from non-accumulative contaminants and contaminant 

mixtures (Table 2). Fish health measures included: (1) gross 

observations for abnormalities; (2) condition and organosomatic indices; and (3) histopathological examination. Gross 

observation and determination of indices based on relative 

fish and organ sizes such as the condition factor (CF), hepatosomatic index (HSI), and splenosomatic index (SSI) are 

relatively simple and indicative of cumulative, organism-level 

changes. However, they are non-specific in terms of causal 

mechanisms and may be transient. Histopathologic changes 

are more complex but may reflect early, subtle alterations and 

may foreshadow subsequent effects at the individual organism 

or population levels. 

In addition to being an indicator of overall organism 

health the SSI is also an indicator of immune system stress 
ued

14 

Table 3. Organochlorine chemical and elemental contaminants measured in composite fish samples.—Contin Contaminant class and analyte Chemical name(s) or atomic symbol Principal uses and sources to aquatic ecosystems Organochlorine chemicals 

p,p'-DDE 

p,p'-DDD (TDE) 

p,p'-DDT 

o,p'-DDE 

o,p'-DDD (TDE) 

o,p'-DDT

Total polychlorinated biphenyls (PCBs) DieldrinEndrinHeptachlor epoxide cis-Chlordane

trans-Chlordane 

cis-Nonachlor

trans-Nonachlor

Oxychlordane (octachlor epoxide) 

2,2-bis (p-chlorophenyl)-1,1-dichloroethylene 

2,2-bis (p-chlorophenyl)-1,1-dichloroethane 

2,2-bis (p-chlorophenyl)-1,1,1-trichloroethane 

2-(o-chlorophenyl)-2-(p-chlorophenyl)-1,1-

dichloroethylene 

2-(o-chlorophenyl)-2-(p-chlorophenyl)-1,1-

dichloroethane 

2-(o-chlorophenyl)-2-(p-chlorophenyl)-1,1,1-

trichloroethane 

Mixture containing as many as 209 mono- through 

octa-chloro-substituted biphenyl congeners. 

1,2,3,4,10,10-hexachloro-6,7-epoxy-1,4,4a,5,8,8ahexahydro-1,4-endnnno-exo-5,8-dimethanonaphthalene 

1,2,3,4,10,10-hexachloro-6,7-epoxy-1,4,4a,5,6,7,8,8aoctahdyro-1,4- endo-endo-5,8-dimethanonaphthalene 

1,4,5,6,7,8,8-heptachloro-2,3-epoxy-3a,4,7,7atetrahydro-4,7-methano-1H-indene 

 1,2,4,5,6,7,8,8-octachloro-2,3,3a,4,7,7a-hexahydro-4,7-

methano-1H-indene (1-

α,2-

α,3aα,4-β,7-β,7aα)

1,2,4,5,6,7,8,8-octachloro-2,3,3a,4,7,7a- hexahydro-4,7-

methano-1H-indene(1-

α,2-β,3aα,4-β, 7-β,7aα)

1,2,3,4,5,6,7,8,8-nonachloro-2,3,3a,4,7,7a- hexahydro4,7-methano-1H-indene (1-

α,2-

α, 3-

α, 3aα, 4-β, 7-β, 

7aα)

 1,2,3,4,5,6,7,8,8-nonachloro-2,3,3a,4,7,7a- hexahydro4,7-methano-1H-indene(1-

α,2-β,3-

α, 3aα,4-β,7-β,7aα)

2,3,4,5,6,6a,7,7-octachloro-1a,1b,5,5a,6, 6a-hexahydro2,5-methano-2H-indeno(1,2-b)oxirene (1aα,1b-β,2-

α,5-

α,5a-β,6-β,6aα) 

p,p'-DDT metabolite 

Insecticide; p,p'-DDT metabolite 

Insecticide

o,p'-DDT metabolite 

o,p'-DDT metabolite 

p,p'-DDT impurityDielectric, hydraulic, and transformer fluids; lubricants; extenders; de-dusting agents; carbonless copy paper Insecticide; aldrin metabolite Insecticide; isodrin metabolite Heptachlor metabolite; technical chlordane constituent/metabolite 

Insecticide; technical chlordane constituent 

Technical chlordane constituent 

Technical chlordane constituent 

Technical chlordane constituent 

cis-Chlordane metabolite 

Environmental Contaminants and their Effects on Fish 
15 

Table 3. Organochlorine chemical and elemental contaminants measured in composite fish samples.—Continued Contaminant class and analyte Chemical name(s) or atomic symbol Principal uses and sources to aquatic ecosystems 

Toxaphene α-Hexachlorocyclohexane (HCH) β-HCH

δ-HCH γ-HCH (Lindane) Hexachlorobenzene (HCB) Mirex

Elemental contaminants 

ArsenicCadmiumChromiumCopperLeadMercuryNickelSeleniumZinc 

Chlorinated camphene mixture averaging 62% chlorine 

by weight 

1,2,3,4,5,6-hexachlorocyclohexane 

1,2,3,4,5,6-hexachlorocyclohexane 1,2,3,4,5,6-hexachlorocyclohexane 1,2,3,4,5,6-hexachlorocyclohexane 

Perchlorobenzene 

1,1a,2,2,3,3a,4,5,5,5a,5b,6-dodecachloro- octahydro1,3,4-metheno-1H-cyclobuta(cd)pentalene 

As

Cd

CrCu

Pb

HgNi

Se

Zn 

Insecticide; herbicide 

Constituent of insecticide mixture containing various 

HCH isomers; also know as α-benzene hexachloride (BHC)

Technical HCH (BHC) constituent 

Technical HCH (BHC) constituent 

Insecticide; technical HCH (BHC) constituent 

Fungicide; industrial intermediate 

Insecticide; fire retardant 

Industrial sources; herbicides; defoliants

Mining, smelting, and other industrial sources; urban 

runoff; sewage discharges 

Mining, tanning, and other refractory and chemical industrial sourcesMining, smelting and other industrial sources 

Mining, smelting, and other industrial sources; urban 

runoff; atmospheric pollution; fishing sinkers; lead shot Herbicides; fungicides; pulp, paper, and textile effluents;open-cycle chloralkali cells; landfills; mining; atmospheric pollution Mining, smelting, and other industrial sources 

Coal-fired powerplants; irrigation return flows 

Mining, smelting, and other industrial sources; urban 

runoff

Materials and Methods 
Table 4.

16 Environmental Contaminants and their Effects on Fish 

Table 4. Monitoring and assessment strategy for polycyclic 

aromatic and polyhalogenated hydrocarbons (PAHs and PHHs). 

a

Total PCBs were determined by gas chromatography with electron-capture detection. b

7-ethoxyresorufin O-deethylase. cH4IIE 

bioassay was performed after reactive cleanup to remove AhRactive PAHs. d

And other planar organic compounds. + responds; 

– does not respond; *AhR-active isomers and congeners only. 

Contaminants 

Endpoint PCDDs & PCBs PAHs d 

PCDFs 

GC-ECD a (carcass) + - -

b EROD activity (liver) * * * 

H4IIE bioassay c

 (carcass) * * -

(Blazer and Dethloff, 2000). Other immune system indicators 

included the measurement of macrophage aggregates (MA) 

in preserved samples of spleen tissue (Table 2). Macrophage 

aggregates, also known as melanomacrophage centers, are 

discrete aggregations of pigment-bearing macrophages found 

in the spleen, kidney, and sometimes liver of advanced teleosts 

(Agius, 1980; Blazer and Dethloff, 2000). These specialized 

cells are thought to be responsible for centralizing foreign 

material and debris for destruction, detoxification or reuse; 

storing waste products; contributing to the immune response; 

and storing and recycling iron (Ellis and others, 1976; Ferguson, 1976). Although they may be affected by many factors, 

MA measurements have responded to contaminant exposure in 

both field and laboratory studies [reviewed by Wolke (1992) 

and Blazer and others (1997)]. 

Measures of reproductive condition included plasma 

vitellogenin (vtg) concentrations, gonadosomatic index (GSI), 

and gonadal histopathology [Table 2; see also review by 

McDonald and others (2000)]. Although reproductive condition in fish can be influenced by many factors (for example 

gender, age, reproductive stage, season, water temperature), 

contaminants, particularly estrogen mimics, have been shown 

to impact fish reproduction in laboratory and field studies 

(Allen and others, 1999; Gimeno and others, 1998). Estrogen 

mimics are theoretically capable of stimulating the production 

of vtg, a precursor of yolk protein, in the livers of oviparous 

vertebrates. A variety of endocrine disrupting compounds 

have been shown to induce vitellogenesis (Servos, 1999; 

Tyler and others, 1998). Vitellogenin production is normally 

associated with female fish; however, vtg may also be present 

in the plasma of male fish, and the detection of levels typical 

of early- to mid-vitellogenic females in male fish has been 

associated with exposure to exogenous estrogens (Bowman 

and others, 2002; Folmar and others, 1996; Folmar and others, 

2001). To document these and other possible reproductive 

alterations and to define baseline concentrations, vtg was measured in both male and female fish. Gonadal histopathology 

[gonadal stage, presence of atretic oocytes, intersex conditions 

(presence of female reproductive tissue in males or viceversa), and presence of pigmented cellular accumulations] and 

GSI were also assessed as measures of reproductive health 

and status (Schmitt and others, 2002b). The GSI relates the 

proportional size of the gonad to body mass and may reflect 

changes resulting from a variety of physiological factors such 

as reproductive stage and environmental stressors, the latter 

including exposure to contaminants. Elevated occurrences 

of atretic (unfertilized and/or reabsorbed) eggs have been 

noted in fish exposed to contaminants (Cross and Hose, 1988; 

Johnson and others, 1988), although other factors may also be 

involved (June 1970; 1977). Feminization of male fish (that 

is, intersex condition) has been reported in laboratory and field 

studies of contaminants (Allen and others, 1999; Gimeno and 

others, 1998; 1997; Jobling and others, 1998). The accumulation of ceroid and lipofuschin pigments, which are oxidation 

products of lipids, in the spleen and liver is considered evidence of oxidative stress and sublethal cellular injury (Blazer, 

2002; Vogelbein, 1993). 

Field Procedures 

Fish Collection 

Fish were collected by DC electrofishing from a boat, 

with the following exceptions: gill nets were used at Station 63 to capture smallmouth bass (Micropterus dolomieu); 

trammel nets were used at Station 65; blue catfish (Ictalurus 

furcatus) were captured by hand at Station 515; and hoop nets 

were used at Station 511 due to high salinity, which precluded 

electrofishing. When possible, these alternative methods were 

avoided to minimize bias resulting from differing collection 

methods. Collection was generally conducted along the shoreline or in backwater areas of the river or reservoir being sampled. Because electrofishing tends to be biased toward larger 

fish (Reynolds, 1983), all specimens of the target species were 

collected, irrespective of size. Those within the desired size 

range were kept and the rest were released. At some sites 

samples representing more than 10 fish of a given species and 

gender were collected. GPS coordinates were recorded at 

the upstream and downstream extents of the area from which 

fish were collected (Table 1). Fish were held on-board in live 

wells and transported to shore for processing, usually within a 

few hours of collection. At some stations, fish were held alive 

overnight in net pens or in aerated tanks containing ambient 

water following night collections. 

Sample Processing 

The methods used to process the fish have been fully 

described (Schmitt, 2002a; Schmitt and others, 1999a; Schmitt 

and Dethloff, 2000). Briefly, a live fish was selected for 

processing and identified to species. A blood sample was 

collected from the posterior caudal artery and vein with a 

heparinized needle and syringe and placed on [wet] ice; from 

this sample, plasma was later obtained for determination of vtg 
17 

concentration. The fish was weighed, measured, and subdued. 

Observations of external features were recorded, and grossly 

visible anomalies were removed by dissection and preserved 

in 10% neutral buffered formalin for histopathological analysis. The abdominal cavity of the fish was dissected open and 

the liver (in species with a discrete liver), spleen, and gonads 

were removed and weighed. Liver weights for carp were not 

determined because this species has a dispersed hepatic organ. 

The liver, gall bladder, posterior and anterior kidneys, gonads, 

mesenteric fat (in certain species), and spleen were examined, 

and the gender of the fish was determined by gonadal observation. Pieces of liver were collected and immediately flashfrozen in a dry ice-ethanol slush, then transferred to a liquid 

nitrogen dry-shipper for storage and shipment; these samples 

were later analyzed for EROD activity. Samples of gonad, 

kidney, spleen, and additional pieces of liver were collected 

and preserved in 10% neutral buffered formalin for histopathological examination, gender confirmation (gonad), and 

macrophage aggregate analysis (spleen). Upon completion of 

the internal examination and dissection, scales (or spines from 

ictalurids) were collected for age determination. Remaining tissues (those not frozen or fixed) were placed back into 

the body cavity and the entire fish was wrapped in aluminum 

foil. The wrapped carcass was placed in a polyethylene bag 

with other carcasses of the same species and gender. These 

samples were chilled and later frozen for analysis of organochlorine chemical residues, elemental contaminants, and 

dioxin-like activity (with the H4IIE bioassay). The entire field 

procedure was typically conducted in 15-20 min (per fish), 

and tissue samples, especially liver for EROD analysis, were 

collected and frozen as rapidly as possible. Following the 

processing of the fish, blood samples were centrifuged and the 

plasma was aspirated and frozen in dry ice. 

Laboratory Analyses 

Composite fish samples were shipped frozen in dry ice to 

private laboratories that are managed through a contract overseen by the Patuxent Analytical Control Facility (PACF) of the 

USFWS. At the laboratories the samples were prepared and 

analyzed for organic and elemental contaminants. These laboratories also prepared the composite samples for H4IIE bioassay analyses. PACF oversaw quality assurance (QA) and quality control (QC) associated with these procedures. The H4IIE 

assays were conducted by the USGS Columbia Environmental 

Research Center, Columbia, Missouri (CERC). Additional 

information on sample preparation and chemical methods are 

presented by Schmitt and others (2002c), Schmitt (2002b), and 

Schmitt (2004). Cryogenically frozen liver samples for EROD 

analysis were also shipped (in dry shippers) to CERC for 

analysis. Cryogenically frozen plasma samples were similarly 

shipped to the Protein Chemistry Research Laboratory of the 

University of Florida, Gainesville, Florida (UF) for analysis of 

vtg. All preserved tissue samples were shipped to the National 

Materials and Methods 

Fish Health Laboratory of the USGS Leetown Science Center, 

Kearneysville, WV (LSC) for histopathological analysis. 

Information on these latter procedures are given by Blazer 

and others (2002) and McDonald and others (2002). Scales 

and spines were processed for age determination as described 

by Jearld (1983), with age (y) estimated from the number of 

completed annuli. 

Composite Sample Preparation 

Carcass samples were stored frozen (-20o C) at the lead 

analytical laboratory (Lab 1) until they were processed. For 

processing, the carcass of each fish in the sample being prepared was sawed into pieces. The pieces of all the fish in the 

sample were then mixed and homogenized in a commercial 

meat grinder. One sub-sample (100 g) of the composite was 

re-frozen (-20o

 C) and shipped frozen to Lab 2 for analysis 

of moisture content and elemental contaminants. A 10-g 

subsample was extracted with methylene chloride, subjected 

to the reactive cleanup procedure described in following sections, ampulated, and shipped to CERC for use in the H4IIE 

bioassay. Another 10-g subsample was retained by Lab 1 for 

analysis of organochlorine chemical residues by gas chromatography with electron capture detection (GC-ECD) and 

gravimetric determination of lipid content. 

Elemental Contaminants and Moisture Content 

At Lab 2 the 100-g sub-samples were freeze-dried; moisture content was determined by weight loss during lyophilization. Freeze-dried fish (25-50 g) was digested in nitric acid. 

Concentrations of total As, Pb, and Se in the digestates were 

determined by graphite furnace atomic absorption spectroscopy (AA). Concentrations of total Hg were determined by 

cold vapor AA. Concentrations of aluminum (Al), barium 

(Ba), beryllium (Be), boron (Bo), Cd, Cr, cobalt (Co), Cu, 

iron (Fe), magnesium (Mg), manganese (Mn), molybdenum 

(Mo), Ni, silver (Ag), strontium (Sr), thallium (Th), vanadium 

(V), and Zn were determined by inductively coupled plasma 

emission spectroscopy (ICPES) without pre-concentration. 

QA measures for elemental analyses included the analysis of 

reagent blanks, duplicate samples, certified reference materials, and fortified (spiked) samples (Table 5). These measures 

indicated minimal contamination in blanks that produced 

upward biases of ≤0.01 μg/g (dw) for all elements except Zn 

(≤0.02 μg/g), none of which would cause detectable additions 

to wet-weight concentrations reported here. Results for the 

other measures were also typical for the elements reported 

here and indicate that the analytical results accurately reflect 

true concentrations in the samples. Dry-weight limits-ofdetection (LODs) were determined individually for each analyte in each sample, but were nominally 5 μg/g for Al; 15 μg/g 

for Fe and Mg; 0.1 μg/g for Be, Cd, and Hg; 0.3 μg/g for Sr; 
c 

18 Environmental Contaminants and their Effects on Fish 

Table 5. Results of quality assurance (Q/A) analyses and limits-of-detection (LODs) for elemental contaminants. 

Element 

Q/A component 

As Cd Cr Cu Hg Ni Pb Se Zn 

Reagent blanks a (μg) 

Min. 0 0 0 0 0 0 0 0 0.01 

Max. 0.04 0.05 0.25 0.25 0 0.25 0 0.20 0.50 

Mean 0.01 0.01 0.07 0.08 0 0.07 0 0.03 0.19 

Max. addition (μg/g dw) 0.00 0.00 0.01 0.01 0 0.01 0 0.01 0.02 

Duplicate analyses (% diff.) 

Min. 0.20 0.20 1.00 1.10 0.08 5.48 0.40 2.32 0.23 

Max. 84.80 110.00 55.20 17.30 22.30 104.00 83.60 26.20 10.40 

Mean 26.10 17.90 21.32 7.94 8.03 35.85 20.72 8.44 4.75 

Reference materials bc (% diff.) 

Min. 73.2 101.0 69.0 85.4 95.0 92.0 86.1 87.9 88.0 

Max. 107.0 135.0 137.0 103.0 122.0 136.0 128.0 111.0 130.0 

Mean 94.0 113.1 98.9 91.2 106.1 110.6 100.9 99.4 98.0 

Cert. conc. (μg/g dw) 21.6 26.7 0.8 106.0 0.3 2.5 0.4 5.6 180.0 

b Fortified sample recoveries (%) 

Min. 83.1 91.3 89.1 91.7 93.6 91.7 95.0 89.1 77.8 

Max. 122.0 115.0 111.0 114.0 124.0 112.0 114.0 106.0 129.0 

Mean 106.1 100.9 98.4 100.2 103.3 102.0 101.3 96.6 106.6 

Nominal LOD (μg/g dw) 0.7 0.15 0.5 0.7 0.15 0.5 0.03 0.7 2.5 

a n = 8 b n = 9 

NRCC TORT-2 (Lobster hepatopancreas)

0.2 μg/g for Pb; 0.5 μg/g for As, Ba, Cr, Cu, Mo, Ni, Se, and 

V; 2 μg/g for B; 1.5 μg/g for Zn; and 0.5 μg/g for Mn (Table 

5). These values, as well as the analytical results, were converted to wet-weight concentrations for statistical analysis and 

reporting. Additional information on the analyses of samples 

for elemental contaminants has been reported elsewhere 

(Schmitt, 2004; Schmitt and others, 2002c). 

Organochlorine Chemical Residues and Lipid 

Content 

At Lab 1, one 10-g subsample of each ground composite sample was mixed with anhydrous sodium sulfate and 

Soxhlet-extracted with hexane, concentrated by rotary evaporation, and dried to constant weight for gravimetric lipid determination. The concentrated lipid extracts were re-dissolved in 

petroleum ether and fractionated on Florisil® in two fractions. 

The first fraction, containing relatively polar organochlorine 

insecticides, was concentrated for quantification of residues 

by dual megabore-column GC-ECD. The second fraction was 

subjected to further concentration and column chromatography on silicic acid to separate HCB, mirex, and PCBs from 

other organochlorine pesticides. Each of these fractions were 

concentrated and analyzed by megabore-column GC-ECD, as 

described by Schmitt (2002b). Precision and accuracy of these 

determinations were ascertained through analyses of duplicates and fortified samples (n = 4), and residue identities were 

confirmed in selected samples. Recovery efficiency ranged 

from 60.2% for hexachlorobenzene (HCB) to 94.4% for mirex, 

but averaged 85-92% for most analytes (Table 6). Based 

on these results, the analyses were determined to accurately 

represent the true residue concentrations in the samples. The 

nominal LOD for individual compounds was 0.01 μg/g wetweight; for multi-component residues (toxaphene and PCBs) 

it was 0.05 μg/g (Table 6). Residue concentrations were not 

adjusted for recovery efficiency. Additional information on 

the organic analyses has been reported elsewhere (Schmitt, 

2002b; Schmitt and others, 2002c). 

H4IIE Rat Hepatoma Cell Bioassay 

The 10-g sub-samples for H4IIE analysis were kept 

frozen at Lab 1 until the initiation of sample processing, full 

details of which can be found in Schmitt and others (2002c). 
Materials and Methods 19 

Table 6. Results of quality assurance (Q/A) analyses and nominal limits of detection (LODs) 

for organochlorine chemical residues. All results in % unless otherwise indicated (ND, not 

determined). 

Fortified sample recoveries Duplicate analyses LOD 

Analyte Min. Max. Mean Min. Max. Mean (μg/g) 

Lipid ND ND 

Moisture ND ND 

o,p’-DDD 82.5 96 

o,p’-DDE 84.0 93.5 

o,p’-DDT 80.0 88.5 

p,p’-DDD 81.0 94.0 

p,p’-DDE 80.0 90.0 

p,p’-DDT 80.0 91.0 

cis-Chlordane 82.5 92.5 

trans Chlordane 82.0 91.5 

cis-Nonachlor 80.0 88.5 

trans-Nonachlor 84.0 92.0 

Oxychlordane 82.5 90.0 

Heptachlor epoxide 81.0 90.0 

Dieldrin 87.0 102.0 

Endrin 85.0 98.0 

HCB 64.0 83.0 

Mirex 90.0 96.5 

-BHC 74.5 87.0 

-BHC 84.5 98.0 

-BHC 77.5 86.5 

-BHC 81.5 91.0 

Toxaphene a ND ND 

Total PCBs b ND ND 

ND 0.4 8.9 4.6 ND 

ND 0.7 2.6 1.5 ND 

90.8 0 0 0 0.01 

90.4 0 0 0 0.01 

84.4 0 0 0 0.01 

86.4 0 29.3 14.0 0.01 

84.4 0 7.8 4.0 0.01 

85.9 0 0 0 0.01 

88.8 0 0 0 0.01 

87.4 0 0 0 0.01 

86.2 0 0 0 0.01 

88.0 0 7.7 1.9 0.01 

87.0 0 0 0 0.01 

85.9 0 0 0 0.01 

93.8 0 0 0 0.01 

89.9 0 0 0 0.01 

74.3 0 0 0 0.01 

94.4 0 0 0 0.01 

82.6 0 0 0 0.01 

90.0 0 0 0 0.01 

83.5 0 0 0 0.01 

84.4 0 0 0 0.01 

ND 0 0 0 0.05 

ND ND ND ND 0.05 a No toxaphene-fortified samples analyzed; all replicates < LOD (0.05 μg/g) 

b No PCB-fortified samples or duplicates analyzed. 

Briefly, samples were thawed, homogenized, and columnextracted with methylene chloride. Percent lipid was determined on a 1% portion of the extract. The remainder was 

concentrated and cleaned up by two-stage column chromatography. Extracts were evaporated, re-dissolved with isooctane, 

ampulated, and shipped to the CERC for analysis. Matrix 

QC samples (blanks and spikes) prepared at Lab 1 and at the 

CERC included ground tissues from laboratory-raised bluegill (Lepomis macrochirus) and samples of a CERC standard 

positive control tissue (carp from Saginaw Bay, Michigan). 

These QC samples were processed concurrently with the 1997 

samples. 

At CERC, the H4IIE bioassay was performed on the 

composite sample extracts according to the method of Tillitt 

and others (1991) as modified for 96-well microtiter plates 

(Tysklind and others, 1994) and described by Birke and Tillitt 

(2000a). The H4IIE cells were seeded at 7000 cells/well in 

300 μL of D-MEM culture media (Tillitt and others, 1991). 

After a 24-h incubation, the cells were dosed with sample 

extracts or standards in isooctane as described in Schmitt and 

others (2002c). At least three 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) curves were analyzed each day. A linear 

regression was performed on the data from each well to 

determine an EROD rate (pmol/min). The amount of protein in each well was determined by the fluorescamine assay 

(Lorenzen and Kennedy, 1993) and the values used to normalize dose to each well and EROD activity. The doses of each 

sample (g-equivalents/mg cellular protein) or TCDD standards 

(pg TCDD/mg of cellular protein) were plotted against EROD 

activity (pmol of resorufin/min/mg cellular protein, henceforth 

pmol/min/mg) to develop dose-response curves. The linear 

portions of these curves were used to compare the relative 
20 Environmental Contaminants and their Effects on Fish 

potencies of the extracts with that of the TCDD standard. The 

determination of TCDD-equivalents (TCDD-EQ) was by slope 

ratio assay (Finney 1980) as described by Ankley and others 

(1991). Variance estimates were based on an additive model 

(Finney 1980) and were calculated as described previously 

(Ankley and others, 1991; Tillitt and others, 1991). 

QA procedures and results are documented in Birke and 

Tillitt (2000a). Samples were analyzed in blocks of about 

40. Limits-of-quantitation (LOQs) and LODs were computed 

separately for each block; LODs were 0-1 pg/g and all LOQs 

were 1 pg/g (rounded to the nearest whole number). The 

results of the QA program indicated that the bioassays accurately reflected the dioxin-like potency of the extracts (Birke 

and Tillitt, 2000a). 

EROD Activity 

Cryogenically frozen liver samples were stored at -80° 

C until the preparation of microsomal fractions. The kinetic 

microsomal assays were conducted in 96-well microtiter 

plates; microsomal preparations were used the day they were 

prepared. The procedures are fully described elsewhere (Birke 

and Tillitt, 2000a; Schmitt and others, 2002c). Briefly, triplicate determinations of EROD activity were performed on 5-μL 

portions of each microsomal preparation; mean EROD activity 

was reported. Protein content was determined using the fluorescamine protein assay (Lorenzen and Kennedy, 1993) in the 

same 96-well microtiter plate as the EROD analyses. A positive control material [liver microsomes from laboratory-raised 

channel catfish injected with 10 mg/kg of benzo(a)pyrene] and 

an additional reference material [liver microsomes of flathead 

catfish (Pylodictis olivaris) obtained from the Missouri River 

near Easley, Missouri] were also analyzed. A linear regression 

was performed on the data from each well to determine an 

EROD rate (pmol/min) and associated variance. The amount 

of protein was used to normalize EROD activity in each well; 

results were reported in units of pmol/min/mg. A rigorous 

QA program was followed for the EROD assays, as described 

in Birke and Tillitt (2000b). LODs ranged from 0 to 0.147 

pmol/min/mg whereas LOQs ranged from 0 to 0.35 pmol/min/ 

mg. The results of this QA program indicated that the assays 

accurately reflected the hepatic EROD rates of the samples 

analyzed. 

Fish Health Indicators 

General Histopathological Analyses 

Fixed tissues (liver, kidney, spleen, gill, gonad, and 

grossly visible lesions) were shipped to the LSC and prepared 

for routine histopathological analysis as described by Blazer 

and others (2002). Paraffin-embedded tissue sections (6-μm) 

mounted on glass slides were stained with hematoxylin and 

eosin (H & E) for microscopic examination. 

Macrophage Aggregates 

Macrophage aggregates and MA pigments in spleen 

sections were stained using Perl’s method (Luna, 1992). 

Using this method, melanin (a melanosome pigment derived 

from tyrosine metabolism) is black; hemosiderin, which is a 

protein-bound iron pigment, stains blue; and ceroid/lipofuscin, which are lipogenic pigments arising from the oxidation 

of unsaturated lipids, are yellow-tan. All MA measurements 

were made with a computer-based image analysis system, 

and included the number of aggregates in 2 mm2 of tissue 

(MAMM) and the mean area occupied by those aggregates 

(MEANAREA). The percentage of tissue occupied by MA 

(TISSOC) was computed from these measurements. 

Quantitative Organism-Level Indicators 

The prevalence of gross external pathological disorders 

was determined with a rating of present (1) or not present 

(0) deduced from the field data. For consistency with other 

monitoring programs that have used this type of assessment 

[for example, Fournie and others, 1996], only certain observations were included (Blazer and others, 2002). Included were 

grossly visible disorders of the eye (exopthalmia, hemorrhage, 

opacity, emboli, missing), opercles (shortening, deformities, 

parasites), and body surface (ulcers, parasites, and raised or 

discolored areas). Disorders of the fins (hemorrhage, fraying, and so forth) and skeleton (curvature) were also included. 

Numerical values were assigned to internal and external 

observations of lesions recorded in the field, and a necropsybased fish health assessment (HAI) score was calculated for 

each fish by summing these values for all organs (Blazer and 

others, 2002). A value was only computed for a fish if there 

were observations for all components. 

Body and organ weights measured in the field were used 

to compute condition factor (CF) and organosomatic indices 

(Blazer and others, 2002; Dethloff and Schmitt, 2000) according to the following formulae: CF = body weight/length3

; HSI 

= liver weight/(total body weight – gonad weight) X 100; and 

SSI = spleen weight/(total body weight – gonad weight) X 

100. The weight of the gonads was subtracted from the body 

weight in the computation of HSI and SSI to minimize the 

effect of the reproductive cycle on these indices (Dethloff and 

Schmitt, 2000). GSI was calculated as GSI = gonad weight/ 

total body weight X 100. 
21 

Reproductive Indicators 

Gonadal Histopathology 

The posterior tip of the gonad was dissected in the field 

and fixed immediately in 10% neutral buffered formalin. 

Transverse sections were processed for routine light microscopy (embedded in paraffin, sectioned at 6 μm, and stained 

with H & E). 

Female gonadal tissue was staged using developmental 

stages (designated 0-5) to classify each section (Blazer, 2002; 

McDonald and others, 2000; Nagahama, 1983; Rodriguez and 

others, 1995; Treasurer and Holliday, 1981). Carp and bass 

ovaries typically contain oocytes in several developmental 

stages and were therefore classified according to the maturity 

of the predominant stage of oogenesis of each tissue sample. 

Ovaries containing only previtellogenic chromatin nucleoli 

and perinuclear oocytes, which were identified by cytoplasm 

that stained basophilic with H & E, were assigned to stage 0. 

Those containing many oocytes with cortical alveoli in addition to the previtellogenic chromatin nucleoli and perinuclear 

oocytes characteristic of stage 0 were assigned to stage 1. 

Ovaries containing primarily oocytes with cortical alveoli and 

yolk globules filling the cytoplasm were classified as stage 2 

(early vitellogenic), whereas those containing larger oocytes in 

which the cortical alveoli were pushed to the periphery of cell, 

yolk globules filled the center, and the chorion of the developing oocytes were thicker than in earlier stages were designated 

as stage 3 (mid-vitellogenic). Ovaries containing oocytes with 

fused yolk globules that appear as a homogenous mass were 

designated as stage 4 (mature). Ovaries containing post-ovulatory follicles, which can be observed for some time after 

ovulation, are typically assigned to stage 5 (spent); none were 

observed, however. After the ovarian tissue samples were 

staged they were further examined by light microscopy for 

atresia and other conditions such as pigment cell accumulations, parasites, and neoplastic changes. One hundred oocytes 

in each sample were counted when possible; those showing 

morphological evidence of resorption or necrosis were quantified, and the percentage of atretic oocytes was calculated. 

Analogous to the procedure used to stage ovaries, male 

gonadal tissue was classified into five developmental stages 

(0–4) according to the maturity of the predominant stage of 

spermatogenesis of each tissue sample(Blazer, 2002; Nagahama, 1983; Schmitt and others, 2002b). Immature, undeveloped, or regressed testes containing only spermatogonia were 

classified as stage 0 (immature) whereas those containing primarily spermatocytes and spermatids were designated as stage 

1 (early spermatogenic). Mid-spermatogenic (stage 2) testes 

containing approximately equal proportions of spermatocytes, 

spermatids, and spermatozoa were classified as stage 2 (midspermatogenic), and those containing primarily mature spermatozoa were classified as stage 3 (late spermatogenic). Stage 

4 (post-spawning or spent) males were not observed. TesticuMaterials and Methods 

lar tissue was also examined microscopically for any abnormalities such as intersex, parasites, neoplasia, and pigmented 

cell accumulations. Male fish were classified as intersex when 

individual or small foci of undeveloped oocytes were observed 

within testicular tissue (that is, when an ovotestis condition 

was detected). 

Pigmented cell accumulations contain ceroid or lipofuscin pigments derived from the oxidation of lipids, which may 

be indicative of oxidative stress. They are visible in histologic 

sections of ovaries and testes as yellowish-brown deposits. 

The amount of ceroid pigment observed in each sample was 

rated on a scale of 0 (no pigment) to 4 (heavily pigmented). 

Vitellogenin 

A sandwich ELISA was used to determine vtg concentrations in plasma samples obtained from largemouth bass, smallmouth bass, and carp (Denslow and others, 1999). Wells of a 

microtiter plate were saturated with 10 μg/mL of monoclonal 

antibodies developed specifically against bass (3G2) and carp 

(2D4) vtg in phosphate-buffered saline (50 μL/well). Plates 

were incubated overnight, then washed with Tris-buffered 

saline-Tween (TBST). Blocking reagent (10% BSA in TBST, 

360 μL/well) was added to each well and allowed to incubate 

for 2 h. Plates were washed with TBST and 50 μL of standard 

or samples were added and incubated overnight. Unknown 

plasma samples were diluted from 1:100 (bass) to 1:500 (carp) 

in 1% BSA-TBST containing 10 kIU/mL of Aprotinin as 

protease inhibitor. Standard curves were prepared by diluting 

purified vtg in diluted plasma from control male fish of the 

same species tested. After overnight incubation, plates were 

washed with TBST and 50 μL of 1:1000 rabbit polyclonal 

antiserum in 1% BSA-TBST was added to each well. After a 

2-h incubation, plates were washed and 50 μL of goat anti-rabbit F(ab)2 alkaline phosphatase conjugate in 1% BSA-TBST 

was added to each well as the secondary antibody. After 

incubation for 2 h, plates were washed and 100 μL of substrate was added to each well and incubated for 30 min. The 

reaction was stopped by addition of 50 μL/well of 3N NaOH. 

Plates were read at 405 nm in an automated microtiter plate 

reader (Spectromax Pro, Applied Biosystems). Concentrations 

of the unknowns were determined from the standard curves. 

The LOD was 0.002 mg/mL of plasma for bass and 0.005 

mg/mL for carp. All assays were performed in triplicate and 

reported as the mean of the three measurements. The coefficient of variation was <10% for all samples analyzed. Interassay variability was routinely measured by analyzing controls 

on several plates and was found to be <10%. 

Data Set Composition and Statistical Analyses 

Species were grouped into larger taxon groupings for 

analysis. These included carp, bass (largemouth and small­
22 Environmental Contaminants and their Effects on Fish 

mouth bass), Morone sp. [white bass (M. chrysops) and striped 

bass (M. saxatilis)], catfish [channel catfish and blue catfish], 

and northern pike (Esox lucius). For indicators based on individual fish (that is, biomarkers and demographic endpoints), 

data are discussed in terms of the magnitudes of the means or 

medians for combined stages or each stage within a gender 

for different species or taxon grouping. Data were graphically summarized with box plots presenting the mean, range, 

median, and 25th and 75th percentiles. Some biomarker results 

were also analyzed more rigorously (see following sections). 

For statistical testing, transformations were applied as necessary to achieve the normality and homogeneity-of-variance 

required for the application of parametric statistical methods. 

Because the RGB data set is relatively small, the transformations applied were those used in the analysis of the much more 

extensive 1995 MRB data set (Schmitt, 2002a). The length, 

weight, and age data were not transformed. Concentrations of 

contaminants and TCDD-EQ in composite samples, as well 

as EROD activities and vtg concentrations in individual fish, 

were log-transformed (only vtg in female fish was analyzed 

statistically). HAI scores were rank-transformed. Raw data 

can be obtained at <http://www.cerc.usgs.gov/data/data.htm>. 

Descriptive statistics (arithmetic mean, minimum, maximum, and standard error) were computed for length, weight 

and age data for species and taxon groupings at each station. 

Fish from which only regenerated scales were collected (24 

fish; 22 carp, one largemouth bass, and one northern pike) 

were reported as ND (not determined) and excluded from the 

analysis and interpretation of age data and all analyses that 

included age as a factor. Fish for which the field gender identification could not be verified histologically (including four 

fish of the targeted species) were reported as NG (no gonad) 

and likewise excluded from analyses that included gender as a 

factor. Data for carp and bass were summarized and discussed 

in more detail than other species. Because of the influence 

of fish size on Hg concentrations in predatory fish, length 

and weights of bass were also analyzed statistically (see next 

section). Length and weight data for all other species are 

presented only in tabular form. 

All results for composite samples were converted to, 

analyzed statistically as, and reported as wet-weight concentrations. A value of one-half the LOD was substituted for 

censored values in the computation of un-weighted geometric 

station means, for statistical analyses, and for graphing. Concentrations of many contaminants were below detection limits, 

which limited the extent and rigor of the statistical analyses 

that could be performed. All composite sample data (tissue 

concentrations and H4IIE results) are presented graphically 

and as tabular summaries. For p,p’-DDE, As, Cd, Cu, Hg, Pb, 

Se, and Zn, temporal and geographic differences were also 

examined statistically using ANOVA. For these analyses, logtransformed concentrations in carp, bass, catfish, and Morone 

sp. were combined with historical data for these taxa (Schmitt 

and others, 1999b) representing the four NCBP stations (16, 

63, 65, and 65) in the RGB. These data were analyzed as a 

one-way ANOVA in which samples representing stations, collection years, and taxa were grouped into 113 unique combinations (“treatments”). Selected pair-wise comparisons of leastsquares treatment means representing 1997 concentrations 

contrasted against previous years within each station-taxon 

combination were then conducted using Fischer’s protected 

LSD. Treatments representing the 1997 station-taxon means 

were also compared as part of this analysis. These single 

degree-of-freedom, non-orthogonal contrasts used the pooled 

error mean-square (MSe

) to evaluate differences between- or 

among-samples representing the same taxon. A nominal αlevel of 0.01 was used in these comparisons to protect against 

experiment-wise error. Because concentrations of Hg in 

predatory fish increase with size, age, or both [see for example 

(Wiener and others, 2002)], log-transformed length-adjusted 

(HgL) and weight-adjusted (HgW) concentrations were also 

tested using this procedure. Following the method of Brumbaugh and others (2001), the length- and weight-adjusted 

values were computed by dividing the measured Hg concentration in each composite sample by the mean length (m) and 

weight (kg) of the individual fish in the sample. Brumbaugh 

and others (2001) also analyzed age-adjusted concentrations; 

however, age-adjustment could not be performed because 

there were substantial numbers of re-generated scales among 

the fish from some sites. Because these fish could not be 

aged, the mean age of many composite samples could not be 

estimated. In addition, the NCBP fish collected historically 

were not aged; however, the mean lengths and weights of the 

fish comprising the composite samples were reported. Length, 

weight, Hg, HgW, and HgL were also analyzed separately for 

bass using the one-way analysis. This resulted in a more conservative test with only 16 treatments, fewer degrees-of-freedom, and MSe

-values based on only one taxon. These latter 

analyses were not performed for other contaminants because 

the relationships between fish size and concentration are not as 

well documented. 

Many biomarkers differ among species, gender, and 

reproductive stages (Schmitt, 2002a; Schmitt and Dethloff, 

2000). Accordingly, and using the analyses conducted on the 

larger 1995 MRB data set as a guide (Schmitt, 2002a), a series 

of linear ANOVA models were fit to the individual fish data 

for carp and bass to determine the influence of selected factors 

on biomarker responses in these taxa. In these models, station 

and gender were treated as class variables and age and stage 

were considered continuous variables; the models included 

both main effects and interactions. The results of these analyses were reported as F-values and significance levels and were 

used to guide the graphical presentation and discussion of the 

findings; however, because of the complexity of the models 

relative to the small size of the data set, most means were not 

adjusted for the factors in the models. The exception was the 

MA variables; because of the known increase in MA density 

(MAMM) over time (Blazer and others, 2002), the MA data 

were re-analyzed with a model that included age, and ageadjusted station means for MA parameters in carp and bass 

were estimated based on data from fish that could be aged. 
Results and Discussion 23 

Results are generally presented and discussed in terms of 

the magnitudes of the means or medians either for combined 

genders or for each gender within a species or taxon grouping 

and at different stations within a species or taxon grouping. 

Where appropriate, data were also grouped and reported by 

gonadal stage or age. 

Correlations between biomarker results and concentrations of selected contaminants in carp and bass were also 

examined. For these analyses, mean biomarker responses in 

the individual fish comprised by each composite sample were 

combined with contaminant data for that sample. Spearman 

rank correlations, which are not affected by deviation from 

normality, were computed for samples representing male and 

female carp and bass (together and combined within taxon) 

and for catfish. 

Results and Discussion 

Geographic Distribution and Demographic 

Characteristics of the Fish Samples 

A total of 368 fish representing eight species were collected at the ten stations sampled (Table 7). Carp and bass 

accounted for 77% of the fish collected. Of the remaining 

five species, only channel catfish accounted for more than 

10% of the total (Table 7). Carp were obtained at all 10 stations, largemouth bass were obtained at five stations, and the 

remaining species were obtained at one or two stations (Table 

7; Appendix Table 2). Only three smallmouth bass were 

collected; two females at Station 63 and one male at Station 

514 (Table 7; Appendix Table 2). Alternate predator species 

(northern pike, white bass, blue and channel catfish) were collected at the stations from which bass could not be obtained. 

Size and count data for non-target species are summarized in 

Appendix Tables 2 and 3. 

The mean total length (TL) of carp from the RGB was 

531 mm (range 294-854 g) and the mean weight was 2355 g 

(range 325-9000 g) (Table 8). The mean age for all carp was 

3.2 y (range 1-8 y). On average, females outweighed males 

(2537 vs. 2131 g) and were longer (542 vs. 518 mm) but the 

mean ages of males and females were approximately equal 

(3.1 vs. 3.2 y). The largest male and female carp (mean TL 

and weight) were collected from Stations 515, 514, and 513 

(largest to smallest); the smallest were from Station 65 (Table 

8). Female carp were youngest on average at Stations 64 and 

516 and oldest at Station 63 whereas males were youngest 

at Station 514 and oldest at Station 513. Mean age was not 

consistently related to mean size; that is, the oldest carp were 

not necessarily the largest, indicating growth rate differences among stations (Table 8). Overall, the average lengths, 

weights, and ages of carp from the RGB were similar to those 

obtained from the MRB in 1995 (Schmitt and others, 2002b), 

but the ranges were smaller. 

A total of 75 bass (72 largemouth, three smallmouth) 

were collected at five RGB stations (Table 9). The smallmouth bass were within the size ranges of the largemouth bass 

at the two stations (63 and 514) where they were collected. 

Overall, bass averaged 364 mm in TL (range 218-614 mm) 

and 843 g (range 149-4700 g; Table 9). The mean age for all 

bass was 1.8 y (range 0-5 y). On average, females outweighed 

males (1032 vs. 546 g) and were longer (388 vs. 325 mm) and 

older (2.1 vs. 1.3 y). Female bass were generally largest at 

Station 514 and smallest at Station 512 (Table 9). Male bass 

were, on average, smallest at Station 514 and largest at Station 

16. Female mean age was lowest at Station 512 and highest at 

Station 63 whereas male mean age was lowest at Station 513 

and highest at Station 16 (Table 9). As was true for carp, ranking of stations by mean age did not yield the same order as 

ranking by TL or weight, indicating growth rates differences 

among the stations. Overall, the bass from the RGB were 

similar in length and age to those obtained from the MRB in 

1995 (Schmitt and others, 2002b); however, as was also true 

Table 7. Fishes collected in the Rio Grande Basin (preferred species in bold). 

Species No. fish No. stations Taxon grouping 

Common carp (Cyprinus carpio) 207 10 Carp 

Blue catfish (Ictalurus furcatus) 6 1 Catfish (Ictalurus sp.) 

Channel catfish (Ictalurus punctatus) 42 2 Catfish 

Largemouth bass (Micropterus salmoides) 72 5 Bass (Micropterus sp.) 

Smallmouth bass (Micropterus dolomieui) 3 2 Bass (Micropterus sp.) 

Northern pike (Esox lucius) 16 1 Pike 

Striped bass (Morone saxatilis) 1 1 Morone sp. 

White bass (Morone americana) 21 1 Morone sp. 
-- -- -- 

-- -- -- 

24 Environmental Contaminants and their Effects on Fish 

Table 8. Total lengths, weights, and ages of common carp collected in the Rio Grande Basin, by station and gender 

(M, male; F, female; NG, no gonad sample obtained). Shown are arithmetic means, minima, and maxima. See Table 

1 for station locations and collection dates. 

Station, Total length (mm) Weight (g) Age (y) 

gender No. fish Mean Range No. fish Mean Range No. fish Mean Range 

All 206 531 294-854 207 2355 325-9000 184 3.2 1-8 

F 113 542 321-854 114 2537 450-9000 98 3.1 1-8 

M 92 518 294-673 92 2131 325-6400 85 3.2 1-8 

NG 1 491 1 1600 1 3.0 

16 

F 9 587 381-669 10 2762 798-4190 10 3.2 1-5 

M 10 608 371-710 10 3086 663-4400 10 3.3 1-5 

63 

F 15 493 459-589 15 1343 1150-1800 12 4.3 3-6 

M 10 453 424-482 10 1120 800-1500 8 3.9 3-7 

64 

F 23 496 426-571 23 1870 1100-2800 22 2.4 1-4 

M 16 460 410-569 16 1378 900-2600 15 2.4 2-4 

NG 1 491 1 1600 1 3.0 

65 

F 10 365 321-461 10 663 450-1225 10 3.2 1-4 

M 10 342 294-370 10 493 325-650 10 3.4 2-5 

511 

F 13 533 400-734 13 2046 737-4300 10 3.4 2-5 

M 7 454 365-520 7 1259 628-1677 7 4 3-5 

512 

F 6 530 470-645 6 1942 1500-3100 6 3.5 1-6 

M 8 551 480-635 8 2013 1400-3200 8 3.9 2-8 

513 

F 11 601 417-854 11 4242 1204-8900 11 2.6 1-5 

M 9 674 441-763 9 4776 1481-6400 8 4 1-8 

514 

F 10 681 561-850 10 5285 3400-9000 8 3.4 1-8 

M 10 593 551-706 10 3110 2350-5300 10 2.1 1-5 

515 

F 6 694 627-820 6 4250 3200-6500 2 4 4-4 

M 4 615 554-673 4 3225 2500-4400 2 3 2-4 

516 

F 10 580 560-609 10 2855 2500-3300 7 2.4 1-5 

M 8 526 353-606 8 1956 600-2850 7 3 2-5 

for carp, the range was greater in the MRB, where a few older 

(but not larger) fish were sampled. Some RGB bass were 

heavier than any collected in 1995, indicating rapid growth at 

some sites. 

Accumulative Contaminants, H4IIE Bioassay, 

and EROD Activity 

Composite samples (n = 47) from 10 stations were 

analyzed for organochlorine chemical residues, elemental contaminants, and TCDD-EQ by the H4IIE bioassay. Of these, 

22 samples (47%) from ten stations were carp and 12 samples 

(25.5%) from five stations were bass. The remaining 13 

samples (26%) comprised channel catfish (five samples from 

two stations), northern pike (four samples, one station), striped 

bass (one sample, one station), blue catfish (one sample, one 

station), and white bass (two samples, one station). 

Elemental Contaminants 

Arsenic 

Concentrations of As exceeded detection limits (0.05-

0.15 μg/g) in 38 samples (81%) representing all ten stations 

(Table 10). The greatest concentrations (0.30-0.55 μg/g) were 

in carp and white bass from Stations 65 and 514, in the middle 

RGB, and in largemouth bass from Station 512, in the lower 

RGB (Fig. 2). The maximum concentration was in male carp 

from Station 514 (Fig.2; Table 10). Elsewhere, concentrations 

in carp, channel catfish, white bass, and bass were 0.13-0.15 
Results and Discussion 25 

Table 9. Total lengths, weights, and ages of bass (Micropterus spp.) collected in the Rio Grande Basin, by station and 

gender (M, male; F, female; NG, no gonad sample obtained). Shown are arithmetic means, minima, and maxima. See 

Table 1 for station locations and collection dates. 

Station, Total length (mm) Weight (g) Age (y) 

gender No. fish Mean Range No. fish Mean Range No. fish Mean Range 

All 75 364 218-614 75 843 149-4700 74 1.8 0-5 

F 42 388 218-614 42 1032 149-4700 41 2.1 0-5 

M 30 325 229-456 30 546 175-1165 30 1.3 0-4 

NG 3 402 340-474 3 991 474-1600 3 3.3 3-4 

16 

F 11 404 332-546 11 995 439-2700 11 2.1 1-3 

M 10 360 315-456 10 664 431-1165 10 2.2 1-4 

63 

F 6 365 295-431 6 700 350-1150 6 2.8 2-5 

NG 2 433 391-474 2 1250 900-1600 2 3.5 3-4 

512 

F 7 289 218-366 7 435 149-757 7 1.3 0-3 

M 8 293 250-325 8 449 200-606 8 1.3 0-3 

NG 1 340 1 474 1 3.0 

513 

F 13 426 312-614 13 1417 405-4700 12 1.8 0-5 

M 9 339 301-406 9 559 350-1035 9 0.3 0-2 

514 

F 5 433 348-580 5 1454 700-3100 5 2.4 1-3 

M 3 250 229-277 3 375 175-750 3 1 0-3 

μg/g, but all northern pike samples were <LOD (Fig. 2). Geometric mean As concentrations at all stations were relatively 

low (greatest = 0.22 μg/g at Station 511; Table 11); nevertheless, differences among stations were statistically significant 

in carp and bass, but not in catfish or Morone sp. (Table 12). 

Concentrations of As in carp from Station 514, where levels were highest, were significantly greater than those from 

Station 512, where they were lowest, but few intermediate 

differences were significant (Table 12). In contrast to carp, As 

concentrations in bass from Station 514 were lowest and differed significantly from those at Station 512 (Table 12). 

Concentrations of As in 1997 differed significantly from 

historical levels in at least one taxon at Stations 63, 64, and 

65, but there was only one obvious temporal trend—declining 

concentrations in Morone sp. at Station 65 (Table 13). Historically (1980-86), As concentrations in carp and largemouth 

bass from Stations 16, 63, and 65 were ≤0.27 μg/g and those in 

Morone sp. from Station 65 were 0.3-0.6 μg/g. Concentrations 

at Station 64 were <0.09 μg/g in all past collections, but only 

brown trout (Salmo trutta) and white sucker (Catostomus commersoni) were collected. Nevertheless, these concentrations 

did not differ greatly from those in other species collected in 

1997 (Fig. 2), and are similar to concentrations reported for 

salmonids and catostomids from other U.S. rivers (Schmitt, 

2004; Schmitt and others, 2002c; Schmitt and others, 1999b). 

In the 1980s, As concentrations were also high (>0.3 μg/g) in 

gizzard shad (Dorosoma cepedianum) from Stations 16, 63, 

and 65, but this species was not collected in 1997. Concentrations in carp from Station 514 and white bass from Station 65 

were about the same as those in gizzard shad historically (Fig. 

2). They also are about the same as the greatest concentrations detected in carp from areas of high background As in 

New Mexico by Ong and others (1991) and Roy and others 

(1992). The range of concentrations encountered in RGB fish 

was similar to those collected from the MRB in 1995 (Schmitt 

and others, 2002c) and from the RRN in 1992 (Goldstein and 

DeWeese, 1999). 

Arsenic tends to accumulate in planktivorous clupeids 

such as gizzard shad (and also sculpins, Cottus sp.) to a greater 

degree than in other fishes (Hunter and others, 1981; Schmitt 

and Brumbaugh, 1990; Wagemann and others, 1978) and 

can be further accumulated by piscivores (Hunter and others, 

1981). The occurrence of planktivorous clupeids at some sites 

and the dynamics of the ecosystems in which they occur may 

therefore confound trends for As. Schmitt and others (2002c) 

speculated that the comparatively high concentrations of As in 

largemouth bass from the southern parts of the MRB, especially in storage impoundments and the river systems containing them, at least partly reflected the presence of gizzard shad 

and other planktivorous clupeids. The same may be true in the 

RGB; as noted, previous NCBP collections revealed elevated 

As concentrations in gizzard shad. Ecosystem and hydrologic 

differences among stations may also partly explain the comparatively high concentrations in carp and low concentrations 

in bass from Station 514. Although gizzard shad are present 

in Amistad International Reservoir, the fish analyzed for this 

study were collected from the RG below the dam (Table 1). 

Areas of elevated As occur naturally in some parts of the 

RGB, particularly in streams of central New Mexico that drain 

the Jemez Mountains (Wilcox, 1997). These naturally occur­
0.4 

0.5 

Se (μg/g wet-weight) As (μg/g wet-weight)

0.3 

0.2 

0.1 

0 

2 

1.5 

1 

0.5 

0 

Station 

Figure 2. Concentrations of arsenic (As) and selenium (Se) in 

composite samples of whole fish, by station and taxon. Censored 

values are plotted as 50% of LOD. Stations are ordered from 

upstream to downstream and are grouped by sub-basin. See 

Table 1 for station descriptions and collection dates. Micropterus 

sp., largemouth or smallmouth bass; Morone sp., white or striped 

bass; Ictalurus sp., blue or channel catfish. 

ring concentrations were reflected in the 1997 fish samples as 

well as those of previous investigations. The 1997 concentrations generally paralleled those reported for whole fish collected in 1992-96 from the transboundary portions of the RGB 

by the Binational Study (TNRCC, 1994a; 1994b; 1997) and by 

Davis and others (1995). However, the 1992-96 maxima for 

whole fish (0.2-0.3 μg/g) were lower than what we observed 

(Fig. 2; Table 10). Roy and others (1992) reported As concentrations as great as 0.62 μg/g in fish from the Alameda 

Drain, a RG tributary that drains an area of high background 

As in central New Mexico. In addition to natural sources, As 

is discharged from sewage treatment plants in some parts of 

the RGB where As-rich groundwater occurs (Wilcox, 1997), 

and large amounts of arsenical herbicides are used throughout 

the RGB for agricultural and domestic weed control. Arsenic0.6 

64 63 516 65 515 513 16 511 512 

sp. 

sp. 

sp. 

26 Environmental Contaminants and their Effects on Fish 

514 

Upper RG Mid-RG/Pecos Lower RG 

Common Carp 

Micropterus

Northern Pike 

Morone

Ictalurus

Table 10. Occurrence (percentages of samples and stations) and censoring levels for elemental contaminants in composite samples of whole fish from the Rio Grande 

Basin. Also shown are the maximum concentrations and the samples (station, species, and sex) in which they occurred. See Table 1 for station locations and collection dates. NA, not applicable. 

Maximum 1997 concentration Samples Stations Censoring range (μg/g) Analyte (% of 47) (% of 10) (μg/g) Station Sex Species 

Arsenic (As)Cadmium (Cd)Chromium (Cr)Copper (Cu)Lead (Pb) Mercury (Hg) 

Nickel (Ni)Selenium (Se)Zinc (Zn) 

81

28

100

100

62

96

100

100

100 

100

60

100

100

80

100

100

100

100 

0.05-0.15

0.002-0.003

NA

NA

0.03-0.09

0.023 a

NA

NA

NA 

0.55

0.12

71.8

1.80

0.83

0.46

4.2

1.87

83.6 

514

64

63

64

16

63

512

65

511 

M Common carp 

F Common carp 

M Common carp 

F Common carp 

F Largemouth bass 

M Largemouth bass 

M Common carp 

M White bass 

F Common carp

a Two Hg values censored at 0.023 μg/g 
Results and Discussion 27 

Table 11. Geometric mean, minimum, and maximum concentrations (μg/g, wet-weight) of elemental 

contaminants in fish from stations in the Rio Grande Basin. Censored values were replaced by 50% LOD for the 

computation of station means, but only if at least one value exceeded detection limits. See Table 1 for station 

locations and collection dates. 

Element 

Station As Cd Cr Cu Hg Ni Pb Se Zn 

16 Mean 0.20 0.03 1.08 0.62 0.20 0.24 0.21 0.66 29.8 

(n = 4) Min. 0.15 <0.03 0.71 0.36 0.20 0.18 0.08 0.56 11.5 

Max. 0.24 0.07 1.59 1.23 0.20 0.31 0.83 0.74 75.3 

63 Mean 0.17 0.02 9.51 0.67 0.24 1.06 0.04 0.45 22.8 

(n = 6) Min. 0.09 <0.02 5.46 0.42 0.15 0.57 <0.06 0.32 11.5 

Max. 0.25 0.08 71.76 1.16 0.46 2.14 0.10 0.54 75.2 

64 Mean 0.08 0.03 1.12 1.06 0.09 0.48 0.08 0.29 51.1 

(n = 8) Min. <0.09 <0.02 0.28 0.74 <0.02 0.19 <0.07 0.17 28.4 

Max. 0.14 0.12 9.86 1.80 0.18 1.88 0.26 0.69 81.0 

65 Mean 0.14 <0.03 0.51 0.74 0.04 0.43 <0.07 0.86 31.9 

(n = 4) Min. <0.09 <0.02 0.38 0.56 <0.02 0.25 <0.07 0.25 19.1 

Max. 0.44 <0.03 0.68 0.95 0.18 0.77 <0.07 1.87 52.0 

511 Mean 0.22 <0.03 11.43 0.49 0.09 0.34 0.21 0.43 30.4 

(n = 5) Min. 0.18 <0.03 5.82 0.29 0.07 0.18 0.11 0.28 12.6 

Max. 0.27 <0.03 33.98 0.91 0.13 0.70 0.42 0.60 83.6 

512 Mean 0.13 <0.03 13.57 0.60 0.09 0.99 0.08 0.62 31.1 

(n = 4) Min. 0.05 <0.02 10.00 0.43 0.07 0.37 <0.05 0.59 14.6 

Max. 0.30 <0.03 18.31 0.82 0.12 4.21 0.21 0.65 61.5 

513 Mean 0.14 <0.03 28.51 0.88 0.09 0.95 0.11 0.61 25.4 

(n = 4) Min. 0.06 <0.03 10.73 0.59 0.07 0.34 0.07 0.47 13.3 

Max. 0.26 <0.03 70.15 1.63 0.14 3.29 0.18 0.68 57.2 

514 Mean 0.13 0.02 4.02 0.90 0.16 0.28 0.05 1.22 19.5 

(n = 5) Min. 0.04 <0.02 2.81 0.66 0.07 0.23 <0.03 1.10 11.1 

Max. 0.55 0.06 10.36 1.56 0.40 0.47 0.20 1.54 59.0 

515 Mean 0.10 0.04 16.57 1.12 0.15 0.50 <0.09 0.78 43.2 

(n = 3) Min. <0.14 <0.03 14.40 0.91 0.14 0.49 <0.03 0.35 16.6 

Max. 0.17 0.07 21.80 1.27 0.16 0.50 <0.09 1.24 73.4 

516 Mean 0.09 0.02 10.34 0.90 0.06 0.39 0.07 0.43 34.8 

(n = 4) Min. <0.05 <0.02 6.90 0.54 0.05 0.36 <0.05 0.32 17.9 

Max. 0.22 0.07 16.19 1.29 0.06 0.42 0.13 0.59 73.7 

containing compounds are also used to defoliate cotton, which 

is grown in many parts of the RGB, and As and other elemental contaminants are released during the smelting of metals 

and the combustion of coal. Consequently, As was among the 

eight elements associated with the combustion of fossil fuels 

for which Van Metre and others (1997) reported increasing 

concentrations in sediment cores from Amistad Reservoir. In 

contrast, As concentrations in cores from Llano Grande Lake 

did not change appreciably from 1989 to 2001 (Mahler and 

Van Metre, 2002). Arsenic was also among the elemental 

contaminants detected at elevated concentrations in fish (liver 

samples) and other matrices obtained from the RG below 

the Creede, Colorado mining district in 1992-93 (Carter and 

Anderholm, 1997; Levings and others, 1998); however, 1997 
28 

Table 12. Least-squares mean concentrations (all in μg/g) of p,p’-DDE, mercury (Hg), length-adjusted Hg (HgL), weight-adjusted Hg (Hg), arsenic (As), 

cadmium (Cd), lead (Pb), zinc (Zn), and selenium (Se). Within each group of taxon-station means, values followed by the same letter are not significantly different (P < 0.01, Fisher’s protected LSD). Also shown are ANOVA F-values, degrees-of-freedom (df), and significance levels (*0.01 < P  0.05; **P  0.01) 

See Table 1 for station locations and collection dates. Taxon, DDE Hg HgL c HgW d As Cd Pb Cu Zn Se station Carp

16 0.39 a 0.20 a 0.34 a 0.07 acde 63 0.02 b 0.16 a 0.34 a 0.13 ae 64 0.05 c 0.12 a 0.24 a 0.07 e 65 0.02 b 0.07 a 0.21 a 0.13 ae 511 0.52 a 0.09 a 0.19 a 0.06 bcde 512 0.07 c 0.08 a 0.14 a 0.04 bcde 513 0.21 ac 0.07 a 0.11 a 0.02 bcde 514 0.09 c 0.11 a 0.17 a 0.03 bce 515 0.15 c 0.16 a 0.25 a 0.04 bce 516 0.29 ac 0.06 a 0.11 a 0.03 bce 

Catfish a 

511 1.43 a 0.09 a 0.22 a 0.14 a 515 0.10 b 0.14 a 0.37 a 0.16 a 516 0.11 b 0.05 a 0.12 a 0.08 a 

Bass b 

16 0.38 a 0.20 a 0.52 a 0.24 a 63 <0.01 b 0.30 a 0.79 a 0.37 a 512 0.09 cd 0.11 a 0.38 a 0.25 a 513 0.15 d 0.12 a 0.31 a 0.13 a 514 0.05 c 0.19 a 0.62 a 0.32 a 

Morone sp. 

63 <0.01 a 0.28 a 0.39 a 0.12 a 65 0.03 b 0.02 b 0.07 b 0.10 a 

0.18 abc 0.06 ac 0.09 ab 1.06 a 67.7 a 0.73 abc 

0.17 abc 0.08 a 0.07 ac 1.14 a 69.1 a 0.47 ad 

0.12 bc 0.04 a 0.10 abc 1.03 a 62.6 a 0.29 d 

0.14 abc 0.01 b 0.04 a 0.73 a 32.3 b 0.66 abc 

0.21 abc 0.01 b 0.24 b 0.79 a 67.0 a 0.54 ac 0.06 b 0.01 b 0.15 bc 0.79 a 61.5 ab 0.61 ab 

0.23 ac 0.02 bc 0.15 bc 1.08 a 46.8 ab 0.68 abc 

0.51 a 0.03 ac 0.10 ab 1.31 a 48.8 ab 1.38 b 

0.11 bc 0.06 ac 0.04 a 1.25 a 69.7 a 1.17 bc 

0.11 bc 0.05 ac 0.11 ab 1.14 a 56.9 ab 0.56 ac 

0.24 a 0.02 a 0.18 a 0.30 a 13.8 a 0.35 a 

0.08 a 0.02 a 0.05 a 0.91 b 16.6 a 0.35 a 

0.07 a 0.01 a 0.04 a 0.71 ab 21.3 a 0.32 a 

0.21 ab 0.01 a 0.47 a 0.36 a 13.2 a 0.60 ab 

0.21 ab 0.01 a 0.04 a 0.53 ab 15.3 a 0.51 a 

0.27 b 0.01 a 0.04 a 0.47 ab 15.8 a 0.64 ab 

0.08 ab 0.01 a 0.08 a 0.71 ab 13.8 a 0.55 ab 

0.06 a 0.01 a 0.03 a 0.76 b 12.7 a 1.15 c 

0.09 a 0.01 a 0.03 a 0.54 a 19.2 a 0.32 a 

0.14 a 0.01 a 0.04 a 0.75 a 31.5 a 1.13 b 

ANOVA-F 41.67 ** 5.44 ** 5.97 ** 7.10 ** 4.21 ** 11.53 ** 5.79 ** 7.97 ** 13.45 ** 7.03 ** 

df 67, 45 61, 42 61, 42 61, 42 54, 40 54, 40 54, 40 37, 28 37, 28 48, 23 

Environmental Contaminants and their Effects on Fish 

a Ictalurus sp. b Micropterus sp. 

μg/g/m 

d μg/g/kg c 
29 

concentrations in fish from Station 64 (RG at Alamosa, CO), 

which is downstream of these sources, were low (Fig. 2). In 

contrast, concentrations of As were 2- to 10-fold greater (1-3 

μg/g) in several species of fish collected in 1996 from resacas 

along the lower RG (Mora and others, 2001) than the greatest 

concentrations in 1997 samples from the RGB (Fig. 2). Mora 

and others (2001) attributed the elevated concentrations in 

the resacas to municipal wastes; however, given the intensity 

of the agricultural activity in the lower RG valley and the 

fact that cotton and vegetables are among the crops produced 

(Mora and others, 2001), arsenical pesticides, defoliants, or 

both may also have contributed to the elevated concentrations. 

Mora and Wainwright (1997), citing data from the Texas 

Agricultural Extension Service, noted that almost 85,000 kg 

of “harvest aids”, a category which includes defoliants, were 

used in the region comprising the lower RG valley in 1994. 

The U.S. Department of Agriculture also reported that 98,000 

lbs (>44,000 kg) of arsenical herbicides and 1.8 million lbs 

(0.8 million kg) of arsenical defoliants were used on cotton in 

Texas and Oklahoma during 1976 (Eichers and others, 1978). 

Elevated concentrations of As in fish as a result of natural 

sources have prompted human consumption advisories in parts 

of New Mexico (Wilcox, 1997). However, the As concentrations in the RGB fish collected in 1997 (0.14–0.55 μg/g) 

were not sufficiently high to represent a hazard to the fish or 

to piscivorous wildlife according to current criteria (USEPA, 

1999). A review by Jarvinen and Ankley (1999) included 

several laboratory studies in which As effects were evaluated 

relative to whole-body concentrations. For example, concentrations of 8.1-13.5 μg/g were associated with loss of equilibrium and 5.4 μg/g caused increased mortality in rainbow trout 

(Oncorhynchus mykiss) fingerlings (McGreachy and Dixon, 

1990; McGreachy and Dixon, 1992). Adult bluegill experienced reduced survival and growth at 11.6 μg/g (Gilderhus, 

1966). Arsenic concentrations in all fish from the RGB were 

below these effect levels. In addition, As is largely accumulated by freshwater fish as organo-arsenical compounds, which 

are less toxic than inorganic forms (Law, 1996; Maeda, 1994). 

Selenium 

Selenium was detected in all samples from all stations at 

concentrations of 0.17-1.87 μg/g, with the maximum occurring in male white bass from Station 65 (PR at Red Bluff 

Lake; Table 10; Fig. 2). Concentrations ≥0.5 μg/g were present in at least one sample from all stations, and all samples 

from Station 514 (RG at Amistad Reservoir) as well as two 

from each of Stations 515 (RG at Langtry) and 65 contained 

>1.0 μg/g (Fig. 2). At Station 65, both of the greatest concentrations (>1.5 μg/g) were in samples representing male 

fish (carp and white bass); concentrations in corresponding 

samples of female fish were lower (Fig. 2). At Station 514, 

Se concentrations in largemouth and smallmouth bass did not 

differ appreciably (Fig. 2) even though one of the largemouth 

bass samples comprised relatively small fish; however, these 

Results and Discussion 

differences were not tested statistically (data not shown). Concentrations in male and female carp from this site were also 

similar to each other (Fig. 2). At Station 63, the only other 

site from which both largemouth and smallmouth bass were 

collected, concentrations in bass were uniformly low (about 

0.5 μg/g). The greatest Se geometric station mean was for 

Station 514 (1.25 μg/g; Table 11). Other stations with means 

>0.5 μg/g included Stations 16, 65, 512, 513, and 515 (Table 

11). Concentrations of Se were greatest in bass from Station 

514, carp from Stations 65, 515, and 514, and white bass from 

Station 65 (male only; Fig. 2). Differences among or between 

stations were statistically significant in all taxa except catfish 

(Table 12). 

The greatest Se concentrations in fish from the RGB (1-2 

μg/g) exceeded those from all stations sampled in the MRB 

during 1995 except one (Schmitt and others, 2002c). Concentrations in RGB carp also exceeded those in carp collected 

from the RRN in 1994 by Goldstein and DeWeese (1999), 

which averaged 1.18 μg/g dw (about 0.35 μg/g ww). However, 

although high compared to other parts of the RGB, the maximum RGB concentrations (1-2 μg/g) are typical for fish from 

seleniferous areas supporting irrigated agriculture. In addition, they are lower by at least 50% than the 3-5 μg/g reported 

for carp and bass collected in 1995 from one MRB site heavily 

affected by irrigated agriculture (Schmitt, 2004; Schmitt and 

others, 2002c; 1999b). 

Concentrations of Se in 1997 were generally within the 

range of 1980-86 NCBP values at Stations 16, 64, and 65 

(Schmitt and others, 1999b) when comparisons are restricted 

to the same species. In fact, the only statistically significant 

and obvious temporal trend for Se (in Morone sp. from Station 65) has been downward (Table 13). Relative to historical 

values for Station 64 (RG at Alamosa), the 1997 concentration 

(0.69 μg/g) in northern pike was high; however, northern pike 

had not been collected previously at this site, and the concentration is within the range of reported values for this species in 

other U.S. rivers (Schmitt and others, 2002c; Schmitt and others, 1999b). Selenium concentrations >1.0 μg/g and in white 

bass from Station 65 confirmed previous findings; levels of 1.5 

to >3.0 μg/g were reported in white bass from Station 65 in 

1978-84 (Lowe and others, 1985; Schmitt and others, 1999b). 

Concentrations in carp from Station 63 (mean = 0.47 μg/g) 

have not changed appreciably over the last decade (Table 13) 

and are about the same as levels in carp from the RG in New 

Mexico and Bosque del Apache NWR during the 1980s (Ong 

and others, 1991). The USFWS reported elevated Se concentrations in carp from 10-Mile Reservoir and Bitter Lake NWR, 

which are on the Pecos River in New Mexico (unpublished 

data, USFWS, Albuquerque, NM). The USFWS also detected 

elevated concentrations in the eggs of fish-eating birds at Bitter Lake NWR. 

In the RG mainstem, the Binational Study (TNRCC, 

1994b; 1997) reported Se concentrations >1.0 μg/g in whole 

carp and channel catfish collected from 1992 through 1996 

below El Paso (near Station 516), in white bass and carp at 

Langtry (near Station 515), and in carp and bass below Amis­
—Continued

30 

Table 13. Least-squares mean concentrations (all in ug/g) of p,p’-DDE, mercury (Hg), length-adjusted Hg (HgL), weight-adjusted Hg (HgW), arsenic (As), 

cadmium (Cd), lead (Pb), zinc (Zn), and selenium (Se). Within each group of station-taxon means, values followed by asterisks (*) differ significantly (*0.01 < P  

0.05;**P  0.01, Fisher’s protected LSD) from 1997 means. Also shown are ANOVA F-values, degrees-of-freedom (df), and significance levels. ND, no data/not 

measured. See Table 1 for station locations and 1997 collection dates. 

Station, taxon	 Year DDE Hg HgL HgW As Cd Pb Cu Zn Se 

16, carp 1981 0.44 0.05 * 0.11 0.04 0.15 0.03 0.10 0.75 41.0 0.40

 1997 0.39 0.20 0.34 0.07 0.18 0.06 0.09 1.06 67.7 0.73 16, bass a 1980 2.57 ** 0.24 0.64 

1984 2.09 ** 0.18 0.50 

1986 2.73 ** 0.26 0.57 

1997 0.38 0.20 0.52 

0.29 0.06 0.01 0.10 * 0.31 12.7 0.57

0.26 0.11 0.01 0.07 ** 0.31 13.2 0.44 

0.17 0.17 <0.01 * 0.12 * 0.61 11.6 0.49

0.24 0.21 0.01 0.47 0.36 13.2 0.60 

63, carp 1972 0.11 ** 0.16 0.51 

1973 0.11 ** 0.11 0.30 

1974 0.09 ** ND ND 1978 0.07 ** 0.26 0.67 

1980 0.04 * 0.16 0.39 

1984 0.01 0.11 0.24 1986 0.01 0.13 0.31 1997 0.02 0.16 0.34 

0.35 0.13 0.03 ** 0.17 ND ND 0.64 * 

0.20 0.05 0.03 ** 0.05 ND ND 0.23

ND ND ND ND ND ND ND

0.38 * 0.04 * 0.13 0.28 ** 0.98 84.7 0.50

0.20 0.12 0.08 0.30 ** 1.15 68.9 0.41

0.06 0.08 0.07 0.25 ** 0.73 51.8 0.29

0.13 0.14 0.04 0.18 * 1.93 52.2 0.34

0.13 0.17 0.08 0.07 1.14 69.1 0.47 

63, bass a 1970 0.23 ** 0.52 

1971 0.15 ** 0.60 

1972 0.09 ** 0.28 

1973 0.05 ** 0.05 ** 

1978 0.10 ** 0.65 

1980 0.03 ** 0.17 

1984 0.01 0.12 1997 <0.01 0.30 

1.10 0.25 ND ND ND ND ND ND

1.41 0.48 0.13 0.03 * 0.05 ND ND ND

0.73 0.31 0.27 0.03 0.05 ND ND 0.66 

0.16 ** 0.08 ** 0.17 0.03 0.05 ND ND 0.20 * 

1.69 0.57 0.16 0.01 0.18 ** 0.44 16.3 0.56

0.50 0.29 0.27 0.01 0.10 0.49 12.9 0.41

0.45 0.44 0.27 <0.01 * 0.01 * 0.53 13.8 0.36

0.79 0.37 0.21 0.01 0.04 0.53 15.3 0.51 

63, Morone sp. 1971 0.17 ** 0.63 1.55 0.69 ** 0.18 0.03 * 0.05 ND ND ND

 1997 <0.01 0.28 0.39 0.12 0.09 0.01 0.03 ND ND ND 

Environmental Contaminants and their Effects on Fish 
31 

Table 13. Least-squares mean concentrations (all in ug/g) of p,p’-DDE, mercury (Hg), length-adjusted Hg (HgL), weight-adjusted Hg (HgW), arsenic (As), 

cadmium (Cd), lead (Pb), zinc (Zn), and selenium (Se). Within each group of station-taxon means, values followed by asterisks (*) differ significantly (*0.01 < P  0.05;**P  0.01, Fisher’s protected LSD) from 1997 means. Also shown are ANOVA F-values, degrees-of-freedom (df), and significance levels. ND, no data/not 

measured. See Table 1 for station locations and 1997 collection dates.—Continued 

Station, taxon	 Year DDE Hg HgL HgW As Cd Pb Cu Zn Se 

64, carp 1970 0.23 ** 0.03 * 0.06 * 

1971 0.11 ** 0.08 0.30 

1972 0.24 ** 0.05 0.13 

1997 0.05 0.12 0.24 

0.02 * ND ND ND ND ND ND

0.34 ** 0.04 * 0.03 0.19 ND ND ND

0.05 0.08 0.03 4.65 ** ND ND 0.38

0.07 0.12 0.04 0.10 ND ND 0.29 

65, carp 1986 0.08 ** 0.04 0.13 0.09 0.01 ** 0.01 0.28 ** 3.25 ** 52.9 0.63

 1997 0.02 0.07 0.21 0.13 0.14 0.01 0.04 0.73 32.3 0.66 

65, Morone sp. 1974 0.47 ** ND ND 

1978 0.17 ** 0.12 ** 0.33 * 

1980 0.14 ** 0.06 0.14 

1984 0.08 * 0.06 0.22 

1986 0.16 ** 0.14 ** 0.31 * 

1997 0.03 0.02 0.07 

ND ND ND ND ND ND ND 

0.15 0.66 * 0.01 0.10 2.91 ** 20.1 3.05 * 

0.05 0.60 * 0.01 0.10 1.00 11.2 ** 1.96

0.22 0.24 <0.01 * 0.06 0.94 14.2 * 1.50 

0.09 0.32 <0.01 ** 0.14 * 5.68 ** 13.0 * 1.00

0.10 0.14 0.01 0.04 0.75 31.5 1.13 

ANOVA-F -- 41.67 ** 5.44 ** 5.97 ** 7.10 ** 4.21 ** 11.53 ** 5.79 ** 7.97 ** 13.45 ** 7.03 ** 

df -- 67, 45 61, 42 61, 42 61, 42 54, 40 54, 40 54, 40 37, 28 37, 28 48, 23 

a Micropterus sp. 

Results and Discussion 
32 Environmental Contaminants and their Effects on Fish 

tad International Reservoir (Davis and others, 1995). However, and although it is an important component of fossil fuel 

emissions, Se was not among the eight elements analyzed in 

sediment cores from Amistad Reservoir by Van Metre and others (1997). Mora and others (2002) reported elevated concentrations in several avian prey species of the peregrine falcon 

sampled in 1996 from the Big Bend area, and concluded that 

Se probably originating from agricultural activities was among 

the contaminants potentially responsible for impaired reproduction in this Federally listed species. Further downstream, 

concentrations of Se in fish collected in 1996 from resacas 

along the lower RG were <0.8 μg/g (Mora and others, 2001), 

as were those in contemporaneously collected fish from the 

lower reaches of the river itself (Davis and others, 1995; 

TNRCC, 1994b; 1997). In the upper RG, Se was detected at 

elevated concentrations in sediment, but one sample of carp 

liver collected in 1992-93 was among the greatest measured by 

the NAWQA program (Carter and Anderholm, 1997; Levings 

and others, 1998). 

Several studies that evaluated effects relative to wholebody Se concentrations in fish were included in the review 

by Jarvinen and Ankley (1999). Among these were several 

showing that concentrations of 8-16 μg/g (dw; estimated 

1.6-3.2 ww assuming 80% moisture) were associated with 

reproductive failure in fathead minnows (Pimephales promelas) (Schultz and Hermanutz, 1990) and bluegill (Baumann 

and Gillespie, 1986; Coyle and others, 1993; Hermanutz and 

others, 1992). Coyle and others (1993) suggested that maternally transferred Se could affect hatching success and larval 

survival, and noted that it is essential to examine multiple life 

stages to correctly assess toxicity/tissue concentration relationships (Jarvinen and Ankley 1999). According to the criteria of 

Lemly (1996; 2002), whole-body Se concentrations should not 

exceed 4 μg/g dry weight (about 0.8 μg/g wet-weight assuming 80% moisture) to avoid toxicity to the fish and 3 μg/g dry 

weight (about 0.6 μg/g wet-weight) to protect piscivorous 

wildlife. Individual samples and three station means (Stations 65, 514, and 515) exceeded Lemly's fish threshold, and 

three additional station means, along with numerous samples, 

approached or exceeded the wildlife value (Fig. 2; Table 11). 

In addition, Se was among the contaminants identified as 

potential threats to Laguna Atascosa and other NWRs in the 

RGB (USFWS, 1986), a concern that appears to remain warranted. 

Mercury 

Mercury (as total Hg) was detected (>0.023 μg/g ww) 

in 45 of 47 samples (96%) from all 10 stations (Table 10). 

Concentrations were generally greatest in predatory fishes; the 

exception was Hg in northern pike from Station 64, which varied more than most (Fig. 3). Concentrations were >0.25 μg/g 

in largemouth bass and striped bass from Stations 63; however, 

levels in smallmouth bass and carp from this site were lower 

(<0.20 μg/g; Fig. 3), which is typical for these species (Bahn0.5 

0.4 

0.3 

0.2 

0.1 

0 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

64 63 516 65 515 513 16 511 512 

Upper RG 

ML 

FL 

FS 

FL 

MS 

ML 

FL 

ML 

FS 

MS 

FL 

ML 

ML 

FL 

FS 

ML 

MS 

FL 

sp. 

sp. 

sp. 

514 

Mid-RG/Pecos Lower RG 

Common Carp 

Micropterus

Northern Pike 

Morone

Ictalurus

Station 

Figure 3. Concentrations of mercury in composite samples of 

whole fish, by station and taxon. Shown are unadjusted concentrations (Hg), length-adjusted concentrations (HgL), and weightadjusted concentrations (HgW). Also shown for Stations 63 and 

514 are individual samples comprising male (M) and female (F) 

largemouth (L) and smallmouth (S) bass referred to in the text. 

Censored values are plotted as 50% of LOD. Stations are ordered 

from upstream to downstream and are grouped by sub-basin. See 

text for computations and Table 1 for station descriptions and collection dates. Micropterus sp., largemouth or smallmouth bass; 

Morone sp., white or striped bass; Ictalurus sp., blue or channel 

catfish. HgW (μg/g/kg) HgL (μg/g/m) Hg (μg/g wet-weight) 
33 

ick and others, 1994; Brumbaugh and others, 2001; Goldstein 

and others, 1996; Goldstein and DeWeese, 1999; Schmitt and 

others, 2002c; 1999b). The maximum 1997 geometric station 

mean (0.24 μg/g) also occurred at Station 63 (Table 11). The 

next greatest was 0.20 μg/g (Station 16), with all others ≤0.15 

μg/g (Table 11). Elephant Butte Reservoir (Station 63) has a 

well-documented history of Hg contamination, and fish consumption advisories have been issued for sport fish (Appendix 

Table 1). 

The maximum 1997 concentration of total Hg in an individual sample was 0.46 μg/g, in male largemouth bass from 

Station 63 (Table 10). Concentrations in female largemouth 

bass and smallmouth bass from Station 63 were lower (Fig. 

3). However, the male largemouth bass from Station 63 were 

larger on average (433 mm, 1250 g) than the females (379 

mm, 775 g) and the smallmouth bass (338 mm, 550 g; Table 

9). Consequently, some of the relative differences among 

the three samples were removed by adjusting for fish length 

(Fig. 3). Adjusting for weight reversed the relative concentrations in male and female largemouth bass, but all values 

for smallmouth bass were lower than those in largemouth 

bass (Fig. 3). These results indicate that the among-species 

concentration differences at Station 63 were not strictly related 

to differing fish sizes among the samples. Largemouth and 

smallmouth bass were also obtained from Station 514. At 

this site, concentrations were greatest in female largemouth 

bass (0.40 μg/g) and lowest in male largemouth bass (0.07 

μg/g); concentrations in smallmouth bass were intermediate 

(Fig. 3). The mean length and weight of the male largemouth 

bass from Station 514 were considerably lower (that is, the 

fish were smaller) than the other two samples (Table 9). As 

was true for Station 63, most among-sample differences were 

removed by adjusting for fish length, but not weight (Fig. 

3), primarily because the male largemouth bass sample, with 

the lowest total Hg concentration, also comprised small fish 

(237 mm, 188 g) relative to the other two samples (277-432 

mm, 750-1454 g). Stations 515 (RG at Langtry) and 514 

(below Amistad Dam) are downstream from the Terlingua 

(Texas) mining district, where Hg was extracted from cinnabar 

deposits for nearly a century (Sharpe, 1980). Mora and others 

(2002) reported elevated concentrations of Hg in several avian 

prey species of the peregrine falcon sampled in 1996 from the 

Big Bend area, and concluded that Hg probably originating 

from the Terlingua district was among the contaminants potentially responsible for impaired reproduction in this species. 

Although total Hg concentrations in bass from Station 514 

were high relative to most other sites in the RGB, those in carp 

and catfish from Station 515 were not (Fig. 3). 

Despite apparent among-station differences for total Hg 

in bass, only differences in Morone sp. and carp were statistically significant (Table 12). Un-adjusted (Hg) and lengthadjusted (HgL) total Hg concentrations in Morone sp. were 

significantly greater at Station 63 than at Station 65 (Fig. 3; 

Table 12). The weight-adjusted concentrations (HgW) did not 

differ significantly among stations for any predator species; 

however, weight-adjusted differences in carp were statistically 

Results and Discussion 

significant (Table 12). Statistical analysis of the bass alone 

yielded similar results (Table 14). In addition, the relative 

rankings of the stations within each taxon grouping in both 

analyses was similar for Hg, HgL, and HgW (Tables 12 and 

14); however, the magnitude of the among-sample differences 

at each station were smaller for the adjusted values (Fig. 3; 

Tables 12 and 14), indicating that some of the observed differences were related to fish size. Nevertheless, most relative 

differences remained after adjustment, which suggests that 

spatial differences were not entirely artifacts of fish size. 

Concentrations of total Hg in bass changed significantly 

at several stations in collections spanning the period 1970-97, 

and these differences were consistent for Hg, HgL, and HgW 

(Table 13). In addition, the same results were obtained when 

the analysis was restricted to bass (Table 14). Overall, Hg 

concentrations in bass at Station 63, Morone sp. at Stations 

63 and 65, and carp at Station 64 differed significantly among 

years (Tables 13 and 14), but there were no clearly evident 

temporal trends at any of these stations. As noted for the 

geographic differences, the ordering of the years within taxa 

was identical for Hg, HgL, and HgW even though the mean 

size of bass from Stations 16 and 63 varied substantially from 

1970-97 (Table 14). The size differences were not statistically 

significant, however (Table 14). In addition, the size range of 

the bass from these stations was smaller than that spanned by 

the NAWQA program’s national Hg pilot study (Brumbaugh 

and others, 2001) in which HgL and HgW computed in this 

manner were also used to normalize concentrations relative to 

fish size. Overall, the results of these analyses indicate that 

the among-year concentration differences were also not overly 

influenced by differences in the sizes of the fish comprised by 

the samples. 

In 1997, total Hg concentrations in largemouth bass from 

Station 16, smallmouth bass and carp from Station 63, and 

white bass from Station 65 were within the range of 1980-86 

concentrations in those species (Schmitt and others, 1999b). 

Concentrations of total Hg at Station 64 were relatively low 

in the 1980s and in 1997, but no species from Station 64 were 

common to collections during this period. However, a composite sample of brown trout collected in 1987 from the RG 

near the Colorado-New Mexico state line contained 0.20 μg/g 

(Roy and others, 1992). At Station 63, total Hg concentrations 

in both male and female largemouth bass returned to levels 

reported during the 1970s (Lowe and others, 1985; May and 

McKinney, 1981) following a decade-long decline (Table 13) 

(Schmitt and Brumbaugh, 1990; Schmitt and others, 1999b). 

Total Hg concentrations in carp were also greater in 1997 than 

during the 1980s, but the differences were not as great as in 

bass (Table 13). Concentrations of Hg in fish collected during the 1980s by Ong and others (1991) from the RG in New 

Mexico were universally low; levels in whole carp from the 

RG upstream of Elephant Butte Reservoir were all <0.025 μg/ 

g , and carp from the reservoir contained only 0.10 μg/g. The 

cycling of Hg in Elephant Butte Reservoir is related to water 

level fluctuations and the associated growth and decomposition of riparian vegetation as well as to the possible presence 
-- -- -- 

34 

Table 14. Geometric mean concentrations of mercury (Hg) in bass a from Stations 16 and 63, 1970-1997. Shown are means for measured Hg

concentrations as well as for length-normalized (HgL) and weight-normalized (HgW) values. Also shown are mean total lengths (TL), weights (Wt), relative ranking of Hg means for each station, numbers of samples, and ANOVA 

F-values with degrees-of-freedom. Within stations, means followed by 

the same lower-case letter are not significantly different (P > 0.05, Fisher’s protected LSD) b. See Table 1 for station locations and 1997 collection dates. 

Station, year TL (mm) Wt (g) Hg (μg/g) Hg rank HgL (μg/g/m) HgL HgW (μg/g/kg) HgW n rank rank Station 63

1970

1971

1972 1973 1978

 1980  1984 

1997 b

Station 16 

1980 1984 

1986

1997 

F 11, 4 

472 

425 

381 

317 

383 

340 267 

383

376 363 

454

382 

2.42 

a

a

a

a

a

aAaA

aA

aAaA

aA

aA 

2041

1247

907 

590 

1134

590 272 

858 

816 680 

1547

830 

3.06 

a

a

a

a

a

aAaA

aA

aAaA

aA

aA 

0.52 

0.60 

0.28 

0.05 

0.65 

0.17 0.12 

0.30 

0.24 0.18 

0.26

0.20 

5.26 

abc

ab

abcd

e

a

cdeAdeA

abcdA 

aAaA

aA

aA

* 

6

7

4

1

8

3

2

5

21

3

4 

1.10 

1.41 

0.73 

0.16 

1.69 

0.50 

0.45 

0.79 

0.64 0.50 

0.57

0.52 

6.82 

abc

ab

abc

d

a

cA

cA

abcA

aAaA

aA

aA

** 

6

7

4

1

8

3

2

5

31

2

4 

0.25 

0.48 

0.31 

0.08 

0.57 

0.29 

0.44 

0.37 

0.29 0.26 

0.17

0.24 

9.62 

bcd

ab

abcd

e

a

bcdA 

abcA

abcdA 

abAaA

aA

bA

** 

2

7

4

1

8

36

5

3

2

1

4 

1

2

1

1

1

11

3

1

1

1

2

16 

* 0.05 < P  0.10; **P  0.05 a All largemouth bass (Micropterus salmoides) except for one 1997 sample of smallmouth bass (M. dolomieui) from Station 63. b Between stations, pairs of means representing the same year all contain the same upper-case subscript (A), indicating that none differ significantly (P>0.05, Fisher’s protected LSD). 

Environmental Contaminants and their Effects on Fish 

of sulfide-rich springs and atmospheric sources (Garcia and 

Kidd, 1979). Strong and prolonged thermal stratification 

and an associated anoxic hypolimnion characterize the lake 

in summer, which enhances Hg methlyation (Canavan and 

others, 2000). In addition, ecosystem structure and dynamics 

profoundly influence Hg accumulation; food chain length is 

particularly important (Cabana and others, 1994). The extent 

to which theses factors changed over time and their attendant 

effects on Hg in predatory fish is not known. 

In the transboundary portions of the RGB, 1997 total Hg 

concentrations were generally similar to those reported for 

whole fish collected during 1992-96 by the Binational Study 

(TNRCC, 1994b; 1997) and by Davis and others (1995). They 

were also similar to concentrations in fish from resacas along 

the lower RG (Mora and others, 2001). Mercury was the only 

elemental contaminant measured in USEPA’s National Study 

of Chemical Residues in Fish (Bahnick and others, 1994; 

USEPA, 1992a; 1992b); concentrations were 0.49 μg/g in 

white bass fillets and 0.11 μg/g in whole carp obtained in 1987 

from the Rio Grande at El Paso, both of which were greater 

than 1997 whole-fish concentrations at Station 516 (Fig. 3). 

Similarly, concentrations of 0.38-0.93 μg/g were documented 

in fillet samples of white bass, largemouth bass, and carp 

obtained in 1993 from the Mercedes diversion canal, along 

the AC upstream of Harlingen, Texas [unpublished TDH data 

summarized by Mora, 2001]. Further downstream, USEPA 

reported 0.10 μg/g in whole channel catfish from the AC at 

Harlingen, which is nearly identical to 1997 concentrations 

in this species from Station 511 (Fig. 3). In the upper RG, 

concentrations in 1997 samples of striped bass and smallmouth bass from Station 63 were similar to those reported 

for white bass (0.21 μg/g) obtained from Elephant Butte and 

Caballo reservoirs in 1996 by Caldwell and others (1999), but 

1997 concentrations in largemouth bass and striped bass were 

greater (Fig. 3). Forage species (Dorosoma sp.) from both 

reservoirs contained 0.05-0.13 μg/g in 1996 (Caldwell and 

others, 1999). 

In the upper RGB, highly elevated concentrations of 

Hg were reported in sediment, but not fish (liver samples), 

collected at sites below the Creede, Colorado mining district 

in 1992-93 by the NAWQA program (Carter and Anderholm, 

1997; Levings and others, 1998). Concentrations of total Hg 

in northern pike collected in 1997 from Station 64, which 

is downstream of the mining district, were 0.09-0.18 μg/g 

(Fig. 3). These concentrations are within the 1970-86 range 

(<0.01-0.21 μg/g) reported in NCBP brown trout from this 

site (Schmitt and others, 1999b). Similarly, brown trout fillets obtained in 1987 from the Rio Mora, a headwater stream 

in northeastern New Mexico sampled as a reference site by 

USEPA (1992b), contained 0.07 μg/g. Concentrations in 

whole carp and channel catfish collected from the RGB in 

1997 were also generally similar to those reported for the RRN 

by Goldstein and others (1996). 

The 1997 concentrations of total Hg in fish from Station 

63 (ca. 0.2-0.5 μg/g) remained high relative to other RGB 

stations, and fish consumption advisories were consequently 
35 

in effect (Appendix Table 1). Similar concentrations were 

present in predatory fish collected from many of the MRB 

stations sampled in 1995 (Schmitt and others, 2002c), from 

many northeastern U.S. lakes (Yeardley and others, 1998), and 

from Oregon streams (Peterson and others, 2002). Nationally, 

concentrations of total Hg in fish obtained by USEPA in 1987 

from waters affected by point-sources were as great as 1.2-1.8 

μg/g (Bahnick and others, 1994; USEPA, 1992a). Concentrations were also as great as 1.8 μg/g in largemouth bass fillets 

and 5.8 μg/g in white bass fillets obtained from NAWQA 

Study Units elsewhere in the U.S. (Brumbaugh and others, 

2001). Relative to all samples in the USEPA study, the greatest 1997 values from Stations 63 and 514 were about at the 

50th percentile, with the caveat that fillet concentrations in the 

1997 samples would probably have been higher than those in 

whole fish. Mercury was among the eight elements associated 

with the combustion of fossil fuels for which Van Metre and 

others (1997) reported increasing concentrations in sediment 

cores from Amistad Reservoir. Thus, and in contrast to most 

other persistent contaminants, elevated Hg concentrations in 

predatory fish (and attendant consumption advisories) remain 

in effect in the RGB and throughout the U.S. 

Overt toxicity in adult fish is usually not associated with 

the Hg concentrations now typically encountered in the wild; 

however, in laboratory studies behavioral effects have been 

documented in fish containing 0.7-5.4 μg/g of total Hg (whole 

fish) (Kania and O'Hara, 1974; Wiener and Spry, 1996). 

Jarvinen and Ankley (1999) reviewed laboratory studies 

evaluating the effects of Hg on reproduction in freshwater fish. 

Among them were studies that reported reduced reproduction 

at whole body concentrations of 4.47 μg/g total Hg in fathead 

minnows (Snarski and Olson, 1982) and 9.4 μg/g in secondgeneration brook trout (Salvelinus fontinalis) (McKim and 

others, 1976). Hatching success and embryo survival may be 

affected by maternally transferred Hg; these parameters were 

negatively correlated with total Hg in wild walleye containing concentrations similar to those in fish from the RGB (Fig. 

3) and MRB (Schmitt and others, 2002c). Consequently, the 

thresholds for behavioral and reproductive effects are probably 

much lower than the ~5 μg/g total Hg threshold for toxicity in 

adult fish (Wiener and others, 2002; Wiener and Spry, 1996). 

In wildlife, dietary total Hg concentrations as low as 

0.3 μg/g have been associated with reproductive impairment 

in common loons (Gavia immer) (Barr, 1986), and reproduction in mallards (Anas platyrhynchos) was affected at 

concentrations as low as 0.1 μg/g wet-weight (Heinz, 1979). 

Dietary concentrations of 0.25-1.0 μg/g may also be toxic to 

piscivorous mammals (studies reviewed by Wolfe and others, 

1998). Consequently, guidelines for concentrations in fish to 

protect piscivorous wildlife range from 0.5 to 1.0 μg/g ww 

total Hg (Eisler, 1987; Thompson, 1996); however, values as 

low as 0.1 μg/g for mammals and 0.02 μg/g for birds have 

been derived from water quality criteria and bioaccumulation 

factors (Yeardley and others, 1998). In addition, the previously held belief that Se protects vertebrates from the adverse 

effects of Hg (Bäckström, 1969, cited by Wiener and others, 

Results and Discussion 

2002; Ganther, 1978; Ganther and others, 1972; Hoffman and 

Heinz, 1998) has been tempered by findings of Se-enhanced 

Hg embryotoxicity in birds (Heinz and Hoffman, 1998). In 

addition, Se may enhance Hg accumulation (Cuvin-Aralar and 

Furness, 1991; Skerfving, 1978). Therefore, although the substantial amounts of Se present in fish from most RGB stations 

may protect adult birds from toxic effects of Hg, reproductive 

and other effects may be exacerbated. Total Hg concentrations in largemouth bass from Stations 63 and 514 exceeded 

0.3 μg/g, and at least one sample from all stations except 

516 exceeded 0.1 μg/g (Fig. 3). Caldwell and others (1999) 

reported total Hg concentrations in the blood of double-crested 

cormorants (Phalacrocorax auratus) from Elephant Butte 

and Caballo reservoirs that were within the range associated 

with adverse effects in bald eagles, which also nest at both 

impoundments. As noted previously, Hg and Se are also 

among the contaminants implicated in the reproductive impairment of peregrine falcons in the Big Bend reach of the RG 

(Mora and others, 2002). In addition, elevated Hg concentrations were reported in aplomado falcon (Falco femoralis) eggs 

collected in 1996 near Brownsville (Mora and others, 1997) 

and in other piscivorous birds from the Laguna Madre [studies 

cited by (Mora and Wainwright, 1997)]. Collectively, our 

findings and those of the previous investigations cited indicate 

that Hg remains a cause for concern in the RGB, as it also 

does elsewhere in the U.S. 

Lead 

Concentrations of Pb exceeded detection limits (0.03-

0.09 μg/g) in 29 of 47 samples (62%) from eight stations 

(Table 10). Concentrations ranged from 0.04 to 0.83 μg/g, 

with the maximum occurring in female bass from Station 16 

(Fig. 4; Table 10). The only other samples >0.30 μg/g were 

from Station 511. Except for Station 16, all geometric station 

means were ≤0.20 μg/g (Table 11). Concentrations of Pb in 

channel catfish, bass, and carp were generally similar (average 0.12-0.14 μg/g) whereas Pb was below detection limits in 

all white bass samples (Fig. 4). Differences between stations 

with the highest (Stations 511, 512, and 513) and the lowest 

(Stations 63, 65 and 515) concentrations were statistically 

significant, but only in carp (Table 12). 

Temporal differences were also significant; 1997 concentrations in bass from Station 16 were significantly greater than 

in all previous collections (Table 13). In contrast, all other 

significant differences reflected lower 1997 concentrations 

than in the past (Table 13). Concentrations were lower at Stations 63, 64, and 65 than in NCBP collections from 1980-1986 

(Schmitt and others, 1999b). Concentrations of Pb in most 

1997 samples from the transboundary portions of the RG were 

about the same as those reported for 1992-96 by the Binational 

Study (TNRCC, 1994b; 1997). At Station 16 (RG at Mission, 

TX), 1997 levels were similar to those in earlier collections 

except for the one bass sample with a very high concentration. 

The Binational Study reported an almost identically elevated 
36 Environmental Contaminants and their Effects on Fish 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1

0

0.14 

0.12 

0.1 

0.08 

0.06 

0.04 

0.02 

0 

64 63 516 65 515 513 16 511 512 

sp. 

sp. 

sp. 

514 

Upper RG Mid-RG/Pecos Lower RG 

Common Carp 

Micropterus

Northern Pike 

Morone

Ictalurus

Cd (μg/g wet-weight) Pb (μg/g wet-weight)

Station 

Figure 4. Concentrations of lead (Pb) and cadmium (Cd) in 

composite samples of whole fish, by station and taxon. Censored 

values are plotted as 50% of LOD. Stations are ordered from 

upstream to downstream and are grouped by sub-basin. See 

Table 1 for station descriptions and collection dates. Micropterus 

sp., largemouth or smallmouth bass; Morone sp., white or striped 

bass; Ictalurus sp., blue or channel catfish. 

Pb concentration (0.76 μg/g) in whole largemouth bass collected from a nearby site in 1992 (TNRCC, 1994b), and Davis 

and others (1995) reported a level of 0.41 μg/g in one sample 

of hardhead catfish (Arius felis) from the AC downstream of 

Harlingen, and concentrations of 1.0-1.8 μg/g were reported 

in fillet samples of several species obtained from the Laguna 

Madre during the 1980s [unpublished TDH data summarized 

by Mora (2001)]. Concentrations were greater (1.6 μg/g) 

in blue tilapia (Oreochromis aurea) from a resaca along the 

lower RG that receives municipal wastes and urban runoff 

(Mora and others, 2001) than any recent values for fish from 

either the RG or AC. Concentrations of this magnitude have 

previously been reported only from sites contaminated by 

smelters and mine tailings (Schmitt, 2004; Schmitt and others, 

2002a; Schmitt and others, 1993). Collectively, these data 

indicate inputs of anthropogenic Pb in the lower RGB. However, it should also be noted that sporadically high concentrations may reflect the heterogeneous distribution of Pb within 

the fish; Pb accumulates in hard tissues such as bone and 

scales, and small pieces of these tissues in the small volume 

of material from each sample digested and analyzed can 

profoundly affect measured concentrations (May and McKinney, 1981; Schmitt and Finger, 1987). In the upper RGB, Pb 

is among the contaminants detected by NAWQA at elevated 

concentrations in fish (liver samples) and other media obtained 

in 1992-93 at sites below the Creede, Colorado mining district 

(Carter and Anderholm, 1997; Levings and others, 1998), 

but 1997 concentrations in samples from Station 64 (RG at 

Alamosa, CO), which is downstream from this district, were 

not elevated (Fig. 4). In addition, Pb was the only one of the 

eight elements associated with the combustion of fossil fuels 

for which Van Metre and others (1997) reported decreasing 

concentrations in sediment cores from Amistad Reservoir, but 

only in the Devil’s River arm of the reservoir. Concentrations 

also declined in sediment cores from Llano Grande Lake for 

the period 1989-2001 (Mahler and Van Metre, 2002). 

Because Pb does not bioaccumulate it represents a hazard 

to lower trophic level organisms; its presence in fish usually 

does not represent a threat to piscivorous wildlife (Henny and 

others, 1994; Henny and others, 2000). It may be associated 

with harmful effects in the fish, however. Several Pb studies 

were included in the recent, comprehensive review by Jarvinen 

and Ankley (1999). Among these was Holcombe and others 

(1976), which determined that Pb concentrations of 0.4 μg/g 

were associated with reduced hatchability in third generation brook trout embryos. Holcombe and others (1976) also 

reported reduced growth at various life stages in third generation brook trout at whole-body concentrations of 4.0-8.8 μg/g. 

Effects on heme synthesis in fish have been associated with 

carcass Pb concentrations exceeding 1.0 μg/g and varying 

indirectly with Zn burden (Dwyer and others, 1988; Schmitt 

and others, 1984; Schmitt and others, 1993). Concentrations 

of Pb in the female bass from Station 16 approached this latter 

value and exceeded the lowest toxicity thresholds of Holcombe and others (1976), but all others were much lower. 

Cadmium 

Concentrations of Cd exceeded detection limits (0.02-

0.03 μg/g) in only 28% of the samples from 60% of the 

stations sampled (Table 10). The maximum concentration 

(in female carp from Station 64) was 0.12 μg/g (Fig. 4; Table 

10), and all geometric station means were <0.05 μg/g (Table 

11). Concentrations in carp generally exceeded those in other 

taxa (Fig. 4). Among-station differences were statistically 

significant in carp, but not in other taxa; concentrations were 

significantly greater at Stations 16, 63, and 515, and 516 than 

at Stations 65 and 511-514 (Table 12). Concentrations in 

RGB carp were similar to those from agricultural areas of the 
Results and Discussion 37 

MRB (Schmitt, 2002b) and the RRN (Goldstein and DeWeese, 

1999). In contrast, carp from sites in the MRB affected by 

mining and industrial sources contained 0.20 to >0.50 μg/g 

(Schmitt, 2002b). Although some temporal differences were 

also statistically significant (Table 13), concentrations of Cd 

in NCBP fish from the RG were also comparatively low in 

the past and no long-term trends were evident; all 1980-86 

concentrations were <0.13 μg/g and most were <0.10 μg/g 

(Schmitt and others, 1999b). Concentrations in the 1997 

samples were similar to most of those reported by the Binational Study (TNRCC, 1994b; 1997) and by Davis and others 

(1995) for the transboundary portions of the RG and AC. 

Cadmium was among the elements detected by NAWQA at 

elevated concentrations in 1992-93 fish (liver samples) and 

other matrices from the RG below the Creede, Colorado mining district (Carter and Anderholm, 1997; Levings and others, 

1998), which is upstream of Station 64. As noted, the 1997 

maximum concentration (0.12 μg/g) occurred at this site (Fig. 

4), but this is still relatively low compared with other areas 

affected by mining and industry [compare to Schmitt, 2004; 

Schmitt and others, 2002c)]. Concentrations were ± 0.34 μg/g 

in all carp from Elephant Butte Reservoir and the RG in New 

Mexcio analyzed by Ong and others (1991). 

Birds and mammals are comparatively resistant to Cd; 

dietary toxicity thresholds in the studies reviewed by Eisler 

(1985) were >100 μg/g. Nevertheless, Eisler (1985) suggested that a Cd concentration of 2 μg/g in fish is evidence of 

contamination, that 5 μg/g is potentially life-threatening to the 

fish, and that 13-15 μg/g is a threat to higher trophic levels. 

The review by Jarvinen and Ankley (1999) cited only one laboratory study that evaluated the effects of Cd exposure relative 

to whole-body concentrations in fish; levels >2.8 μg/g were 

associated with decreased spawning and number of embryos 

produced in flagfish (Jordanella floridae) (Spehar, 1976). All 

1997 Cd concentrations were below these benchmarks (Fig. 

4). However, recent studies have shown that Cd can act as 

an estrogen mimic in rats (for example, Johnson and others, 

2003), indicating that reproductive effects in fish and wildlife 

may occur at exposure levels previously believed to be safe. 

Zinc 

Zinc was detected in all samples; concentrations were 

11.1-83.6 μg/g, with the maximum occurring in female carp 

from Station 511 (Fig. 5; Table 10). The range of concentrations and the geometric mean values were similar for all 

stations (Fig. 5; Table 11). As in 1980-86, bass from Stations 

16 and 63 and one white bass sample from Station 65 contained 10-20 μg/g whereas 1997 concentrations in carp were 

uniformly >40 μg/g (Fig. 5). Concentrations in 1997 carp 

from the RGB were similar to those collected in 1992 from the 

RRN (Goldstein and DeWeese, 1999) and in the 1980s from 

the RG in New Mexico (Ong and others, 1991); however, they 

were lower than 1995 concentrations in carp from many sites 

in the MRB (Schmitt and others, 2002c). Differences among 

Ni (μg/g wet-weight) Cr (μg/g wet-weight) Cu (μg/g wet-weight) Zn (μg/g wet-weight)

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

2 

1 

0 

80 

70 

60 

50 

40 

30 

20 

10 

0 

5 

4 

3 

2 

1 

0 

0.5 

1.5 

64 63 516 65 515 513 16 

sp. 

sp. 

sp. 

514 511 512 

Upper RG Mid-RG/Pecos Lower RG 

Common Carp 

Micropterus

Northern Pike 

Morone

Ictalurus

Station 

Figure 5. Concentrations of zinc (Zn), copper (Cu), chromium (Cr), 

and nickel (Ni) in composite samples of whole fish, by station and 

taxon. Censored values are plotted as 50% of LOD. Stations are 

ordered from upstream to downstream and are grouped by subbasin. See Table 1 for station descriptions and collection dates. 

Micropterus sp., largemouth or smallmouth bass; Morone sp., 

white or striped bass; Ictalurus sp., blue or channel catfish. 
38 Environmental Contaminants and their Effects on Fish 

stations were statistically significant only for carp; Zn concentrations were significantly greater at Stations 16, 63, 511, and 

515 than elsewhere (Table 12). 

Overall, 1997 Zn concentrations in the transboundary 

portions of the RGB paralleled those reported for 1992-96 by 

the Binational Study (TNRCC, 1994b; 1997) and by Davis and 

others (1995). The only significant temporal changes were in 

white bass from Station 65; concentrations were about twofold greater in 1997 than in previous collections (Table 13). 

Station 64 was again difficult to compare temporally because 

of species changes, but relative to other stations sampled, Zn 

concentrations were about the same as or only slightly greater 

than those in comparable species from other locations. Zinc 

was among the elements detected at elevated concentrations 

in fish (liver samples) and other media by NAWQA in 1992­

93 samples from the upper RG below the Creede, Colorado 

mining district (Carter and Anderholm, 1997; Levings and 

others, 1998), upstream of Station 64. Zinc was also among 

the eight elements associated with the combustion of fossil 

fuels for which Van Metre and others (1997) reported increasing concentrations in sediment cores from Amistad Reservoir, 

but concentrations in cores from Llano Grande Lake did 

not change appreciably from 1989 to 2001 (Mahler and Van 

Metre, 2002). 

In life-cycle (larvae-to-adult) studies with Zn the growth 

and survival of American flagfish (Jordanella floridae), a cyprinid, were affected at concentrations of 40-64 μg/g [Spehar, 

1976 as cited by Jarvinen and Ankley, 1999]. These concentrations are within the range of those commonly encountered 

in wild carp but greater than levels typical for other fishes 

(Schmitt and others, 1999b). In the RGB, only concentrations 

in carp exceeded the threshold values (Fig. 5; Table 12), so it 

is unlikely that either fish or higher trophic level organisms 

were adversely affected by Zn (Eisler, 1993). 

Copper 

Copper was detected in all samples at concentrations of 

0.29-1.80 μg/g, with the maximum occurring in female carp 

from Station 64 (Fig. 5; Table 10). Concentrations >1.5 μg/g 

were also present in carp from Stations 513 and 514 (Fig. 

5). All geometric station means were <1.2 μg/g (Table 11). 

Differences among stations were statistically significant only 

in catfish and bass (Table 12). In catfish, Cu concentrations 

were significantly greater at Station 516 than at Station 511; 

and in bass concentrations at Station 514 exceeded those at 

Station 16 (Table 12). Concentrations in 1997 samples from 

the transboundary portions of the RGB were similar to most 

1992-96 values reported for whole fish by the Binational 

Study (TNRCC, 1994b; 1997) and by Davis and others (1995). 

Exceptions were 1996 concentrations of 3.7 μg/g in white bass 

and 2.4 μg/g in largemouth bass from sites near Brownsville 

(TNRCC, 1997), which exceeded all 1997 concentrations (Fig. 

5). All recent RGB concentrations (1992-97) were within the 

range of values reported for whole fish of the same species 

from the MRB (Schmitt and others, 2002c) and the RRN 

(Goldstein and DeWeese, 1999), and the RG in New Mexico 

(Ong and others, 1991) however. 

Concentrations of Cu in fish from the RGB collected in 

1997 were generally similar to those reported for 1980-86 

by the NCBP (Schmitt and others, 1999b), and few temporal 

differences were statistically significant (Table 13). The only 

exceptions were downward trends at Station 65; 1997 concentrations carp were significantly lower than in 1986, and 1997 

concentrations in white bass were lower than in both 1978 

and 1986, when they were especially high (Table 13). Except 

for some elevated concentrations in brown trout in the past 

(Schmitt and others, 1999b), the ranges of Cu concentrations 

were similar in 1980-86 and 1997 at Station 64. Copper was 

among the elements detected at elevated concentrations by 

NAWQA in 1992-93 samples of fish (liver samples) and other 

media collected from the RG below the Creede, Colorado 

mining district (Carter and Anderholm, 1997; Levings and 

others, 1998), upstream of Station 64. Cu was also among the 

eight elements associated with the combustion of fossil fuels 

for which Van Metre and others (1997) reported increasing 

concentrations in sediment cores from Amistad Reservoir, 

but concentrations in cores from Llano Grande Lake did 

not change appreciably from 1989 to 2001 (Mahler and Van 

Metre, 2002). 

Copper concentrations in brown trout collected by the 

NCBP at Station 64 increased steadily from 1.26 μg/g in 

1978 (first year that Cu was measured) to 5.72 μg/g in 1986 

(Schmitt and others, 1999b). On a dry-weight basis, the greatest of these concentrations (10-22 μg/g) were within the range 

reported for this species obtained from mining-contaminated 

sites in Montana in which biological effects were documented 

(Farag and others, 1995). Brown trout were not collected at 

Station 64 in 1997, but the 1997 concentrations in northern 

pike and carp were all <2.0 μg/g (Fig. 5). 

The ecological relevance of the 1997 Cu concentrations 

is difficult to judge because few studies linking whole-body 

concentration to effects on fish have been conducted. Of those 

reviewed by Jarvinen and Ankley (1999), only the work of 

Stouthart and others (1996) was potentially relevant; these 

authors determined that Cu concentrations of 11.1-11.7 μg/g 

were associated with reduced survival of carp larvae, and concentrations of 42 μg/g reduced egg survival. These concentrations are at least 10-fold greater than levels encountered in RG 

fish. No criteria are available against which to evaluate risks 

of Cu in fish to wildlife (Eisler, 1997). 

Chromium and Nickel 

Concentrations of Cr and Ni were not determined by 

the NCBP (Schmitt and others, 1999b). Although they were 

measured as part of the 1995 MRB BEST project, they were 

not reported (Schmitt and others, 2002c). Nevertheless, 

results for Cr and Ni are presented here because of interest in 

the effects of these metals on wildlife [for example, Outridge 
Results and Discussion 39 

and Scheuhammer, 1993] and because they are released to the 

RG from metal plating sites in El Paso (Appendix Table 1) 

and from the combustion of fossil fuels; both Cr and Ni were 

among the eight elements associated with the combustion of 

fossil fuels for which Van Metre and others (1997) reported 

increasing concentrations in sediment cores from Amistad 

Reservoir, but concentrations in cores from Llano Grande 

Lake did not change appreciably from 1989 to 2001 (Mahler 

and Van Metre, 2002). We detected Cr in all 1997 samples at 

concentrations of 0.4-71.8 μg/g, with the maximum occurring 

in male carp from Station 63 (Fig. 5; Table 10). Comparatively high concentrations (40-70 μg/g) also characterized both 

samples of largemouth bass from Station 513 (Fig. 5). These 

maxima are about 100-fold greater than most other recently 

reported concentrations in fish from the RGB (Davis and 

others, 1995; Ong and others, 1991; Roy and others, 1992; 

TNRCC, 1994b; 1997), and most 1997 concentrations were 

10-fold greater. Geometric mean concentrations were >4.0 

μg/g at Stations 63, 511, 512, 513 (maximum, 28.5 μg/g), 

514, 515, and 516 (Table 11). According to Eisler (1986), 

Cr concentrations >4 μg/g dry-weight (about 1.0 μg/g wetweight) in the tissues and organs of fish and wildlife indicate 

environmental contamination, but the significance of these 

concentrations is not clear. Most 1997 samples and station 

means exceeded 1.0 μg/g (Fig. 5; Table 11). Whole-body Cr 

concentrations in carp from the mostly rural RRN averaged 

only about 0.8 μg/g; concentrations in liver were lower (mean 

= 0.6 μg/g) whereas those in muscle were higher (mean = 1.0 

μg/g) (Goldstein and DeWeese, 1999). A recent review of the 

literature (Jarvinen and Ankley 1999) found no studies linking 

whole-body Cr concentrations to survival or growth effects in 

freshwater fishes. 

All RGB samples also contained detectable Ni (Table 10). 

Concentrations were 0.2-4.2 μg/g, with the maximum occurring in male carp from Station 512 (Fig. 5; Table 10). One 

sample of largemouth bass from Station 513 contained substantially more Ni than most others (3.2 μg/g; Fig 5). As was 

also true for Cr, these maxima are greater (by about 10-fold) 

than most recent values reported by other RGB studies (Davis 

and others, 1995; TNRCC, 1994b; 1997). Geometric station 

means were greatest for Stations 63, 512, and 513 (0.95-1.05 

μg/g); all others were <0.5 μg/g (Fig 5; Table 11). These 

means are also many-fold greater than the mean whole-body 

Ni concentration in carp (0.14 μg/g) from the mostly rural 

RRN (Goldstein and DeWeese, 1999). Nickel was among 

a large group of mining-derived metals detected at elevated 

concentrations in water, but not in sediments or fish, from the 

upper RG in 1992-93 by NAWQA (Carter and Anderholm, 

1997; Levings and others, 1998). Criteria for the protection of 

fish and piscivorous wildlife have not yet been established for 

Ni in fish, and no studies linking whole-body concentrations 

and effects were reviewed by Jarvinen and Ankley (1999). 

Data for additional metals (Al, B, Ba, Be, Fe, Mg, Mn, 

Mo, Sr, V), along with all other data from this study, are available at <http://www.cerc.usgs.gov/data/data.htm>. Of these, 

B, Be, and Mo were below detection limits in all samples. 

Organochlorine Chemical Residues 

DDT and its Primary Metabolites 

Although DDT use in the United States was curtailed in 

1972, it continued to be used for mosquito control in Mexico 

through 1997 (Environmental Health Perspectives, 1997; 

Mora and others, 1997). In addition, residues of this persistent 

organochlorine insecticide and its metabolites remain present 

in the environment from historical use and as a consequence 

of atmospheric transport from elsewhere in the world. In U.S. 

rivers, elevated concentrations persist in cotton-growing areas 

and near former sites of production and formulation (Schmitt 

and others 2002c). At the RGB sites sampled in 1997, concentrations of p,p’-DDT, the parent compound, were below the 

detection limit (<0.01 μg/g) in all samples (Table 15), indicating the presence of only weathered material from historical 

use (Aguillar, 1984; Schmitt and others, 1999b). Further 

evidence of weathering is that on average p,p’-DDE (DDE), 

the most persistent metabolite, constituted 94.3% of the total 

p,p’- homologs detected (Fig. 6). Residues of DDE remained 

widespread in the RGB, however; they were detected (>0.01 

μg/g) in 43 of the 47 samples analyzed and in at least one 

sample from all stations (Fig. 7; Table 15). The greatest concentrations (individual samples and geometric station means) 

were from Stations 16 and 511, in the lower RGB (Figs. 6 

and 7; Tables 15 and 16). Individual samples containing 

comparatively high concentrations (≥0.50 μg/g) included carp 

from Station 16 (0.50 μg/g) and carp and channel catfish from 

Station 511 (range 0.67–1.60 μg/g; Fig. 7). Concentrations 

of DDE differed significantly among stations in all taxa, and 

there was a general gradient from upstream to downstream; 

levels in fish from sites in the upper parts of the RGB were 

generally low relative to those from downstream stations (Fig. 

7; Table 12). The exception was Station 512 (RG at Brownsville), where concentrations were also low (Fig. 7). Whole 

fish from the upper RGB sampled in 1992-93 by NAWQA 

contained traces of DDE (Carter and Anderholm, 1997; Levings and others, 1998), as did several 1997 samples from Stations 63 and 64 (Fig. 7; Table l5). Residues of p,p’-DDE were 

also detected in SPMDs from four of the six transboundary 

RG sites sampled by Moring (1999) in July-August 1997. 

Environmental residues of p,p’-DDD result from the 

anaerobic metabolism of p,p’-DDT and to the historical use 

of the insecticide Rhothane® (Table 3). In 1997 p,p’-DDD 

represented 5.7% (mean) of total DDT and was detected 

(>0.01 μg/g) in 23 of 47 samples from six stations (Table 15). 

Concentrations were uniformly low, however; all samples contained ≤0.084 μg/g (Table 15) and all geometric station means 

were <0.05 μg/g (Table 16). Residues of p,p’-DDD were 

detected in SPMDs from only one of the six transboundary 

RG sites (downstream of Hildalgo) sampled by Moring (1999) 

in July-August 1997. 

In the upper RG, total DDT concentrations at NCBP 
40 

Table 15. Occurrence (percentages of samples and stations with concentrations exceeding the LOD (0.01 μg/g for individual compounds and 0.05 μg/g for 

toxaphene and total PCBs) of organochlorine chemical residues in composite samples of whole fish from the Rio Grande basin. Also shown are the maximum concentrations and the samples (station, species, and sex) in which they occurred. See Table 1 for station locations and collection dates. ND, not detected. 

Samples Stations Maximum 1997 concentration Analyte(s) (% of 47) (% of 10) μg/g Station Sex Species 

p,p’-DDT p,p’-DDD (TDE) p,p’-DDETotal p,p’-homologs a 

o,p’-DDT 

o,p’-DDD (TDE) o,p’-DDEDieldrinEndrincis-Chlordane trans-Chlordane cis-Nonachlortrans-NonachlorOxychlordane Heptachlor epoxide Total chlordane-related residues a, b 

ToxapheneMirexHexachlorobenzene (HCB) Total PCBs Hexachlorocylohexane (HCH) a, c 

0

49

91

91

6

6

0

17

0

17

9

926

0

0

26

13

0

0

0

0 

0

60

100

90

10

10

0

20

0

30

20

20

40

0

0

40

20

0

0

0

0 

<0.01

 0.08 

 1.60 

 1.69 

 0.06 

 0.03 

0

 0.05 

<0.01

 0.07 

 0.02 

 0.03 

 0.07 

<0.01

<0.01

 0.21 

2.4 

<0.01

<0.01

<0.05

<0.01 

ND

511

511

511

511

511

ND

511

ND

511

511

511

511

ND

ND

511

511

ND

ND

ND

ND 

ND ND 

M Channel catfish 

J Channel catfish 

J Channel catfish 

J Channel catfish 

M Channel catfish 

ND ND 

M Channel catfish 

ND ND 

M Channel catfish 

M Channel catfish 

M Channel catfish 

M Channel catfish 

ND ND 

ND ND 

M Channel catfish 

J Channel catfish 

ND ND 

ND ND 

ND ND 

ND ND 

Environmental Contaminants and their Effects on Fish 

a Censored values represented by 50% LOD in totals b Sum of cis- and trans-chlordanes and nonachlors; oxychlordane; and heptachlor epoxide 

Sum of -, -, -, and -hexachlorocyclohexane c 
Results and Discussion 41 

Total p,p'-DDT (

μg/g wet-weight) 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

64 63 516 65 

DDT 

DDE 

DDD 

515 514 513 16 511 512 

Upper RG Middle RG/Pecos Lower RG 

Station 

Figure 6. Geometric mean concentrations of p,p’-DDT, DDE, and DDD in composite samples of whole fish, by station. (Note: censored 

values are represented by 50% LOD in the computation of means and totals but are not shown). Stations are ordered from upstream to 

downstream and are grouped by sub-basin. See Table 1 for station descriptions and collection dates. 

Stations 63, 64, and 65 were historically ≤0.2 μg/g; most Station 63 (RG at Elephant Butte Reservoir) concentrations were 

<0.02 μg/g (Schmitt and others, 1999b). This trend continued 

through 1997 (Fig. 6). Ong and others (1991) reported only 

traces of DDE in samples of biota and sediments collected 

during the 1980s from the RG in New Mexico and Bosque del 

Apache NWR. Concentrations of DDT in recently deposited 

sediments (1990s) from Elephant Butte Reservoir were also 

comparatively low, having declined steadily since the 1970s 

(Van Metre and others, 1997). 

Concentrations of DDT were historically elevated in parts 

of the middle RG. Quillback carpsuckers (Carpiodes carpio) 

collected from the RG near Las Cruces, New Mexico in 1976 

contained 2.7 μg/g of DDE (White and others, 1983), and carp 

collected from several sites near Las Cruces in 1987 contained 

1.3-6.3 (Roy and others, 1992). Western kingbirds (Tyrannus 

verticalis) collected in 1987 from some sites in this area also 

contained 1.4-5.1 μg/g (Roy and others, 1992). Fish from 

the Big Bend region collected during the 1980s contained as 

much as 8.7 μg/g of p,p’-DDE (Mora and others, 2002; Mora 

and Wainwright, 1998). Concentrations were also elevated 

in peregrine falcon eggs from the Big Bend area, which may 

have affected their reproduction; and in northern roughwinged swallows (Stelgidopterys serripennis), which forage 

over the river on emergent aquatic insects (Irwin, 1989; Mora 

and others, 2002). Concentrations of DDE in fish collected in 

1997 from Station 515 and 516, which are respectively below 

and above Big Bend, were 0.10-0.32 μg/g (Figs. 6 and 7). 

Relative to the 233 samples analyzed nationwide by NAWQA, 

the greater of these concentrations approached the 95th percentile (Wong and others, 2000). However, sediment cores 

from Amistad International Reservoir revealed comparatively 

low concentrations of total DDT and p,p’-DDE in sediments 

deposited since 1970 (Van Metre and others, 1997). 

The lower RGB has a long history of contamination by 

DDT and its metabolites, but the trend in concentration has 

been uniformly downward; concentrations of DDE declined 

significantly in at least one taxon at all four NCBP stations 
42 Environmental Contaminants and their Effects on Fish 

2 

1.5 

1 

0.5 

0 

3 

2.5

2

1.5

1

0.5

0

0.25 

0.2 

0.15 

0.1 

0.05 

0 

0.06 

0.05 

0.04 

0.03 

0.02 

0.01

0

64 63 516 65 515 513 16 

Upper RG 

sp. 

sp. 

sp. 

514 511 512 

Mid-RG/Pecos Lower RG 

Common Carp 

Micropterus

Northern Pike 

Morone

Ictalurus

Dieldrin (μg/g wet-weight) Total Chlordanes (μg/g wet-weight) Toxaphene (μg/g wet-weight) p,p'-DDE (μg/g wet-weight)

Station 

Figure 7. Concentrations of p,p’-DDE, toxaphene, total chlordanes (sum of cis- and trans-chlordanes and nonachlors, 

heptachlor epoxide, and oxychlordane), and dieldrin in composite samples of whole fish, by station and taxon. Censored 

values are represented by 50% of LOD. Stations are ordered 

from upstream to downstream and are grouped by sub-basin. 

See Table 1 for station descriptions and collection dates. 

Micropterus sp., largemouth or smallmouth bass; Morone sp., 

white or striped bass; Ictalurus sp., blue or channel catfish. 

sampled in 1997 (Table 13). This finding is consistent with 

most other recent investigations in the RGB and elsewhere. 

Relatively high DDE concentrations typified fish collected 

near the Port of Harlingen (near Station 511) over two decades 

[1975-1995; (Mora and Wainwright, 1998)]. For example, 

concentrations of DDE were >20 μg/g in whole fish and as 

great as 71 μg/g (mean = 34 μg/g) in laughing gulls (Larus 

atricilla) collected from Llano Grande Lake in 1978-79 

(White and others, 1983). Concentrations also exceeded 

10 μg/g in many samples of fish obtained from the AC near 

Harlingen in 1978-79 (White and others, 1983). Concentrations remained typically 2-9 μg/g in fillet samples of many fish 

species from Llano Grande Lake and the AC during the 1980s 

[unpublished TDH data summarized by Mora (2001)]. Whole 

channel catfish obtained from the AC in 1987 also contained 

3.2 μg/g of DDE (USEPA, 1992b), about twice the levels in 

1997 samples of this species from Station 511 (Figs. 6 and 7). 

However, by 1994 the arithmetic mean DDE concentration 

in fish from the AC near the Port of Harlingen had declined 

to 0.46 μg/g (Davis and others, 1995). Concentrations also 

continued to decline in Llano Grande Lake sediment cores 

from 1980 to 2001 (Mahler and Van Metre, 2002). Despite the 

declines, the TDH has issued a consumption advisory against 

eating fish from the AC and other waters near Station 511; 

DDE is among the contaminants in the advisory (Appendix 

Table 1). At Station 16 (RG at Mission, TX), the NCBP site 

geographically closest to Stations 511, 512, and 513, concentrations of total DDT in fish also consistently exceeded 1.0 

μg/g, primarily as p,p’-DDE, in the past. The geometric mean 

concentration of total p,p’-DDT at this site was the second 

highest among all NCBP sites sampled in 1986, and the station 

mean exceeded the 85th percentile concentration nationwide 

(Schmitt and others, 1999b). Total DDT concentrations 

remained comparatively high at this station in 1997 (Fig. 6). 

Concentrations of p,p’-DDE in our fish samples from 

the transboundary portions of the RGB were generally about 

the same as those reported during the 1990s by the Binational 

Study (TNRCC, 1994b; 1997) and by Davis and others (1995). 

In contrast, fish obtained in 1996 from some resacas along the 

lower RG in Texas and Mexico contained 5-10 μg/g (Mora 

and others, 2001). Carp collected in 1997 from resacas and 

settling basins by Wainwright and others (2001) also contained 

as much as 2.1 μg/g of p,p’-DDE, and the eggs of fish-eating birds from all sites contained DDE residues. In addition, 

Wainwright and others (2001) noted that concentrations in 

the eggs of greenback heron (Butorides virescens) tended to 

increase from west to east (upstream to downstream); eggs 

from Falcon International Reservoir contained only 0.06-0.28 

μg/g (ww) whereas those from further downstream contained 

as much as 19.38 μg/g. Relative to the AC and other waters 

of the lower RGB, 1978-79 DDE concentrations in fish from 

Falcon and Amistad International Reservoirs were low [≤0.5 

μg/g; White and others (1983)]. 

Nationally, the greatest DDE concentration detected 

by USEPA in 1987 samples was 14 μg/g (Kuehl and others, 

1994). Relative to all stations sampled by USEPA, the greatest 
Results and Discussion 43 

1997 concentration (1.6 μg/g in channel catfish from Station 511) was equivalent to about the 90th percentile (USEPA, 

1992a) and also exceeded the 95th percentile concentration 

based on the 233 samples analyzed nationwide by NAWQA 

(Wong and others, 2000). Collectively, these findings indicate 

that although concentrations of residues derived from DDT 

have declined greatly over the last two decades, substantial 

quantities remain in the lower RGB as they also do in the 

cotton-growing regions of the lower MRB (Schmitt and others, 2002c). The 1997 data from the AC along with those of 

Mora and others (2001) also indicate that pockets of elevated 

concentrations remain despite lower overall levels relative to 

the past. Sediment cores from Falcon International Reservoir 

also revealed declining concentrations in the 1970s followed 

by stable levels through 1995, indicating continuing low-level 

inputs (Van Metre and others, 1997). 

Concentrations of total DDT in fish >0.15 μg/g are 

considered potentially harmful to the brown pelican, a sensitive avian species (Anderson and others, 1975), and wildlife 

criteria as low as 0.20 μg/g have been proposed (Newell and 

others, 1987). According to studies reviewed by Blus (1996), 

concentrations of 1-3 μg/g are potentially hazardous to most 

piscivorous birds. Studies of fish reviewed by Jarvinen and 

Ankley (1999) associated toxic effects with whole-body concentrations as low as 0.05 μg/g. Other studies indicated lower 

toxicity; egg survival was reduced in rainbow trout containing 

1.27 μg/g of total DDT (Hopkins and others, 1969), and in 

juvenile green sunfish (Lepomis cyanellus) and pumpkinseed 

(L. gibbosus) reduced survival occurred at 24 μg/g (Hamelink 

and others, 1971). In 1997, concentrations of total DDT 

(mostly DDE) were >1.0 μg/g in channel catfish from Station 

511, and individual samples from Stations 16, 511, 513, and 

516 exceeded 0.15 μg/g (Figs. 6 and 7). 

Technical DDT contains o,p’-DDT as an impurity (Table 

3), and residues of this compound and its metabolites also 

remain widespread (Schmitt and others, 2002c; Schmitt and 

others, 1999b; Schmitt and others, 1985). Traces of o,p’-DDT 

(0.03-0.06 μg/g) and o,p’-DDD (0.02-0.03 μg/g) were present 

exclusively in the three channel catfish samples from Station 

511, but no o,p’-DDE residues were detected (<0.01 μg/g) in 

any samples (Table 15). Although the o,p’- homologs have 

historically been considered relatively benign, o,p’-DDD is 

weakly estrogenic (Ackerman and others, 2002; Guillette and 

others, 1994; Toppari and others, 1995), as are many other 

pesticides and their metabolites (Tyler and others, 1998). 

Hence, the total risk to fish and wildlife represented by environmental residues derived from DDT and other organochlorine pesticides no longer used in the U.S. are unknown. 

Cyclodiene Insecticides 

Chlordane and Heptachlor 

Chlordane is a mixture of cyclopentadiene-derived compounds that was widely used as a soil insecticide. Concentrations are typically greatest in fish from corn-growing regions, 

urban areas where termites are present, and near production 

and formulation facilities (Schmitt, 2002b; Schmitt and others, 

1999b). Although chlordane is no longer used in the U.S., it 

continued to be used for termite control in Mexico through 

1997 (Environmental Health Perspectives, 1997). Six chlordane-related components and metabolites were measured in 

the 1997 samples: cis-chlordane, trans-chlordane, cis-nonachlor, trans-nonachlor, oxychlordane, and heptachlor epoxide 

(Fig. 8; Table 3). Oxychlordane is a metabolite of cis-chlordane, and heptachlor epoxide is a metabolite of the insecticide 

heptachlor. Both of the latter are also minor constituents of 

technical chlordane; environmental residues of heptachlor 

epoxide derive from insecticidal use of both heptachlor and 

chlordane. 

Total 1997 chlordane concentrations (sum of six components) were greatest in channel catfish from Station 511 (Fig. 

7). Concentrations of oxychlordane and heptachlor epoxide 

were below the LOD (0.01 μg/g) in all samples (Fig. 8; Table 

15). Concentrations of the other four compounds exceeded the 

LOD (0.01 μg/g) in fish from four lower RGB stations: 511, 

512, 514, and 16 (Fig. 8; Table 16). Residues of cis-chlordane 

were detected in channel catfish and carp from Station 511 at 

concentrations of 0.017–0.073 μg/g; the highest concentrations were in channel catfish (Fig. 8; Table 16). Traces (<0.02 

μg/g) were also detected in bass from Station 512 and carp 

from Station 16 (Fig. 8). Trans-chlordane (0.015–0.022 μg/g) 

and cis-nonachlor (0.019–0.033 μg/g) were also detected in 

channel catfish from Station 511 (Table 16). Traces of transchlordane were present in carp from Station 512 whereas bass 

from Station 16 contained traces of cis-nonachlor. Transnonachlor was the most frequently detect chlordane constituent; it was detected (0.036–0.073 μg/g) in 12 of 47 samples 

from four stations (Table 15). Channel catfish from Station 

511 contained the highest concentrations. Concentrations in 

bass from Stations 16 and 512 were 0.02-0.03 μg/g, and other 

species from Stations 16, 511, 512, and 516 contained traces 

(0.01-0.02 μg/g). No chlordane components exceeded detection limits (0.01 μg/g) in any 1997 samples from Stations 63, 

64 65, 513, 514, or 515 (Figs. 7 and 8; Table 16). 

The 1997 results agree with those of most recent studies. Residues of trans-nonachlor and trans-chlordane were 

each detected in SPMDs from only one the six transboundary 

RG sites sampled by Moring (1999) in July-August 1997; 

both sites were on the lower RG (downstream of Laredo and 

downstream of Brownsville). Ong and others (1991) reported 

only traces of chlordane constituents in samples of biota and 
nued

44 

Table 16. Geometric mean, minimum, and maximum concentrations (μg/g wet-weight unless otherwise indicated) of organochlorine chemical residues in fish 

from stations in the Rio Grande Basin. Censored values were replaced by 50% LOD for the computation of station means, but only if at least one value exceeded detection limits. See Table 1 for station locations and collection dates.—Conti 

trans- cis- trans- Total	 TCDD- p,p’- p,p’- p,p’- Total cis-	 Toxa- Total Station DDD DDE DDT DDT a	 Chlor- Chlor- Nona- Nona- Chlor- Dieldrin phene PCBs EQ dane dane chlor chlor dane b (pg/g) 

16

(n = 4) 

63(n = 6) 

64(n = 8) 

65(n = 4) 

511(n = 5) 

512(n = 4) 

513(n = 4) 

Mean

Min.

Max.

Mean

Min.

Max.

Mean

Min.

Max.

Mean

Min.

Max.

Mean

Min.

Max.

Mean

Min.

Max.

Mean

Min.

Max. 

 0.02 

 0.01 

 0.02 

<0.01

<0.01<0.01 0.01

<0.01 0.02<0.01

<0.01<0.01 0.04

 0.03  0.08 0.01

<0.01 0.03 0.01

 0.01  0.02

 0.38 

 0.30 

 0.50 

 0.01 

<0.01

 0.02 

 0.05 

0.03 

 0.09 

0.03 

0.02 0.04 

 0.87 

 0.41 

 1.60  0.08 

0.07 

 0.10  0.18 

 0.13 

 0.26 

<0.01

<0.01

<0.01

<0.01

<0.01<0.01<0.01

<0.01<0.01<0.01

<0.01<0.01<0.01

<0.01<0.01<0.01

<0.01<0.01<0.01

<0.01<0.01 

0.40

0.31

0.52

0.02

0.02

0.03

0.07

0.04

0.12

0.04

0.030.05

0.92

0.44

1.69

0.10

0.08

0.13

0.20

0.150.28 

 0.01 

 0.01 

 0.01 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

 0.03 

 0.02 

 0.07 

 0.01 

<0.01

 0.02 

<0.01

<0.01<0.01 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

 0.01 

<0.01

 0.02 

 0.01 

<0.01

 0.01 

<0.01

<0.01<0.01 

 0.01 

<0.01

 0.01 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

 0.01 

<0.01

 0.03 

<0.01

<0.01

<0.01

<0.01

<0.01<0.01 

 0.01 

<0.01

 0.03 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

 0.02 

 0.01 

 0.07 

 0.01 

<0.01

 0.02 

<0.01

<0.01<0.01 

 0.04 

<0.03

 0.05 

<0.03

<0.03

<0.03

<0.03

<0.03

<0.03

<0.03

<0.03

<0.03

 0.09 

 0.05 

 0.21 

 0.05 

<0.03

 0.06 

<0.03

<0.03<0.03 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

 0.023 

 0.012 

 0.050 

 0.010 

<0.01

 0.014 

<0.01

<0.01<0.01 

 0.04 

<0.05

 0.11 

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

 1.08 

 0.56 

 2.40 

<0.05

<0.05

<0.05

<0.05

<0.05<0.05 

<0.05 0.9 

<0.05 0.2 

<0.05 2.0 

<0.05 1.0 

<0.05 0.5 

<0.05 2.0 

<0.05 0.9 

<0.05 0.4 

<0.05 2.0 

<0.05 2.5 

<0.05 2.0 

<0.05 3.0 

<0.05 2.5 

<0.05 1.0 

<0.05 5.0 

<0.05 2.6 

<0.05 1.0 

<0.05 6.0 

<0.05 0.9 

<0.05 0.3 <0.05 2.0 

Environmental Contaminants and their Effects on Fish 
45 

Table 16. Geometric mean, minimum, and maximum concentrations (μg/g wet-weight unless otherwise indicated) of organochlorine chemical residues in fish 

from stations in the Rio Grande Basin. Censored values were replaced by 50% LOD for the computation of station means, but only if at least one value exceeded detection limits. See Table 1 for station locations and collection dates.—Continued 

trans- cis- trans- Total TCDD- p,p’- p,p’- p,p’- Total cis- Toxa- Total Station Chlor- Chlor- Nona- Nona- Chlor- Dieldrin phene PCBs EQ DDD DDE DDT DDT a dane dane chlor chlor dane b (pg/g) 

514(n = 5) 

515(n = 3) 

516(n = 4) 

Mean

Min.

Max.

Mean

Min.

Max.

Mean

Min.

Max. 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

 0.02 

<0.01

 0.05 

 0.06 

 0.03 

0.11 

 0.13 

 0.10 

0.22 

 0.18 

 0.10 

 0.32 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01 

0.07

0.04

0.12

0.14

0.11

0.23

0.21

0.11

0.37 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

 0.01 

<0.01

 0.01 

 0.03 

 0.03 

0.03 

 0.03 

 0.03 

0.03 

 0.03 

 0.03 

 0.04 

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01 

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05 

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05 

 1.2 

 0.4 

2.0 

 1.8 

 1.0 

3.0 

 1.3 

 0.2 

 3.0 

a p,p’ homologs; censored values represented as 0.005 μg/g (50% LOD).b Sum of cis-chlordane, trans-chlordane, cis-nonachlor, trans-nonachlor, heptachlor epoxide and oxychlordane; heptachlor epoxide, and oxychlordane were not detected in any samples 

but were include in total chlordane at 0.005 μg/g (50% LOD). 

sediments collected during the 1980s from the RG in New 

Mexico and Bosque del Apache NWR as did fish sampled 

in 1992-93 from the upper RGB by the NAWQA program 

(Carter and Anderholm, 1997; Levings and others, 1998). 

Residues of 0.01-0.03 μg/g were detected in whole fish from 

some sites in the lower RG sampled in 1992-93 by the Binational Study (TNRCC, 1994b; 1997) and by Davis and others 

(1995); however, higher concentrations were reported in fish 

fillets from some of these sites. In contrast, Wainwright and 

others (2001) found <0.01 μg/g of trans-nonachlor in 16 carp 

samples obtained from lower RGB resacas and settling basins 

in 1997. These latter values were about 10-fold lower than our 

1997 maxima for fish from the lower RG and AC (Table 16). 

Wainwright and others (2001) also detected trans-nonachlor in the eggs of fish-eating birds from all sites investigated 

(maximum = 0.15 μg/g wet-weight). Chlordane concentrations of 0.72-0.93 μg/g were reported in fillet samples of 

several fish species obtained from Llano Grande Lake during the 1980s [unpublished TDH data summarized by Mora 

(2001)], but 1989-2001 concentrations in sediment cores were 

uniformly low (Mahler and Van Metre, 2002). Whole carp 

and white bass fillets obtained in 1987 by USEPA (1992b) 

from the RG at El Paso contained traces of chlordane components whereas whole channel catfish from the AC contained 

a total of 0.16 μg/g (sum of five components detected), which 

is nearly identical to what was present in 1997 samples of this 

species (Fig. 7). Relative to all 1987 USEPA samples, 1997 

chlordane concentrations in channel catfish from Station 511 

were equivalent to about the 90th percentile (USEPA, 1992a). 

Elsewhere, USEPA reported total chlordane concentrations ≥1.2 μg/g in fish from the most contaminated sites sampled 

in 1987(Kuehl and others, 1994; USEPA, 1992a). Chlordane 

residues in fish from Station 511 were similar in concentration 

and composition to those obtained in 1995 from agricultural 

and urban areas of the MRB (Schmitt, 2002b; Schmitt and 

others, 2002c). Nationally, trans-nonachlor was detected in 

33.8% of the fish samples analyzed by NAWQA; other chlordane components were detected less frequently (Wong and 

others, 2000). The greatest 1997 concentrations of chlordane 

components in channel catfish from Station 511 were all less 

than the 75th percentile concentrations reported by NAWQA 

(Wong and others, 2000). 

Chlordane concentrations at Station 16 peaked at about 

0.20 μg/g in 1978, only two years after NCBP measurements 

began (Schmitt and others, 1999b). Fish samples from this 

site typically contained ≤0.02 μg/g of cis-chlordane, trans-

chlordane, and cis-nonachlor and <0.01-0.08 μg/g of trans­

nonachlor. Concentrations in fish from this site have generally 

declined since the late 1970s (Schmitt and others, 1999b). 

Elsewhere in the lower RG, much higher concentrations were 

detected historically; Mora and Wainwright (1997) indicated 

that concentrations in fish from the Brownsville area peaked 

in 1976 when levels as great as 15.6 μg/g were reported. At 

other NCBP sites in the RGB, chlordane concentrations 

were historically lower than at Station 16. From 1980-86, 

heptachlor epoxide concentrations were below detection and 

Results and Discussion 
46 Environmental Contaminants and their Effects on Fish Total Chlordanes (

μg/g wet-weight) 

0.12 

0.1 

0.08 

0.06 

0.04 

0.02 

0 

64 63 516 65 515 514 513 16 511 512 

Heptachlor epoxide 

Oxychlordane 

cis-Chlordane 

trans-Chlordane 

cis-Nonachlor 

trans-Nonachlor 

Upper RG Middle RG/Pecos Lower RG 

Station 

Figure 8. Geometric mean concentrations of chlordane-related compounds (cis- and trans-chlordanes and nonachlors, heptachlor 

epoxide, and oxychlordane) in composite samples of whole fish, by station. (Note: censored values are represented by 50% LOD in the 

computation of means and totals but are not shown). Stations are ordered from upstream to downstream and are grouped by subbasin. See Table 1 for station descriptions and collection dates. 

oxychlordane was rarely detected in fish from Stations 16, 63, 

64, and 65 at detection limits of 0.005-0.01 μg/g (Schmitt and 

others, 1999b). Concentrations of cis-chlordane, trans-chlordane, cis-nonachlor, and trans-nonachlor were all ≤0.01 μg/g 

at Stations 63, 64, and 65 during this period. 

As noted, total chlordane concentrations (sum of six components) in 1997 were greatest (>0.1 μg/g) in channel catfish 

samples from Station 511; other samples did not exceed 0.059 

μg/g (Fig. 7). The geometric mean total chlordane value at 

Station 511 was 0.098 μg/g, more than twice the mean value 

at Station 512 (0.043 μg/g; Table 16). In addition, concentrations of cis-chlordane in fish from Station 511 exceeded those 

of trans-nonachlor (Fig. 8; Table 16), the opposite of what 

is typical for weathered chlordane residues in fish (Schmitt, 

2002b; Schmitt and others, 2002c). This finding suggests 

comparatively recent chlordane inputs. 

Chlordane residues >0.3 μg/g are of concern for the 

health of predatory fish and fish-eating birds (Eisler, 1990). 

Chlordane is also among the contaminants included in the 

present fish consumption advisory for the AC (Station 511) 

and nearby waters. Elsewhere in the U.S., residual chlordane 

from historical use has been implicated in recent wildlife kills 

(Stansley and Roscoe, 1999), demonstrating the potential of 

this pesticide mixture to inflict ecological injury despite the 

fact that it is no longer used. 

Dieldrin 

Most environmental dieldrin results from the metabolism 

of aldrin, which has not been used in the U.S. since 1974. 

Dieldrin was detected (>0.01 μg/g) in only 17% of the samples 

from two sites (Fig. 7; Table 15). Concentrations were greatest (0.035-0.05 μg/g) in the channel catfish samples (male, 

juvenile, and female) from Station 511 (Fig. 7). The other two 

samples from Station 511 (carp) and three of the four samples 

from Station 512 (carp and largemouth bass) also contained 

traces (<0.015 μg/g; Fig. 7). 
47 

Historically, dieldrin concentrations were below detection 

(<0.01 μg/g) at Stations 63, 64 and 65. In contrast, concentrations at Station 16 were typically as great as 0.2 μg/g (Schmitt 

and others, 1999b). Concentrations of 0.11-0.19 μg/g were 

detected in fillet samples of several species obtained from the 

AC and Llano Grande Lake during the 1980s [unpublished 

TDH data summarized by Mora (2001)]. Dieldrin concentrations in sediment cores representing 1989-2001 from Llano 

Grande Lake were generally low (Mahler and Van Metre, 

2002), and residues were detected in SPMDs from only the 

two lowermost (of six) transboundary RG sites sampled by 

Moring (1999) in July-August 1997. Traces of dieldrin (ca. 

0.01 μg/g) were detected in only three 1992-96 samples collected from the transboundary portions of the RG by the Binational Study (TNRCC, 1994b; 1997), and none was detected 

by Davis and others (1995). Neither was dieldrin detected in 

fish or sediment samples collected in 1992-93 from the upper 

RGB by NAWQA (Carter and Anderholm, 1997; Levings and 

others, 1998), or in the RG in New Mexico during the 1980s 

(Ong and others, 1991). However, dieldrin was detected at 

concentrations ≤0.36 μg/g (75th percentile = 0.006 μg/g) in 

28.9% of 232 fish samples analyzed nationally by NAWQA 

(Wong and others, 2000). Dieldrin was detected in whole 

channel catfish collected by USEPA (1992b) from the AC in 

1987 but not in whole carp or white bass fillets from the RG 

at El Paso. Concentrations were <0.01-0.01 μg/g in all carp 

collected in 2001 from resacas and settling basins along the 

lower RG by Wainwright and others (2001), but dieldrin was 

detected in the eggs of fish-eating birds from all sites investigated and the maximum concentration [0.30 μg/g wet-weight 

in great egrets (Ardea alba) from Falcon International Reservoir] was comparatively high. Dieldrin was not detected in 

our samples from Falcon Reservoir, however. 

Even the greatest 1997 dieldrin concentrations (about 

0.05 μg/g) are 10- to 100-fold lower than toxicity thresholds 

for piscivorous wildlife (Newell and others, 1987; Peakall, 

1996) and probably do not represent a significant threat to 

higher-level consumers or the fish. As cited by Jarvinen 

and Ankley (1999), whole-body residue concentrations of 

0.36-2.13 μg/g were not associated with impaired survival or 

growth of juvenile rainbow trout (Macek and others, 1970; 

Shubat and Curtis, 1986), and only at concentrations of 5.65 

μg/g did juvenile rainbow trout experience reduced survival 

(Shubat and Curtis, 1986). This concentration is at least 10­

fold greater than even the highest contemporary environmental 

levels in fish (Schmitt, 2002b; Schmitt and others, 2002c). 

Endrin 

Endrin, one of the most toxic organochlorine pesticides 

to fish (Johnson and Finley, 1980), was historically used on 

cotton but relatively few other crops. In 1997, concentrations were below the detection limit (0.01 μg/g) in all samples 

(Table 15). Since 1980, traces (≥0.01 μg/g) of endrin have 

been found at Stations 63, 64, and 65 (Schmitt and others, 

Results and Discussion 

1999b). At Station 16, concentrations were 0.02 μg/g in 1980 

and 0.03 μg/g 1984, however (Schmitt and others, 1999b), 

reflecting the agricultural intensity in the lower RGB and 

the past use of endrin. In other years, only trace residues 

were found at Station 16 (Schmitt and others, 1999b). Historically, endrin concentrations were elevated in fish from 

the AC (Mora and Wainwright, 1997), and residues were 

detected in 1987 samples by USEPA (1992b). Concentrations of 0.02-0.04 μg/g were common in fish fillet samples 

obtained from the AC and Llano Grande Lake during the 

1980s [unpublished TDH data summarized by Mora (2001)], 

but concentrations were uniformly low in sediment cores from 

Llano Grande Lake (Mahler and Van Metre, 2002). Endrin 

was also detected (0.01 μg/g) in only one sample collected 

from the transboundary portion of the RG in 1992-96 by the 

Binational Study (TNRCC, 1994b; 1997), none was detected 

by Davis and others (1995), and none was detected in fish or 

sediments collected during 1992-93 from the upper RGB by 

NAWQA (Carter and Anderholm, 1997; Levings and others, 

1998). Nationally, endrin was detected at low concentrations 

(≤0.016 μg/g) and in only 1.7% of 232 fish samples analyzed 

by NAWQA (Wong and others, 2000). In the 1995 study of 

the MRB, endrin was detected only at one site affected by a 

point-source (Schmitt and others, 2002c). 

Other Organochlorine Pesticides 

Toxaphene 

Following the U.S. ban on DDT use, toxaphene was the 

most heavily used insecticide in the U.S.; most was applied to 

cotton (Schmitt and Winger, 1980). Toxaphene use peaked in 

the late 1970s, and it was subsequently banned. Concentrations in NCBP fish samples reflected use patterns, peaking 

in the mid-1980s (Schmitt and others, 1999b). Although 

toxaphene was used mostly on cotton, this pesticide has 

been transported to remote locations and residues have been 

detected in fish from the Arctic and the Great Lakes (Muir and 

others, 1999; Schmitt and others, 1999b). In 1997, toxaphene 

was detected in only six samples from two stations (16 and 

511) in the lower RGB (Fig. 7; Table 15). Concentrations at 

Station 16 were low (<0.1 μg/g) whereas those in fish from 

Station 511 (0.56-2.40 μg/g) were similar to 1995 levels in 

fish from cotton-growing regions of the lower MRB (Schmitt 

and others, 2002c). Toxaphene is therefore among the contaminants included in the present consumption advisory for 

fish from the AC (Station 511) and nearby waters. 

Toxaphene concentrations were historically elevated in 

fish from the lower RGB. Although most 1976-86 samples 

of whole fish from Station 16 contained only 0.3-1.4 μg/g 

(Schmitt and others, 1999b), concentrations >20 μg/g were 

typical of fish collected from the AC and Llano Grande Lake 

in 1978-79 (White and others, 1983). Concentrations were 
48 Environmental Contaminants and their Effects on Fish 

also typically 5-10 μg/g in fillet samples obtained from the 

AC and Llano Grande Lake during the 1980s [unpublished 

TDH data summarized by Mora (2001)]. Toxaphene was 

not detected in any samples collected from the transboundary portion of the RG in 1992-96 by the Binational Study 

(TNRCC, 1994b; 1997), by Davis and others (1995), in fish 

from the upper RGB collected in 1992-93 by NAWQA (Carter 

and Anderholm, 1997; Levings and others, 1998), or in the 

RG in New Mexico during the 1980s (Ong and others, 1991). 

Toxaphene concentrations were also uniformly low in sediment cores from Llano Grande Lake (Mahler and Van Metre, 

2002). In contrast, concentrations were as great as 0.3 μg/g in 

carp collected from resacas and settling basins along the lower 

RG by Wainwright and others (2001). In addition, greenback 

heron eggs from sites along the lower RG contained as much 

as 4.4 μg/g, the greatest concentration ever reported in bird 

eggs from this region (Wainwright and others, 2001). Nationally, toxaphene was detected in only 0.4% of the 233 samples 

analyzed by NAWQA at concentrations ≤0.4 μg/g (Wong 

and others, 2000), a level exceeded by all 1997 samples from 

Station 511. The samples collected in 1987 from the RG at El 

Paso and the AC at Harlingen as part of the National Study of 

Chemical Residues in Fish were not analyzed for toxaphene 

(USEPA, 1992a). Toxaphene concentrations in fish from Stations 63, 64, and 65 collected in 1980-86 by the NCBP were, 

with one exception, ≤0.1 μg/g. In contrast, concentrations at 

Station 16 were historically 0.2-1.0 μg/g (Schmitt and others, 

1999b). It should also be noted that methodological inconsistencies confound comparisons among studies. 

Acute and chronic effects on freshwater fish have been 

noted at whole-body toxaphene concentrations ≥0.4 μg/g 

(Eisler and Jacknow, 1985; Jarvinen and Ankley 1999), a 

level exceeded by all samples from Station 511. Jarvinen and 

Ankley (1999) cited a number of laboratory studies on the 

acute and chronic effects of toxaphene. Among these were 

several by Mayer and others (1978; 1975), who reported that 

adult brook trout containing whole-body concentrations of 

0.4 μg/g produced eggs with reduced viability, and lake trout 

(Salvelinus namaycush) and white suckers containing 0.035-

0.203 μg/g also produced eggs with reduced viability. Survival and growth of freshwater fish (several species) at various 

life stages were reduced at concentrations >0.90 μg/g (Mayer 

and others, 1975; 1978). It should be noted that the composition and potential toxicity of weathered toxaphene can vary 

greatly (Bidleman and others, 1993; Gooch and Matsamura, 

1987; Harder and others, 1983; Ribick and others, 1982), and 

that composition of the residues present in the 1997 samples 

can’t be determined from the low-resolution methods used 

(Ribick and others, 1982). In addition, the concentrations 

reported here should be considered estimates due to the low 

resolution of the analytical methods employed (Schmitt, 

2002b). 

Mirex 

In the southern United States, mirex was used as an 

insecticide to combat red imported fire ants (Solenopsis wagneri). Elsewhere, mirex was used as a flame retardant and as 

a polymerizing agent (Kaiser, 1987). Consistent with previous NCBP findings (Schmitt and others, 1999b), mirex was 

not detected (<0.01 μg/g) in any 1997 samples from the RGB 

(Table 15). Other recent studies have also failed to detect 

mirex in fish from the RGB (Carter and Anderholm, 1997; 

Davis and others, 1995; Levings and others, 1998; Ong and 

others, 1991; TNRCC, 1994b; 1997), and none was detected in 

any of the 233 fish samples analyzed nationally by NAWQA 

(Wong and others, 2000). Mirex was also not detected in 

any 1987 samples from either the RG at El Paso or the AC at 

Harlingen (USEPA, 1992b). In the 1995 study of the MRB 

(Schmitt and others, 2002c), traces of mirex were detected in a 

few samples from Louisiana, however. 

Hexachlorocyclohexanes (HCH) 

Four HCH isomers (α, β, δ, and γ) were measured in 

the 1997 samples. Although mixtures of HCH isomers were 

historically used on cotton and other crops in the U.S., by 

1997 only γ-HCH (lindane) remained in use in North America 

for some agricultural and domestic applications (Li and others, 1996; Poissant and Koprivnjak, 1996). HCH isomers are 

relatively volatile and are rapidly cleared by fish (Butte and 

others, 1991; Willett and others, 1998). Concentrations of all 

isomers in all 1997 samples were below the LOD (0.01 μg/g) 

in 1997 (Table 15). In samples collected from the four NCBP 

sites in the RGB from 1978-1986, α- and γ-HCH concentrations did not exceed 0.02 μg/g, and all 1986 samples contained 

<0.005 μg/g (Schmitt and others, 1999b). Other studies in the 

RGB have also reported only low or non-detected concentrations of HCH isomers. None were detected in the RG in New 

Mexico during the 1980s (Ong and others, 1991). Whole carp 

and white bass (fillets) collected in 1987 from the RG at El 

Paso and whole channel catfish from the AC contained only 

traces of α- and γ-HCH (USEPA, 1992b), and traces of γ-HCH 

were detected in 9% of the fish sampled from the upper RGB 

in 1992-93 by NAWQA (Carter and Anderholm, 1997; Levings and others, 1998). No HCH isomers were detected in any 

1992-96 fish samples from the transboundary RG collected by 

the Binational Study (TNRCC, 1994b; 1997), but traces were 

present in two of 94 samples analyzed by Davis and others 

(1995). Nationally, α- and γ-HCH were both detected at low 

concentrations (≤0.055 μg/g) in 0.4% of the fish samples analyzed by NAWQA, but β-HCH was not detected (<0.005 μg/g) 

(Wong and others, 2000). 
49 

Hexachlorobenzene (HCB) 

Hexachlorobenzene was used as a fungicide; it is also a 

by-product of the production of other chlorinated hydrocarbons. HCB is less toxic to fish than many other persistent 

organochlorines (Schmitt and others, 1999b), but may contain 

toxic impurities, including TCDD. Concentrations of HCB 

were universally below the detection limit (that is, <0.01 

μg/g) in all 1997 samples (Table 15), which is consistent with 

NCBP findings since 1978 (Schmitt and others, 1999b). Other 

investigations have also reported only low HCB concentrations in fish. Whole carp and white bass collected from the 

RG at El Paso in 1987 contained traces (<0.01 μg/g) of HCB, 

but none was detected in whole channel catfish from the AC 

at Harlingen (USEPA, 1992b). No HCB was detected in fish 

collected during 1992-96 from the transboundary portions of 

the RG by the Binational Study (TNRCC, 1994b; 1997) or 

by Davis and others (1995), but traces were present in 9% of 

the fish samples obtained in 1992-93 from the upper RGB by 

NAWQA (Carter and Anderholm, 1997; Levings and others, 1998). None was detected in the RG in New Mexico by 

Ong and others (1991). Nationally, HCB was detected at low 

concentrations (≤0.033 μg/g) in 7.3% of 234 fish samples 

analyzed by NAWQA (Wong and others, 2000). 

Organochlorine Pesticides not Measured 

The low-resolution organochlorine scan used to analyze 

the 1997 samples from the RGB was selected to yield quality 

data for a broad spectrum of compounds at a reasonable cost. 

This approach was justified because concentrations of these 

compounds, most of which are no longer used in the U.S., 

were expected to be low. Results of the 1997 analyses, as well 

as those of the other studies cited, proved this assumption to 

be true, and resources not devoted to the analysis of organochlorine chemicals were available for other aspects of the 

study. Several compounds of potential importance in the RGB 

were not accounted for, however. Among these was Dacthal®, 

an organochlorine herbicide that is used heavily to control 

weeds in ornamental, turf, and vegetable crops. Dacthal® was 

detected in SPMD samples from five of the six sites sampled 

in July-August 1997 by Moring (1999). In 1986, Dacthal® 

concentrations in fish from Station 16 averaged 0.14 μg/g, 

which was the highest nationally (Schmitt and others, 1999b), 

and in 1984 a concentration of 0.45 μg/g was reported in an 

individual sample (Schmitt and others, 1990). The occurrence of this compound was increasing nationally through the 

1980s (Schmitt and others, 1999b), but concentrations in fish 

were not measured by any of the other recent investigations of 

the lower RGB (USEPA, 1992a; TNRCC, 1994b; 1997) and 

none was detected in the upper RGB by NAWQA (Carter and 

Anderholm, 1997; Levings and others, 1998) or in the RG in 

New Mexico by Ong and others (1991). In contrast, residues 

were detected regularly in fish samples (fillets) collected during the 1980s from many sites in the lower RGB [unpublished 

Results and Discussion 

TDH data summarized by Mora (2001)]. Nationally, Dacthal® 

was detected at low concentrations (≤0.067 μg/g) in 3% of 231 

fish samples analyzed by NAWQA (Wong and others, 2000). 

Dacthal® was also detected (0.003-0.03 μg/L) in NASQAN 

water samples from Stations 512 (RG at Brownsville) and 516 

(RG at El Paso) during 1995-97. 

NCBP samples collected in 1986 from Stations 16, 63, 

64, and 65 were also analyzed for residues of pentachloroanisole (PCA), a metabolite of the wood preservative pentachlorophenol; concentrations were below the LOD (<0.005 

μg/g) in all samples (Schmitt and others, 1999b). However, 

carp (whole and fillet) collected from the RG at El Paso in 

1987 contained <0.003 μg/g, and channel catfish from the AC 

at Harlingen contained traces (0.006 μg/g) (USEPA, 1992b). 

PCA was not reported for the upper RG by NAWQA (Carter 

and Anderholm, 1997; Levings and others, 1998), or in the 

middle RG by Ong and others (1991) but it was detected at 

low concentrations (≤0.087 μg/g) in 8.2% of 232 fish samples 

analyzed nationally by NAWQA (Wong and others, 2000). 

Residues of PCA were also detected in SPMD samples from 

two of the six sites sampled in July-August 1997 (below Laredo and below Brownsville) by Moring (1999). 

Endosulfan is among the few organochlorine insecticides 

currently registered for use in the U.S.; more than 38,000 kg/y 

was used in Texas from 1990-93 (Mora and Wainwright, 1997, 

citing other sources). Neither endosulfan nor its metabolites 

were measured by the NCBP or by USEPA (1992a). Endosulfan and its metabolites were measured in samples from 

the upper RG by NAWQA (Carter and Anderholm, 1997; 

Levings and others, 1998) and from transboundary reaches of 

the RG by the Binational Study (TNRCC, 1994b; 1997), but 

all concentrations were below detection limits. Conversely, 

Davis and others (1995) detected traces of endosulfan sulfate 

in one sample collected in 1994. Endosulfan was not among 

the compounds characterized nationally in surficial streambed 

sediments and biota by NAWQA (Wong and others, 2000) nor 

was it measured by Ong and others (1991). 

Total PCBs, H4IIE-Derived Dioxin Equivalents, 

and Ethoxyresorufin O-Deethylase (EROD) 

Activity 

Total PCBs 

Total PCB concentrations were less than the LOD (0.05 

μg/g) in all 1997 samples (Table 15), which confirms the 

findings of previous RGB investigations reporting only low 

PCB concentrations in fish. With only one exception, NCBP 

samples from Stations 16, 63, 64, and 65 have contained ≤0.1 

μg/g of PCBs (as Aroclors® 1248, 1254, and 1260) since 1980 

(Schmitt and others, 1999b). Samples of whole carp and 

white bass from the RG at El Paso and whole channel catfish 

from the AC at Harlingen obtained by USEPA in 1987 also 
50 Environmental Contaminants and their Effects on Fish 

contained relatively low concentrations (0.09-0.24 μg/g) of 

total PCBs (USEPA, 1992b). Similarly, PCB residues were 

detected (0.1-1.0 μg/g) in only 45% of the fish collected 

in 1992-93 from the upper RGB by NAWQA (Carter and 

Anderholm, 1997; Levings and others, 1998) and in only a 

few samples obtained from the RG in New Mexico during 

the 1980s (Ong and others, 1991). Concentrations of 0.10-

0.28 μg/g were detected in fish from several lower RG sites 

sampled by the Binational Study (TNRCC, 1994b; 1997), and 

Davis and others (1995) reported total PCB concentrations of 

0.48-0.58 μg/g in two species of fish obtained in 1994 from a 

site near Brownsville. Total PCB concentrations in sediment 

cores from Llano Grande Lake were uniformly low (Mahler 

and Van Metre, 2002). The maximum concentration in carp 

collected from lower RGB resacas and settling basins in 1997 

by Wainwright and others (2001) was 0.22 μg/g; however, 

great egret eggs from Falcon International Reservoir contained 

as much as 1.50 μg/g. In contrast to most other studies, fillet 

samples of several fish species obtained in 1993 by TDH from 

Donna Reservoir, in the lower RGB, contained as much as 9.6 

μg/g of PCBs [unpublished data summarized by Mora (2001)], 

and concentrations in carp and smallmouth buffalo (Ictiobus 

bubalus) fillets from Donna Reservoir were as great as 24.0 

μg/g in 1993 (Wainwright and others, 2001). These findings 

indicate that there is some localized PCB contamination in the 

lower RGB. Nevertheless, the overall 1997 PCB concentrations in fish from the RGB were lower than most reported 

for the MRB in 1995 (Schmitt and others, 2002c), as was the 

range of values reported by other recent RGB investigations. 

They are also low relative to those reported nationally by 

NAWQA (75th percentile = 0.15 μg/g, maximum = 7.2 μg/g) 

(Wong and others, 2000). 

H4IIE Bioassay 

The H4IIE bioassay detected relatively low dioxin-like 

activity (≤6 pg/g) in all 1997 samples from the RGB (Fig. 9; 

Table 16), indicating minimal contamination by PHH. Only 

three samples contained ≥4 pg/g, all from Stations 511 and 

512 (Fig. 9). TCDD-EQ in female carp (n = 12) ranged from 

<LOD (<0.35 pg/g; four samples) to 3 pg/g, whereas male 

carp (n = 11) ranged from <0.71 to 3 pg/g with one sample 

below the LOQ. TCDD-EQs were <0.94-4 pg/g in female 

bass (n = 6) with one value below the LOQ whereas in male 

bass (n = 6) all values were 1-6 pg/g. In the other 11 samples, 

which comprised channel catfish, Morone sp., and northern 

pike, TCDD-EQs were also relatively low (<0.4-5 pg/g). Most 

1997 TCDD-EQ levels in fish from the RGB were similar to 

those reported for reference sites in previous studies (Giesy 

and others, 1995; Schmitt and others, 2002c; van den Heuvel 

and others, 1996; Whyte and others, 2004). Whole carp and 

white bass (fillets) obtained in 1987 from the RG at El Paso by 

USEPA contained 0.03-0.06 pg/g TCDD-EQ (determined by 

analytical methods), and whole channel catfish from the AC at 

Harlingen contained 2.19 pg/g, the latter about the 30th percenTCDD-EQ (pg/g wet-weight) 

7 

6 

5 

4 

3 

2 

1 

0 

64 63 516 65 515 513 16 511 512 

sp. 

sp. 

sp. 

514 

Upper RG Mid-RG/Pecos Lower RG 

Common Carp 

Micropterus

Northern Pike 

Morone

Ictalurus

Station 

Figure 9. Concentrations of H4IIE bioassay-derived TCDD-EQ in 

composite samples of whole fish, by station and taxon. Stations 

are ordered from upstream to downstream and are grouped by 

sub-basin. See Table 1 for station descriptions and collection 

dates. Micropterus sp., largemouth or smallmouth bass; Morone 

sp., white bass or striped bass; Ictalurus sp., blue or channel 

catfish. 

tile nationally (USEPA, 1992b). Elsewhere USEPA reported 

concentrations as great as 100-400 pg/g from heavily contaminated sites(Kuehl and others, 1994; USEPA, 1992a). Some 

TCDD-EQ values were ≥60 pg/g and levels ≥20 pg/g were 

widespread in fish collected from the MRB in 1995 (Schmitt 

and others, 2002c). 

The dietary threshold for TCDD-EQ toxicity in mammals is 4.4 pg/g (Heaton and others, 1995; Tillitt and others, 

1996) and for avian wildlife it is about 5 pg/g (Nosek and 

others, 1992). One sample from each of Stations 511 and 512 

exceeded both values (Fig. 9). No 1997 samples from the 

RGB approached the threshold for toxic effects in fish, which 

is about 35 pg/g (see Schmitt and others, 2002c and references 

cited therein; Whyte and others, 2004). The universally low 

levels of TCDD-EQ in RGB fish samples are consistent with 

the lack of detectable PCB residues. These findings are also 

consistent with earlier reports indicating only low levels of 

contamination by chlorinated dioxins and PCBs in the lower 

RGB (USEPA, 1992b). 

Ethoxyresorufin O-Deethylase (EROD) Activity 

EROD activity varies among fish species, between 

genders, and with gonadal stage (Schmitt and others, 2002c; 

Whyte and others, 2000). In RGB carp, a statistically significant ANOVA model containing the factors station, gender, and 

gonadal stage explained 56% of the total variance in log-transformed EROD activity, but no individual factors or interactions 
51 

were significant (Table 17); that is, after accounting for all 

other factors in the model, differences among stations were not 

significant. In bass, the ANOVA model was also significant 

and explained 57% of the total variance; however, in contrast 

to carp, EROD activity in bass differed significantly among 

stations after accounting for all other effects. In addition, the 

interaction of station and sex was also significant (Table 17). 

Analysis of the larger 1995 data set from the MRB, which 

spanned a wider range of EROD values, also indicated that 

gender was significant for carp (Schmitt and others, 2002c). 

For all of these reasons, EROD activity was tabulated and 

evaluated by gonadal stage in both carp and bass (Appendix 

Table 4). 

EROD in carp 

In female carp, EROD activity was greatest at Stations 

511 and 512; geometric station means (stages combined) were 

16.8 and 9.44 pmol/min/mg, respectively (Fig. 10; Table 18). 

Mean EROD activity in four stage-0 female carp from Station 

511 was 61.0 pmol/min/mg, ranging from 18.2 to 161.1 pmol/ 

min/mg; the latter was the greatest value measured (Appendix 

Table 4). EROD activity in stage-1 and -2 female carp was 

0-84.0 pmol/min/mg, with a basin-wide mean of 5.0 pmol/ 

min/mg (Appendix Table 4). The greatest station means for 

stage-1 and -2 female carp were for Stations 511 and 512; all 

others were much lower (Appendix Table 4). At Stations 511 

and 512, most EROD values were >10.0 pmol/min/mg, and 

the rate in one stage-2 female carp from Station 65 was 55.5 

pmol/min/mg (Fig. 10; Appendix Table 4). Individual values 

of 5-10 pmol/min/mg were also recorded in other female carp 

from Station 65 and in several from Station 513 (Fig. 10). 

EROD rates in most (84%) stage-1 and -2 female carp were 

0-5 pmol/min/mg, about the same as the 0-4 pmol/min/mg 

range identified as “normal” expression for female carp from 

uncontaminated sites in previous studies (Schlenk and others, 

1996a; Schlenk and others, 1996b; Schmitt and others, 2002c; 

Whyte and others, 2000). Nevertheless, EROD rates in at 

least one fish from all sites exceeded 4 pmol/min/mg, and the 

one elevated value at Station 65 and most from Station 511 

and 512 were >10 pmol/min/mg, indicating that these fish had 

been exposed to exogenous Ah-R agonists. EROD activity 

was as great or greater in many female carp from Station 511 

(Fig. 10) as it was in those from the most contaminated MRB 

sites sampled in 1995 (Schmitt and others, 2002c). 

EROD activity in male carp was similar to, and generally 

paralleled, activity in female carp (Fig. 10; Table 18). Only 

one stage-1 male (from Station 16) was analyzed; it had a 

low EROD rate (4.0 pmol/min/mg), and one carp of unknown 

gender or stage (no gonad was preserved) from Station 64 was 

<0.01 pmol/min/mg (Appendix Table 4). EROD activity in 

stage-2 and -3 male carp ranged from <0.01 to 54.2 pmol/min/ 

mg with a basin-wide mean of 7.8 pmol/min/mg (Appendix 

Table 4). Station means for stages 2 and 3 ranged from 1.1 

pmol/min/mg at Station 64 to 34.6 pmol/min/mg at Station 

Results and Discussion 

511. Stage-2 and -3 means were also >10.0 pmol/min/mg at 

Stations 63 (11.7 pmol/min/mg), 512 (10.1 pmol/min/mg), 

and 516 (11.3 pmol/min/mg). EROD activity in most male 

carp (72%) was <0.01-8.0 pmol/min/mg (Fig. 10; Table 

18). The upper part of this range is slightly greater than the 

0-6 pmol/min/mg identified as indicative of un-induced or 

reference conditions in previous studies (Schlenk and others, 

1996a; Schlenk and others, 1996b; Schmitt and others, 2002c; 

Whyte and others, 2000). Slightly elevated EROD rates (>8.0 

pmol/min/mg) were detected in individual male carp from 

Stations 63, 511, 512, 514, and 516 (Fig. 10). In addition, and 

as described for female carp, EROD rates in male carp from 

Station 511 were as great as those in male carp from the most 

contaminated MRB sites sampled in 1995 by Schmitt and 

others (2002c), indicating substantial exposure of the fish to 

exogenous Ah-R agonists at this site. 

EROD in bass 

As is typical for these fishes (Schmitt and others, 2002c; 

Whyte and others, 2000), EROD rates were greater in bass 

than in carp (Fig. 10; Table 18). EROD activity in stage-1 

female bass ranged from 5.0 to 107.4 pmol/min/mg (Appendix Table 4), with a basin-wide mean of 46.3 pmol/min/mg. 

EROD rates in 95% of stage-1 female bass exceeded 15.0 

pmol/min/mg, and 75% were >30.0 pmol/min/mg. Station 

means for stage-1 female bass ranged from 18.0 pmol/min/mg 

at Station 514 to 77.1 pmol/min/mg at Station 512 (Appendix 

Table 4). The minimum value at all stations except Station 

514 was >15.0 pmol/min/mg, and all values in fish from 

Stations 63 and 512 exceeded 25.0 pmol/min/mg (Fig. 10). 

Although EROD activity in stage-2 female bass was generally less than in stage-1 females, the ranges were similar 

(Appendix Table 4). Elevated EROD rates were also detected 

in stage-0 female bass from Station 512 (55.1 and 67.5 

pmol/min/mg) and in stage-3 females from Station 63 (79.6 

and 147.2 pmol/min/mg), both of which are similar to stage-1 

activity at these stations (Appendix Table 4). EROD rates in 

two stage-4 female bass from Station 514 were slightly greater 

(38.1 and 44.3 pmol/min/mg) than those of stage-1 females 

from this site (Fig. 10; Appendix Table 4). Most EROD values 

in female bass exceeded the 16 pmol/min/mg threshold for 

basal activity established in other studies (Adams and others, 

1994; Schlenk and others, 1996a; Schlenk and others, 1996b; 

Schmitt and others, 2002c; Whyte and others, 2000). In addition, EROD rates in several female bass from Station 513 and 

512 and in most from Station 63 were about the same as those 

from some of the most contaminated MRB sites sampled in 

1995 by Schmitt and others (2002c). Collectively, these data 

indicate that female bass were exposed to exogenous Ah-R 

ligands at all RGB sites from which they were obtained. 

EROD rates in male bass were also greater than for male 

carp (Fig. 10; Table 18), and some exceeded the reference 

threshold for basal activity (22 pmol/min/mg) established by 

other studies (Adams and others, 1994; Schlenk and others, 
—Continued

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52 Environmental Contaminants and their Effects on Fish 

Table 17. Results of preliminary analysis-of-variance investigating the effects of various factors on 

biomarker a responses in carp and bass (Micropterus sp.) from the Rio Grande Basin. Shown are 

degrees-of-freedom (df), F-values with significance levels (*0.01 < P  0.05; **P  0.01), and coefficients 

R2 of determination ( ).— 

Variable, source, and Carp Bass 

(transformation) df F R2 df F R2 

EROD (log) 

Model 28 7.53** 0.56 15 4.91** 0.57 

Station 6 1.08 4 3.78** 

Sex 1 0.27 1 2.23 

Station*Sex 1 0.06 1 0.19 

Stage 1 0.65 1 2.05 

Stage*Station 6 0.57 4 2.15 

Stage*Sex 1 <0.01 1 4.38* 

Stage*Station*Sex 1 0.27 1 <0.01 

Error 163 56 

HAI (rank) 

Model 28 2.47** 0.28 15 2.62** 0.41 

Station 6 0.92 4 2.01 (P = 0.11) 

Sex 1 0.18 1 0.6 

Station*Sex 1 2.13 (P = 0.15) 1 4.91* 

Stage 1 6.65** 1 0.01 

Stage*Station 6 0.87 4 1.49 

Stage*Sex 1 0.09 1 2.02 (P = 0.16) 

Stage*Station*Sex 1 2.04 (P = 0.15) 1 8.61** 

Error 176 56 

CF 

Model 28 0.67 0.09 15 12.94** 0.78 

Station 1 <0.01 4 10.67** 

Sex 1 <0.01 1 40.74** 

Station*Sex 1 <0.01 1 51.20** 

Stage 1 <0.01 1 58.29** 

Stage*Station 6 <0.01 4 12.39** 

Stage*Sex 1 <0.01 1 40.74** 

Stage*Station*Sex 1 <0.01 1 56.17** 

Error 176 56 

HSI 

Model ND ND ND 7 1.1 0.12 

Station ND ND 3 1.9 

Sex ND ND 1 0.92 

Station*Sex ND ND 3 0.35 

Error ND 57 

SSI 

Model 19 4.35** 0.31 8 1.92 (P = 0.07) 0.19 

Station 9 6.26** 4 2.63* 

Sex 1 11.81** 1 1.37 

Station*Sex 9 1.21 3 0.65 

Error 183 63 

MAMM 

Model 38 3.94** 0.52 17 2.03* 0.39 

Station 9 4.75** 4 2.09 (P = 0.10) 

Sex 1 2.00 (P = 0.15) 1 0.24 
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-- 

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Results and Discussion 53 

Table 17. Results of preliminary analysis-of-variance investigating the effects of various factors on 

biomarker a responses in carp and bass (Micropterus sp.) from the Rio Grande Basin. Shown are 

degrees-of-freedom (df), F-values with significance levels (*0.01 < P  0.05; **P  0.01), and coefficients 

R2 of determination ( ).—Continued 

Variable, source, and Carp Bass 

(transformation) df F R2 df F R2 

Station*Sex 8 1.69 (P = 0.11) 3 0.3 

Age 1 0.12 1 0.4 

Age*Station 9 1.3 4 1.83 (P = 0.14) 

Age*Sex 1 0.42 1 0.84 

Age*Station*Sex 8 1.48 (P = 0.16) 3 0.6 

Error 136 53 

MEANAREA (log) 

Model 38 4.86** 0.57 17 1.96* 0.39 

Station 9 5.56** 4 1.93 (P = 0.11) 

Sex 1 1.72 1 0.43 

Station*Sex 8 1.26 3 0.52 

Age 1 1.14 1 0.25 

Age*Station 9 0.91 4 2.11 (P = 0.09) 

Age*Sex 1 0.49 1 1.00 

Age*Station*Sex 8 1.19 3 0.51 

Error 137 53 

TISSOC (log) 

Model 38 5.61** 0.61 17 2.10* 0.40 

Station 9 6.62** 4 1.27 

Sex 1 1.71 1 <0.01 

Station*Sex 8 0.72 3 1.08 

Age 1 2.91 (P = 0.09) 1 2.60 

Age*Station 9 1.06 4 1.74 

Age*Sex 1 1.91 1 0.10 

Age*Station*Sex 8 1.01 3 0.86 

Error 137 53 

Atresia b 

Model 25 2.69** 0.49 17 25.41** 0.95 

Station 1 0.04 3 0.13 

Stage 1 0.73 1 0.46 

Stage*Station 1 0.93 3 0.52 

Age 1 1.55 (P = 0.11) 1 0.02 

Age*Station 1 0.19 3 2.04 (P = 0.13) 

Stage*Age 1 1.29 1 <0.01 

Stage*Age*Station 1 0.15 3 5.01** 

Error 71 23 

GSI 

Model 28 16.26** 0.72 15 8.73** 0.70 

Station 6 4.58** 4 2.42 (P = 0.06) 

Sex 1 0.19 1 7.85** 

Station*Sex 1 0.1 1 3.88* 

Stage 1 11.41** 1 18.44** 

Stage*Station 6 5.09** 4 2.13 (P = 0.09) 

Stage*Sex 1 0.05 1 1.33 

Stage*Station*Sex 1 0.19 1 5.47* 

Error 175 56 
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54 Environmental Contaminants and their Effects on Fish 

Table 17. Results of preliminary analysis-of-variance investigating the effects of various factors on 

biomarker a responses in carp and bass (Micropterus sp.) from the Rio Grande Basin. Shown are 

degrees-of-freedom (df), F-values with significance levels (*0.01 < P  0.05; **P  0.01), and coefficients 

R2 of determination ( ).—Continued 

Variable, source, and Carp Bass 

(transformation) df F R2 df F R2 

Vtg b (log) 

Model 25 19.01** 0.87 17 3.50** 0.71 

Station 1 0.77 3 0.67 

Stage 1 31.14** 1 0.31 

Stage*Station 1 13.58** 3 0.30 

Age 1 1.97 (P = 0.17) 1 0.17 

Age*Station 1 0.09 3 1.28 

Stage*Age 1 12.97** 1 <0.01 

Stage*Age*Station 1 3.88* 3 0.80 

Error 69 23 a

 EROD, ethoxyresorufin O-deethylase activity; CF, condition factor; SSI, splenosomatic index; HSI, hepatosomatic index; HAI, health assessment 

index; TICCOC, percent splenic tissue occupied by macrophage aggregates (MA); MEANAREA, mean area of splenic MA; MAMM, density of splenic 

MA; GSI, gonadosomatic index; Vtg, plasma vitellogenin concentration. 

b

 Only female fish included in analysis 

1996a; Schlenk and others, 1996b; Schmitt and others, 2002c; 

Whyte and others, 2000). Of 30 male bass analyzed, only 

two had EROD rates <20.0 pmol/min/mg (Fig. 10). EROD 

expression in all stage-1 males was ≥33.5 pmol/min/mg with 

a basin-wide mean of 73.2 pmol/min/mg (Appendix Table 4). 

EROD rates were ≥13.0 pmol/min/mg protein in all stage-2 

males with a basin-wide mean of 55.3 pmol/min/mg, and the 

rate in one stage-3 male bass from Station 514 was 55.6 pmol/ 

min/mg (Appendix Table 4). Station means for EROD activity 

ranged from 17.6 pmol/min/mg at Station 514 (all stage 2) to 

78.2 pmol/min/mg at Station 512 (also stage 2; Fig. 10; Table 

18). Mean EROD activity in male bass exceeded background 

rates at Stations 16 (stage 1, 33.5 pmol/min/mg; stage 2, 

62.3 pmol/min/mg) and 513 (stage 2, 56.7 pmol/min/mg). In 

addition, elevated EROD rates (62.9-129.8 pmol/min/mg) 

were detected in two male bass from Station 63 and in one of 

unknown gender and stage from Station 512 (Fig. 10; Appendix Table 4). EROD activity in some of the male bass from 

Stations 16, 513, and 512 was about the same as in those from 

the most contaminated MRB sites sampled in 1995 by Schmitt 

and others (2002c). No male bass were obtained from Station 

63, however. 

EROD in other fishes 

In addition to carp and bass, EROD activity was measured in 47 fish representing five other species (channel 

catfish, blue catfish, northern pike, white bass, and striped 

bass) from five of the 10 stations sampled (Table 18). Extant 

data are available for comparison with all of these species 

except blue catfish (Schmitt and others, 2002c; Whyte and 

others, 2000). Channel catfish were obtained from Stations 

511 and 516. At Station 511, EROD activity was 17.8-71.7 

pmol/min/mg in females, 12.8-27.9 pmol/min/mg in males, 

and 18.9-110.5 pmol/min/mg in juveniles (Table 18). These 

values are substantially greater than the 0.4-14.2 pmol/min/mg 

reported for channel catfish (n = 6) collected in 1995 from two 

stations in the Missouri River sub-basin of the MRB (Schmitt 

and others, 2002c), and is further evidence of exposure of fish 

to exogenous Ah-R agonists at Station 511. EROD rates in 

channel catfish from Station 516 were generally lower than 

those from Station 511 (Table 18), but were nevertheless 

greater than 1995 values for this species (Schmitt and others, 2002c) and therefore suggests some degree of exposure 

to Ah-R agonists at Station 516 as well. EROD rates in blue 

catfish (n = 6), which were obtained only at Station 515, were 

7.2-17.2 pmol/min/mg (Table 18). These rates are within the 

range of those in the congeneric channel catfish from Stations 

511 and 516, but there are no extant data for comparison in 

this species. Northern pike were obtained only from Station 

64; EROD activity was 0.4-1.8 pmol/min/mg in females and 

0.2-2.3 in males (Table 18). These values are almost identical 

to those reported for northern pike from a site on the Missouri 

River in North Dakota sampled in 1995 (Schmitt and others, 

2002c) and, like the 1995 values, are about 10-fold lower than 

previously reported basal EROD rates for this species (Förlin and Celander, 1993; Williams and others, 1997). EROD 

activity in white bass, which were obtained exclusively at 

Station 65, were 2.5-18.1 pmol/min/mg in females and 2.8-8.4 

pmol/min/mg in males (Table 18). These values are similar 

to those observed in white bass and striped bass X white bass 

hybrids obtained from the lower Mississippi River in 1995 

(Schmitt and others, 2002c), but basal rates in this species 

have not been determined (Whyte and others, 2000). A laboratory study (Shayne Washburn and others, 1996) indicated 

a low level of basal EROD activity (1.2 pmol/min/mg) in the 

congeneric striped bass; however, the 1997 results suggest 

that the fish from Station 65 had been exposed to exogenous 

AhR agonists. The only striped bass collected in the RGB 
Results and Discussion 55 

100 Carp-male 

10 

1 

0.1 

0.01

1000 Carp-female

100 

10 

1 

0.1 

0.01

140

120 

100 

80 

60 

40 

20 

0 

160

140

120

100

80

60

40

20

0

sp.-male 

sp.-female 

Micropterus

Micropterus

Upper RG Mid-RG/Pecos Lower RG 

64 63 516 65 515 514 513 16 511 512 

Station 

Figure 10. Hepatic EROD activity in male and female carp and 

bass (Micropterus sp.), by station. Shown for each group are 

points representing individual fish and the mean (red horizontal 

line), median (black horizontal line), interquartile range (box), and 

the 10th and 90th percentiles (whiskers). Stations are ordered 

from upstream to downstream and are grouped by sub-basin. 

See Table 1 for station descriptions and collection dates. pmol/min/mg protein pmol/min/mg protein pmol/min/mg protein pmol/min/mg protein 

(from Station 63) had a very low level of EROD activity (<0.1 

pmol/min/mg; Table 18). 

Accumulative Contaminants, H4IIE, and EROD: 

Summary 

Among the elemental contaminants measured in 1997, 

only Hg, As, and Se concentrations were elevated, and all 

had been documented previously in the RGB. Concentrations of the other elements measured were either generally 

low or elevated in relatively few samples (for example, Pb in 

one bass sample from Station 16). Elevated concentrations of 

Hg and Se are well documented in the RGB; these elements 

are suspected of being involved in the reproductive failure 

of peregrine falcons in Trans-Pecos Texas (Mora and others, 

2002). Concentrations of Hg in bass collected in 1997 from 

Station 63 were 0.30-0.50 μg/g, levels that are potentially 

toxic to piscivorous wildlife and consistent with the present 

consumption advisory for Elephant Butte Reservoir. Concentrations were lower, but nevertheless elevated relative to most 

sites, in other predatory fishes from Station 63. Concentrations of Hg were comparatively low in fish from Station 515, 

which is downstream from BBNP and the Terlingua cinnabar 

mining district of Trans-Pecos Texas. Elevated concentrations 

of Hg were also detected in bass from Station 514 (Amistad 

Dam), which is further downstream, and at Station 16 (RG at 

Mission, Texas). In the past, elevated Hg concentrations were 

also reported in fish from the RG at El Paso (USEPA, 1992b), 

but 1997 concentrations at Station 516 were universally <0.06 

μg/g (Fig. 3). Overall, Hg concentrations in bass from the 

RGB were generally similar to those collected from the MRB 

in 1995 (Schmitt and others, 2002c). Temporal trends in Hg 

concentrations were not clearly evident in the RGB, and both 

temporal and geographic differences were confounded to some 

degree by fish size differences. 

Comparatively high concentrations of Se (>1.0 μg/g) 

characterized fish from Stations 65, 514, and 515 (Fig. 2) 

and confirmed previous findings for the central parts of the 

RGB; however, the 1997 Se concentration in Morone sp. from 

Station 65 was significantly lower in 1997 than in previous 

NCBP collections (Schmitt and others, 1999b). In addition, 

Se concentrations in all RGB fish were considerably lower 

than the 3-5 μg/g reported for a site in the MRB with a history of contamination from irrigated agriculture (Schmitt and 

others, 2002c). Nevertheless, mean concentrations at Stations 65, 514, and 515 and individual fish from several other 

sites exceeded the 0.8 μg/g threshold for fish toxicity and 0.6 

μg/g for wildlife proposed by Lemly (1996; 2002). Naturally 

occurring Se mobilized by irrigated agriculture is among the 

contaminants suspected in the reproductive failure of peregrine 

falcons in Trans-Pecos Texas (Mora and others, 2002), as 

noted in the preceding paragraph. 
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56 Environmental Contaminants and their Effects on Fish 

Table 18. Geometric means and ranges of EROD activities (pmol/min/mg protein), by species, station and gender. 

Censored values were represented by 50% of the limit-of-quantitation in the computation of geometric means. See 

Table 1 for station locations and collection dates. 

Species and Female Male Juvenile a 

station n Range Mean n Range Mean n Range Mean 

Common carp 

16 10 0.76-3.35 1.31 10 0.72-4.47 2.89 0 

63 8 0.18-4.95 1.81 9 0.18-27.2 4.76 0 

64 19 0.07-2.72 0.25 14 0.08-7.84 0.34 1 0.07 

65 10 0.03-55.5 1.90 10 0.03-7.38 1.54 0 

511 13 0.06-161.1 16.8 7 20.1-54.2 32.6 0 

512 6 3.19-15.1 9.44 8 2.0-18.8 8.70 0 

513 11 0.49-7.98 1.80 9 2.58-6.88 3.97 0 

514 10 0.12-3.42 1.42 10 0.06-8.74 3.46 0 

515 6 0.02-2.98 0.86 4 0.57-4.50 2.13 0 

516 10 0.81-2.83 1.49 8 2.26-28.0 9.24 0 

Largemouth 

bass 

16 11 9.09-58.5 28.6 10 33.5-132 54.8 0 

63 4 38.5-81.0 57.7 0 2 62.9-130 90.3 

512 7 28.1-107 67.4 8 46.2-107 75.9 1 111.2 

513 13 15.5-96.6 37.2 9 19.7-83.5 50.6 0 

514 5 5.00-44.3 21.3 2 13.0-22.3 17.0 0 

Smallmouth 

bass 

63 2 79.6-147 108 0 0 

514 0 1 52.6 0 

Striped bass 

63 1 <0.1 0 0 

White bass 

65 18 2.50-22.4 9.52 3 2.77-8.36 4.34 0 

Blue catfish 

515 3 7.19-17.2 10.3 0 3 9.75-12.2 11.0 

Channel catfish 

511 8 17.9-71.7 40.6 3 12.8-27.9 20.3 9 18.9-111 38.9 

516 11 6.12-23.1 12.2 2 15.8-19.5 17.5 9 9.3-25.4 14.1 

Northern pike

 64 6 0.14-1.77 0.52 10 0.18-2.30 0.69 0 a May include fish of undetermined gender from which no gonad sample was obtained. 

Arsenic and Se concentrations are naturally elevated in 

some parts of the RGB, and these high background concentrations are reflected in fish. In addition, naturally occurring 

concentrations may be further increased by leaching due to 

irrigation, and As is discharged from sewage treatment plants 

where groundwater is rich in As (Wilcox, 1997). Arsenic is 

released during the smelting of metals, and both As and Se are 

released during the combustion of fossil fuels. Arsenic-containing pesticides and defoliants are also used extensively in 

parts of the RGB, and As accumulates in planktivorous fishes 

and higher trophic level species that consume them. Consistent with previous studies, As concentrations were comparatively high (0.37-0.55 μg/g) in carp and other fishes from 

Stations 65 and 514; comparable concentrations were reported 

in fish collected from the southwestern parts of MRB in 1995 

(Schmitt and others, 2002c). At least one 1997 sample from 

every station except 64 contained >0.2 μg/g. The As in fish 

from the RGB is not considered a hazard to the fish or to fisheating wildlife based on current criteria. Other than As and 

Hg, concentrations of elemental contaminants associated with 

mining were comparatively low, even at Station 64. This site 

is downstream from the Crede, Colorado mining district where 

elevated metals from historical mining and related activities 

have been reported in the past. 

Comparatively high concentrations of the organochlorine pesticides DDT (as p,p’-DDE), chlordane, dieldrin, and 

toxaphene were detected in fish from at least one site in the 

lower RGB, which also generally confirmed the findings 

of previous and contemporaneous investigations. No other 

residues were detected. Although concentrations were lower 

than in the past, residues derived from DDT continued to be 

detected throughout the RGB in 1997. Most of this material 

was p,p’-DDE, indicating weathered insecticide rather than 

the influx of new material. Concentrations remained sufficiently high to represent a threat to fish-eating birds. At least 

one sample from Stations 16, 513, and 516 contained >0.15 

μg/g, and all channel catfish samples from Station 511 (AC 

at Harlingen) contained 1-2 μg/g, which is within the range 
57 

of toxicity for most avian wildlife (Blus, 1996). Except for 

Station 516, residues in fish from the upper RGB were low, as 

they also were in 1992 samples from the upper RGB analyzed 

by NAWQA (Carter and Anderholm, 1997; Levings and others, 1998). In contrast, concentrations at Station 16, on the 

lower RG, remained relatively high (0.32-0.52 μg/g) in 1997. 

Nevertheless, these levels represent a substantial decrease 

since the 1980s, when they were consistently >1.0 μg/g 

(Schmitt and others, 1999b). In general, 1997 findings for 

DDT in the lower RGB and at El Paso confirmed the results 

of other recent investigations of this area (Davis and others, 

1995; Moring, 1999; TNRCC, 1994b; 1997; USEPA, 1992b; 

Wainwright and others, 2001), and the AC and nearby waters 

remain under a fish consumption advisory. 

Chlordane was among the few other organochlorine 

chemical residues detected at potentially toxic concentrations 

in 1997. During the 1970s, traces of chlordane components 

(≤0.01-0.3 μg/g) were detected in some samples from Stations 

63, 64, and 65 (Schmitt and others, 1999b) but a trend toward 

non-detections that began in the 1980s continued through 

1997 at these stations; concentrations of all six components 

measured were <0.01 μg/g in all samples. Traces of chlordane 

constituents were detected in fish samples from some sites 

in the upper RGB sampled during the 1980s and early 1990s 

(Carter and Anderholm, 1997; Levings and others, 1998; Ong 

and others, 1991). At Station 16, concentrations of chlordane components ranged from <0.005 to 0.09 μg/g in NCBP 

collections, but 1997 concentrations were slightly lower; all 

component concentrations were ≤0.01 μg/g except for two 

trans-nonachlor values. In contrast, 1997 chlordane concentrations were >0.1 μg/g in fish from Station 511, where they 

were also elevated in all recent and contemporaneous studies 

(Davis and others, 1995; TNRCC, 1994b; USEPA, 1992b). 

Like DDT, chlordane is part of the consumption advisory for 

fish from these waters, and 1997 concentrations remained 

sufficiently elevated to also represent a threat to fish-eating 

wildlife. The chlordane residues in fish from Station 511 were 

similar in concentration and composition to those in fish from 

agricultural regions of the MRB sampled in 1995 (Schmitt and 

others, 2002c). 

Toxaphene concentrations were historically elevated 

in the lower RGB, but levels in the upper basin were low; 

concentrations did not exceed 0.3 μg/g at Stations 63, 64, 

and 65 during the 1970s, and levels in all but one sample 

collected in the 1980s were ≤0.1μg/g. In 1997, toxaphene 

was not detected at these sites, nor were residues detected in 

fish collected from the upper RGB in 1986-1992 (Carter and 

Anderholm, 1997; Levings and others, 1998; Ong and others, 

1991) or from the transboundary segments of the RG during 

the mid-1990s (TNRCC, 1994b; 1997). In contrast, toxaphene 

concentrations at Station 16 were elevated in the 1970s (0.3-

1.4 μg/g) but decreased through the 1980s (to 0.2-0.3 μg/g in 

1986). Wainwright and others (2001) also reported concentrations as great as 0.3 μg/g in carp from resacas and settling 

basins and as much as 4.4 μg/g in greenback heron eggs. All 

1997 samples from Station 16 contained ≤0.11 μg/g; however, 

Results and Discussion 

toxaphene concentrations were 0.5-2.5 μg/g in fish from Station 511, which is consistent with recently reported concentrations at this site (Davis and others, 1995; TNRCC, 1994b; 

1997). These values are also similar to those in fish collected 

from cotton-farming regions of the MRB in 1995 (Schmitt 

and others, 2002c) and may be sufficiently high to represent a 

threat to fish; however, the toxicity of weathered toxaphene is 

highly variable and cannot be determined from the 1997 data. 

Toxaphene is also a component of the consumption advisory 

for fish from the AC and nearby waters. 

In 1997, PCBs were not detected in any samples from the 

RGB and the greatest TCDD-EQ concentrations were only 3-6 

pg/g (in fish from Stations 511 and 512). Concentrations of 

TCDD-EQ at reference sites in previous studies ranged from 

below detection to 6 pg/g (Giesy and others, 1995; Schmitt 

and others, 2002c; van den Heuvel and others, 1996; Whyte 

and others, 2004), and 6 pg/g was suggested as a threshold 

for toxicity to fish-eating wildlife (Giesy and others, 1995). 

These results were consistent with recent and historical findings of low levels of PCB and dioxin contamination in the 

RGB (Carter and Anderholm, 1997; Levings and others, 1998; 

Ong and others, 1991; Schmitt and others, 1999b; TNRCC, 

1994b; 1997; USEPA, 1992b; Wainwright and others, 2001), 

and indicate minimal risk to wildlife from TCDD and similar 

compounds. 

Based on data from the 1995 MRB study and an extensive review of the literature (Schmitt and others, 2002c; Whyte 

and others, 2000) considered 0-4 pmol/min/mg to be the normal range of EROD expression in female carp and 0-6 pmol/ 

min/mg the normal range in male carp. The normal range for 

bass was considered 0-16 pmol/min/mg in females and 0-22 

pmol/min/mg in males. Relative to these ranges, EROD rates 

were induced above basal levels and therefore indicative of 

exposure to exogenous AhR agonists at all stations and were 

greatest at Stations 511 and 512. Although basal rates have 

not been determined for channel catfish, EROD rates were 

also greater than those observed in other studies and at other 

RGB stations in channel catfish from Station 511. Rates also 

exceeded basal levels in male carp from Stations 63 and 516, 

in one female carp from Station 65, and possibly also in white 

bass from Station 65 (basal rates have not been determined 

in this species). EROD rates in individual male and female 

bass were elevated at Stations 512 and Station 513; in bass 

(female and unknown gender) from Station 63; and in male 

bass from Station 16. In addition, there was a general gradient 

of increasing EROD activity from upstream to downstream. 

Considering the low concentrations of total PCBs and TCDDEQ at all sites, the EROD findings suggest that some fish 

from all RGB stations had been exposed to PAH. Moring 

(1999) detected PAH in SPMD samples from five of the six 

transboundary RG stations sampled in July-August 1997, and 

both concentrations and numbers of compounds detected also 

generally increased from upstream to downstream. Although 

PAH emanate from a variety of sources (Schmitt, 1998), the 

extensive oil and gas extraction and transportation industry of 

the RGB cannot be overlooked as a potential source. 
58 Environmental Contaminants and their Effects on Fish 

Fish Health Indicators 

Organism-Level Indicators 

External Gross Lesions 

Of the 368 fish examined, 28% had some type of external lesion (Table 19). Lesion frequencies ranged from 2% at 

Station 65 to 75% at Station 515. Of the 207 carp examined, 

29% had external lesions. Percentages were lowest at Stations 

16, 65, and 512 and highest at Stations 516 and 515 (Table 

19). Carp from Station 65 were notable for a lack of external 

lesions (0%) whereas the percent occurrence at Station 515 

was 3-9 times greater than that at most other stations (Table 

19). In bass, 27% had external lesions. Percentages were 

lowest at Stations 16 (1%) and 513 (14%) and were ≥50% at 

Stations 512 and 514 (Table 19). 

External lesions were not analyzed statistically. Overall, percentages of external lesions for carp and bass were 

similar to each other and to the percentage for all fish (Table 

19). Greater percentages of bass from Stations 512 and 514 

than from Stations 16, 63, and 514 had lesions (Table 19). As 

noted, the greatest percentages of all fish and of carp with 

lesions were from Station 515, but bass were not collected 

there. Lesions occurred most commonly on bass (≥50%) at 

Stations 512 and 514 whereas the occurrence on carp was low 

(7%) at Station 512 and similar to the basin average at Station 

514 (Table 19). No other taxa were collected at the latter two 

stations, however. The incidence of external lesions on carp 

and bass from Station 16 and carp and white bass from Station 

65 was also low (Table 19). 

Overall, the percentages of fish with external lesions were 

similar to what was reported for the MRB in 1995 (Blazer and 

others, 2002), which were generally higher than the proportions reported by other studies. However, and as noted by 

Blazer and others (2002), external lesion frequencies are difficult to compare because protocols differ among investigators. 

As also reported for the MRB, most external lesions were 

identified as eroded, frayed, or hemorrhagic fins. Tissues from 

a total of 24 external lesions (16 from carp, eight from bass) 

were fixed and analyzed microscopically; most were identified 

as inflammatory responses, often associated with parasites. 

Two carp from Station 515 had unusual inflammatory reactions in the epidermis or hypodermis associated with refractile, 

crystalline material (Fig. 11). In addition, two carp from each 

of Stations 516 and 514 had papillomas of the body surface or 

oral cavity (Fig. 11). 

Health Assessment Index 

Statistically significant ANOVA models that included the 

factors station, gender, stage, and their interactions accounted 

for 28% of the total HAI variation (rank-transformed) in carp 

and 41% in bass (Table 17). Differences among gonadal 

stages were statistically significant in both taxa, as was the 

interaction of station, stage, and sex in bass (Table 17). However, after accounting for all other effects, differences among 

stations were not significant in carp and only approached 

significance (P = 0.11) in bass; differences between genders 

were not significant in either taxon (Table 17). These results 

are similar, but not identical, to those obtained from the analysis of the much larger 1995 MRB data set (Blazer and others, 

2002) in which differences between carp genders were also 

detected. Results for male and female fish were nevertheless 

combined for presentation. 

Overall, HAI scores for carp and bass were distributed 

similarly. Most (90%) of the scores for carp ranged from 20 to 

90, indicating that the fish had 1-3 lesions (Fig. 12; Table 20). 

The basin-wide mean HAI score was 46 and all station means 

Table 19. Percentages (%) of carp, bass (Micropterus spp.), and all fish from the Rio Grande Basin 

with external lesions, by station. Also shown are the numbers of fish with lesions (EL) and of fish in 

each category examined (n). See Table 1 for station locations and collection dates. 

Station 

n 

Carp 

EL % 

Bass All fish 

n EL % n EL % 

16 20 2 10 21 2 10 41 4 10 

63 25 8 32 8 2 25 34 10 29 

64 40 14 35 0 0 0 56 26 46 

65 20 0 0 0 0 0 41 1 2 

511 20 6 30 0 0 0 40 9 23 

512 14 1 7 16 9 56 30 10 33 

513 20 6 30 22 3 14 42 9 21 

514 20 6 30 8 4 50 28 10 36 

515 10 9 90 0 0 0 16 12 75 

516 18 8 44 0 0 0 40 12 30 

Total 207 60 29 75 27 27 368 103 28 
Results and Discussion 59 

Figure 11. Histologic appearance of external gross lesions observed in carp from the Rio Grande Basin. Inflammatory responses 

included chronic inflammation (thin arrows) throughout a thickened epidermis (A) and within the dermis (B) of carp from Station 515. 

H & E stain (X 165). C. Higher magnification (X 412) of a portion of (A) illustrating the refractile, crystalline material that appears to elicit 

this inflammatory response (thick arrows). D. Papilloma in carp from Station 514. H & E stain (X 165). 

were between 30 and 70 except for Station 65 (mean = 28; 

Fig. 12). Station means for carp were >60 at Stations 511 and 

515, indicating that these fish had, on average, two external 

lesions. Except for Station 515, 80-100% of the carp from 

each station received ratings <100, indicative of three or fewer 

abnormalities (Fig. 12; Table 20). For bass, most HAI scores 

(90%) were between 10 and 110, indicative of 1-5 abnormal 

ratings (Fig. 12; Table 20). The basin-wide mean for bass was 

53, but the only station mean <50 was for Station 512 (mean 

= 33; Fig. 12). Means for Stations 514 and 63 were >60, and 

no individual bass from these sites scored <40. The lowest 

mean HAI score for carp was at Station 65; for bass Station 

512 was lowest (Fig. 12). Mean HAI scores were >60 for bass 

from Stations 514 and 63. The greatest means for carp and 

bass did not occur at the same stations; however, bass were 

not collected at Stations 511 or 515, where scores in carp were 

generally greatest. 

The HAI had not been used to evaluate carp prior to the 

1995 MRB study (Schmitt, 2002a). In contrast, HAI had been 

used to evaluate largemouth bass in several previous investigations (Adams and others, 1993; Coughlan and others, 1996). 

Based on these previous studies, Blazer and others (2002) 

considered an HAI score ≤20 as indicative of an unimpacted 

or minimally impacted site. Relative to this value, most of the 

individual fish and all carp and bass station means indicated 

some impact. Although the overall range and distribution of 

HAI scores in carp and bass from the RGB were similar to 

those reported for the MRB 1995 (Blazer and others, 2002), 

fish from greater percentage of RGB stations had scores >20. 
60 Environmental Contaminants and their Effects on Fish 

140

120

100

80

60

40

20

0

140

120

100

80

60

40

20

0

sp. 

Carp 

Upper RG Mid-RG/Pecos Lower RG 

Micropterus

64 63 516 65 515 514 513 16 511 512 

Station 

Figure 12. Health Assessment Index (HAI) scores of male and 

female carp and bass (Micropterus sp.), by station. Shown for 

each group are points representing individual fish and the mean 

(red horizontal line), median (black horizontal line), interquartile 

range (box), and the 10th and 90th percentiles (whiskers). Stations are ordered from upstream to downstream and are grouped 

by sub-basin. See Table 1 for station descriptions and collection 

dates 

Condition and Organosomatic Indices 

Condition Factor 

An ANOVA model that included station, sex, stage, 

and all interactions of these factors was highly significant in 

bass and explained 78% of the variation in CF (Table 17). In 

contrast, no CF differences in carp were statistically significant, and these factors combined accounted for only 9% of the 

variation (Table 17). These results are consistent with results 

obtained for the larger 1995 data set for the MRB (Blazer and 

others, 2002). Data were therefore combined for carp, but are 

presented separately for bass (Fig. 13). The basin-wide mean 

CF for RGB carp was 1.4. Station means ranged from 1.2 

(Stations 63 and 512) to 1.6 (Stations 513 and 514) (Fig. 13); 

however, CF in carp did not differ significantly among stations 

(Table 17). Across all stations, 90% of carp CFs were between 

1.1 and 1.8 (Fig. 13). Individual values of 0.7-0.9 occurred at 

Stations 16, 63, 511, and 516, whereas values >2.0 occurred at 

Stations 64, 511, and 514 (Fig. 13). Station means and individual values were generally similar to previously published 

CFs in carp from other locations (BEST Program, unpublished 

data from the Columbia River Basin; Blazer and others, 2002; 

Carlander, 1969; Solé and others, 2002), but individual values 

>2.0 are unusual and indicate rapid growth. 

In contrast to carp, among-station differences for CF in 

bass were statistically significant (Table 17). The basin-wide 

mean was 1.5, with station means ranging from 1.4 at Stations 

16 and 63 to 1.8 at Station 514 (Fig. 13). For the RGB, 90% 

of the bass examined had CF values between 1.2 and 2.0 (Fig. 

13), and the among-station and between-gender differences 

appear to have been heavily influenced by two widely separate 

values, both in males: the CF of one fish from Station 513 was 

only 0.9 whereas one from Station 514 was 3.5 (Fig. 13). All 

other CF values were more uniform, and differences among 

stations were small. The means and individual CFs of RGB 

bass were generally similar to previously reported values for 

other locations, but means >1.6 and individual values >2.0 are 

unusual (BEST Program, unpublished data from the Columbia 

River Basin; Blazer and others, 2002; Carlander, 1977). These 

relatively high values also indicate rapid growth in bass. 

Hepatosomatic Index (HSI) 

Liver weights were not obtained for any fish from Station 

63. Statistical analysis indicated that HSI in bass did not differ 

significantly among stations or between genders (Table 17). 

Nevertheless, because of the role of the liver in vitellogenesis 

and hormone homeostasis, HSI data are presented separately 

for males and females to facilitate comparisons with other 

endpoints (Fig. 14). The basin-wide mean HSI for female bass 

(all largemouth) was 1.0%. Station means ranged from 0.9% 

(Station 16) to 1.1% (Station 514). Most individual values 

were between 0.6 and 1.5%; only one fish from each of Stations 513 and 514 were >1.5% (Fig. 14). All individual values 

were within the range reported for female bass in previous and 

ongoing studies (BEST Program, unpublished data from the 

Columbia River Basin; Blazer and others, 2002). 

The overall mean HSI for male bass was similar to that 

for females (0.9%). Station means ranged from 0.7% (Station 16) to 1.0% (Stations 512, 513, 514), and most individual 

values were between 0.6% and 1.4%; only one fish (Station 

513, 2.3%) exceeded 1.4% (Fig. 14). The livers of male 

bass from Station 16 were comparatively small (Fig. 14). As 

was true for female bass, the range of individual values was 

similar to that reported for male bass collected in previous and 

concurrent studies (Blazer and others, 2002), indicating that 

values <1.0% are not abnormal for bass. In addition, and in 

contrast to findings for sites in the MRB (Blazer and others, 

2002), at no station were HSI values in RGB bass indicative of 

liver enlargement characteristic of chronic chemical exposure. 
Results and Discussion 61 

Table 20. Distribution of Health Assessment Index (HAI) scores in carp and bass (Micropterus sp.) from the Rio Grande Basin, by 

station. See Table 1 for station locations and collection dates. 

HAI score 

Taxon and station 0 10-30 40-60 70-90 100-120 130 Total 

n 

n % n % n % n % n % n % 

Bass 

16 3 14 4 19 6 29 6 29 2 10 0 0 21 

63 0 0 0 0 5 63 1 13 1 13 1 13 8 

512 0 0 11 69 4 25 1 6 0 0 0 0 16 

513 2 9 6 27 9 41 2 9 2 9 1 5 22 

514 0 0 0 0 4 50 3 38 0 0 1 13 8 

Carp 

16 1 5 2 10 12 60 5 25 0 0 0 0 20 

63 1 4 7 28 10 40 4 16 3 12 0 0 25 

64 4 10 18 45 11 28 7 18 0 0 0 0 40 

65 1 5 8 40 11 55 0 0 0 0 0 0 20 

511 0 0 1 5 10 50 5 25 4 20 0 0 20 

512 0 0 5 36 5 36 3 21 1 7 0 0 14 

513 0 0 5 25 9 45 4 20 1 5 1 5 20 

514 2 10 6 30 7 35 3 15 1 5 1 5 20 

515 0 0 1 10 4 40 1 10 3 30 1 10 10 

516 0 0 8 44 2 11 7 39 1 6 0 0 18 

Totals 

Bass 5 7 21 28 28 37 13 17 5 7 3 4 75 

Carp 9 4 61 29 81 39 39 19 14 7 3 1 207 

Bass and carp 14 5 82 29 109 39 52 18 19 7 6 2 282 

Many of our HSI values for bass and most of those from the 

MRB (Blazer and others, 2002) were lower than the average 

relative liver weight for resting fish (1-2%) suggested by Gingerich (1982). They were also smaller than the values reported 

for male and female largemouth bass (treatments and controls) 

exposed in Florida to paper mill effluents (Sepúlveda and others, 2001; 2003). However, the Florida fish were sampled in 

the spring, during the reproductive season, when liver enlargement is normal. In addition, the formula used to compute the 

HSI for the Florida fish differed slightly from that used for the 

RGB and MRB fish. 

Splenosomatic Index (SSI) 

An ANOVA model that included station, sex, and the 

interaction of these factors was statistically significant and 

explained 31% of SSI variation in carp (Table 17). Differences between genders and among stations were statistically 

significant (Table 17); mean SSI was lower (that is, spleen size 

was smaller) in females than in males. In bass, the ANOVA 

model was marginally significant (P = 0.07) and accounted 

for only 19% of the variation, but there was some evidence 

supporting among-station differences (Table 17). The biological reason for differences between genders is not known, 

but similar findings were reported for carp collected from the 

MRB in 1995 (Blazer and others, 2002) and from the CRB in 

1997 (BEST Program, unpublished data from the Columbia 

River Basin). 

The basin-wide mean for SSI in male carp was 0.28%. 

Station means ranged from 0.15 (Station 515) to 0.44% 

(Station 516), and most values (90%) were between 0.08 

and 0.49% (Fig. 15). The lowest values (0.01%) occurred 

at Stations 512 and 515; all other SSI values in male carp 

were ≥0.08% (Fig. 15). Individual male carp with SSI values 

>0.5% were captured at Stations 64, 514, and 516 (Fig. 15). 

The maximum SSI value in male carp (1.5%) occurred at 

Station 64, but all other values for this station were within the 

range of other stations. In female carp the basin-wide mean 

for SSI was 0.20%. Station means ranged from 0.13% at Station 512 to 0.28% at Station 514, and 90% of the individual 

values were between 0.08 and 0.32% (Fig. 15). Similar to 

male carp, very low values (0.01%) characterized one female 

carp from each of Stations 512 and 515 and the maximum 

value (0.80%) was obtained from Station 64 (Fig. 15). Values 

>0.4% also occurred at Stations 16 and 514, and most female 

carp from Stations 514 and 516 had SSIs >0.2%. (Fig. 15). 

Therefore, and as also described for males, the means, medians and individual values were greatest at Stations 514 and 

516 (Fig. 15). 

Because SSI values in bass did not differ significantly 

between genders (Table 17) they were combined for analysis 

(Fig. 15); however, SSI values are tabulated by gender and age 
62 Environmental Contaminants and their Effects on Fish 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

0.0

4 Micropterus sp.-male

Carp 

3 

2 

1 

0 

2.5 

2.0 

1.5 

1.0 

0.5 

0.0

64 63 516 65 515 514 513 16 511 512

Micropterus sp.-female

Upper RG Mid-RG/Pecos Lower RG 

Station 

Figure 13. Condition factor (CF) of male and female carp and bass 

(Micropterus sp.), by station. Shown for each group are points 

representing individual fish and the mean (red horizontal line), 

median (black horizontal line), interquartile range (box), and the 

10th and 90th percentiles (whiskers). Stations are ordered from 

upstream to downstream and are grouped by sub-basin. See 

Table 1 for station descriptions and collection dates. 

%

%

2.5 Micropterus sp.-male 

2.0 

1.5 

1.0 

0.5 

0.0 

2.5 

2.0 

1.5 

1.0 

0.5 

0.0 

64 63 516 65 515 514 513 16 511 512 

Upper RG Lower RG 

Micropterus sp.-female 

Mid-RG/Pecos 

Station 

Figure 14. Hepatosomatic index (HSI) values of male and female 

bass (Micropterus sp.), by station. Shown for each group are 

points representing individual fish and the mean (red horizontal 

line), median (black horizontal line), interquartile range (box), and 

the 10th and 90th percentiles (whiskers). Stations are ordered 

from upstream to downstream and are grouped by sub-basin. See 

Table 1 for station descriptions and collection dates. 

in Appendix Table 5. SSI in bass ranged from <0.1% (many 

fish at Stations 16, 512, and 513) to 0.8% (male largemouth 

bass from Station 16; Fig. 15; Appendix Table 5). The basinwide mean for SSI in bass was 0.11%, with station means 

ranging from 0.07 % at Station 512 to 0.22% at Station 514 

(Fig. 15). All but four values were between 0.04 and 0.22% 

(Fig. 15); fish with unusually large spleens (>0.35%) were 

obtained from Stations 16 (n = 1), 513 (n = 1), and 514 (n = 

2) (Fig. 15). Only at Station 514 were SSI values consistently 

high, however; this station had the highest median and the 

only mean >0.12% (Fig. 15). 

Overall, the range of SSI values in bass and carp from the 

RGB was similar to those observed in fish collected in 1995 

from the MRB (Blazer and others, 2002), and the overall pattern was similar. That is, differences among stations were not 

clearly evident, but a few fish from a few sites had unusually 

large spleens relative to most others. 
Results and Discussion 63 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

1.2 

1.4 

1.6 Carp-male 

%

1.0 

Carp-female 

% % 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

64 63 516 65 515 513 16 511 512 

sp. 

0.0 

0.2 

0.4 

0.6 

0.8 

Station 

514 

Micropterus

Upper RG Mid-RG/Pecos Lower RG 

Cellular and Histopathological Indicators 

Splenic macrophage aggregates, as described and 

illustrated by Blazer and Dethloff (2000), were quantified 

and measured and are described in the following paragraphs. 

Gonadal tissue was examined microscopically. A high incidence of ovotestis in male bass from several sites, and ovarian 

pathologies including parasites and necrosis in carp and bass 

from others were observed. Ovotestis was described and illustrated by McDonald and others (2000). 

Macrophage Aggregates 

Three MA parameters [density or number of aggregates per mm2 (MAMM), mean size of aggregates (in μm2

; 

MEANAREA), and percent of tissue occupied (TISSOC) by 

macrophage aggregates] were quantified in 199 carp and 75 

bass. Statistical analysis indicated that different factors were 

significant for the three parameters (Table 17). In addition, 

data for MEANAREA and TISSOC required log-transformation prior to analysis due to heterogeneous variances. Results 

are nevertheless presented in the same format (combined genders for bass, separate for carp) for the three parameters. The 

MA parameters are summarized by station, gender, and age in 

Appendix Table 5. Station means adjusted to the basin-wide 

mean age of carp (3.2 y) and bass (1.8 y) are shown in Table 

21. 

MAMM 

Preliminary ANOVA indicated that MAMM differed 

significantly among stations in carp, but no other factors or 

interactions were significant; however, several other factors 

approached significance, and the preliminary model explained 

52% of the total variation (Table 17). The overall model was 

also significant for bass and accounted for 39% of total variation in MAMM (Table 17), and differences among stations 

approached statistical significance (P = 0.06). The evidence 

supporting an association with age was weak in both taxa, as 

also was the evidence supporting differences between genders 

(Table 17). 

The basin-wide mean for MAMM in male carp was 8.8 

MA/mm2

. Station means ranged from 4.0 MA/mm2 at Station 

513 to 13.6 MA/mm2 at Station 512 (Fig. 16). Most values 

(90%) were between 1.2 and 16.0 MA/mm2

, but no MAs were 

detected in two male carp from Station 514. Overall, MAMM 

in male carp tended to be greatest at Stations 65 and 512 and 

lowest at Stations 513 and 514, but there was considerable 

variation within the stations such that only Station 512 differed 

Figure 15. Splenosomatic index (SSI) values of male and female 

carp and bass (Micropterus sp.), by station. Shown for each 

group are points representing individual fish and the mean (red 

horizontal line), median (black horizontal line), interquartile range 

(box), and the 10th and 90th percentiles (whiskers). Stations are 

ordered from upstream to downstream and are grouped by subbasin. See Table 1 for station descriptions and collection dates. 

noticeably (higher) than the others (Fig. 16). In female carp 

the basin-wide mean for MAMM was 7.8 MA/mm2

. Station 

means ranged more widely in female than in male carp, from 

1.8 MA/mm2

 at Station 513 to 16.2 MA/mm2 at Station 512 

(Fig. 16), but the range of individual values was similar. No 

MAs were detected in five female carp, from Stations 16 (n = 

1), 513 (n = 2), and 514 (n = 2; Fig. 16). Overall, MAMM in 
64 Table 21. Age-adjusted station means for splenic macrophage aggregate (MA) parameters in carp and bass (Micropterus sp.; not collected at Stations 64, 65, 511, 515, or 516). Shown are arithmetic means for MA density (MAMM) and geometric means for mean MA area (MEANAREA) and percent tissue occupied (TISSOC) adjusted to the 

basin-wide mean age for each species (3.2 y for carp, 1.8 y for bass) using analysis-of-covariance (ANCOVA). Also shown are ANCOVA 

F-values and degrees-of-freedom 

(df) for the analyses (**P 

≤ 0.01; *0.01 < P 

≤ 0.05; ns, P > 0.05). Within each column means followed by the same letter are not significantly different (P > 0.05, Fisher’s 

protected LSD). See text for description of variables and statistical procedures and Table 1 for station locations and collection dates. 

Carp 

Station MAMM MEANAREA 

(No./mm2) (μm) 

Bass 

TISSOC MAMM MEANAREA TISSOC

(%) (No./mm2) (μm) (%) 

16 8.35 bcd 1756 ab 

63 8.97 bcd 4063 bcd 

64 9.15 cd 3431 bc 

65 10.09 cd 1911 ab 511 7.82 bcd 3258 bc 

512 14.51 e 6393 d 

513 2.95 a 1484 a 

514 3.46 a 1430 a 

515 12.29 cde 3945 cd 

516 6.54 bc 3989 bc 

ANCOVA 

1.11 bc 7.70 a 3685 a 2.67 a 

3.22 c 5.37 b 3364 a 1.66 abc 

2.99 c -- -- --

1.73 bc -- -- --

1.79 bc -- -- --

8.49 d 4.97 b 3146 a 1.44 bc 

0.30 a 4.91 b 2984 a 1.25 bc 

0.30 a 4.31 b 2477 a 0.89 bc 

4.73 cd -- -- --

2.46 bc -- -- --

d d Model 9.95** a 9.18** a 

Station 10.38** b 7.70** b Age 2.18ns c 15.41** c 

14.50** a 5.73** d 4.03** 5.23**

13.19** b 5.65**e 1.10ns e 3.16* e 

18.01** c 1.68ns f 9.99** f 6.64* f 

a df = 10, 166; b df = 9, 166; c df = 1, 166; d df = 5, 68; e df = 4, 68; f df = 1, 68 

tion differences were more evident. The overall pattern was 

the same; however, MAMM was generally lowest at Stations 

513 and 514 and greatest at Station 512 (Fig. 16). Because the 

effect of age on MAMM was not significant, the age-adjusted 

station means (sexes combined) reflected the overall pattern of 

the unadjusted means for males and females. That is, stations 

from which carp contained the greatest MA densities (512 

and 515) differed significantly from those with the smallest 

(513, 514), with considerable overlap among the other stations 

(Table 21). Overall, the range of MAMM values and station 

means in RGB carp was about the same as what was reported 

in carp collected from the MRB in 1995 (Blazer and others, 

2002). 

The basin-wide mean for MAMM in bass was 5.7 MA/ 

mm2

. Station means were lowest at Station 514 (4.3 MA/ 

mm2) and highest at Station 16 (7.8 MA/mm2; Fig. 16), but the 

other three station means were similar (4.8–5.7 MA/mm2

), and 

90% of MAMM values were between 2.4 and 9.4 MA/mm2. 

MAs were present in all bass, but all MAMM values were <12 

MA/mm2. Eight of the 12 highest values were at Station 16 

(Fig. 16). As was true for carp, the effect of age on MAMM 

in bass was not significant (Table 21) and the age-adjusted station means for bass are essentially the same as the unadjusted 

values (Fig. 16). MA density in bass from Station 16 was 

significantly greater than at all other stations from which bass 

were collected (Table 21). In addition, and in contrast to carp, 

the range of MAMM values in bass from the RGB was smaller 

than what was reported for MRB bass collected in 1995, 

where individual bass with MAMM values >12 MA/mm2 were 

common and two station means exceeded 10 MA/mm2 (Blazer 

and others, 2002). 

MEANAREA 

In contrast to MAMM, preliminary statistical analysis 

indicated that age was weakly associated (P = 0.09) with 

log-transformed MEANAREA in carp, but not in bass (Table 

17). ANOVA models that included terms for station, sex, 

age, station, and the interactions of these factors explained 

57% of total MEANAREA variation in carp, but only 39% in 

bass (Table 17). Further analysis indicated that in bass, a 1-y 

increase in age was associated with a multiplicative change of 

1.17 (16%) in median MEANAREA. MEANAREA in carp 

differed significantly among stations after accounting for age 

whereas in bass only age was significant (Table 21). 

Except for the two fish from Station 514 in which no 

MAs were observed (see MAMM), individual MEANAREA 

values in male carp ranged from 351 μm2

 at Station 65 to 

14,709 μm2 at Station 512 (Fig. 17). Ninety percent of the 

male carp examined had MEANAREA values of 600-8,500 

μm2, but only four (one from Station 63 and four from Station 512) exceeded 8,000 μm2

. MEANAREA was <6,000 

μm2 in all male carp from Stations 16, 65, 513, and 514 (Fig. 

17). The basin-wide mean for MEANAREA in male carp 

Environmental Contaminants and their Effects on Fish 

female carp was less variable than in males, and among-sta-
10 

15 

10 

0 

8 

14 

Results and Discussion 65 

30 Carp-male 

sp. 

Carp-female 

Upper RG Mid-RG/Pecos Lower RG 

Micropterus

16000

μ

m2 μ

m2 μ

m2 

Carp-male 

sp. 

Carp-female 

Upper RG Mid-RG/Pecos Lower RG 

Micropterus

14000 25

12000

20

10000

8000

6000

2 2 2 #/mm #/mm #/mm 

4000

5

2000

0 

0 

25 14000

12000

20

10000

15

8000

6000

4000

5

2000

0 

0

14000

12000 12

10000

10

8000

6000

6

4000

4

2000

2 0 

64 63 516 65 515 514 513 16 511 512

Station 

Figure 16. Splenic macrophage aggregate density (MAMM) in 

male and female carp and in bass (Micropterus sp.), by station. 

Shown for each group are points representing individual fish 

and the mean (red horizontal line), median (black horizontal line), 

interquartile range (box), and the 10th and 90th percentiles (whiskers). Stations are ordered from upstream to downstream and are 

grouped by sub-basin. See Table 1 for station descriptions and 

collection dates 

was 3,319 μm2, with station means ranging from 1,453 μm2


at Station 514 to 7,349 μm2

 at Station 512 (Fig. 17). Overall, 


MEANAREA in male carp tended to be greatest at Station 512 


and lowest at Stations 65 and 514. In female carp the range of 


MEANAREA values was similar to that of males. Except for 


the five fish from Stations 16, 513, and 514 in which no MAs 


64 63 516 65 515 514 513 16 511 512

Station 

Figure 17. Splenic macrophage aggregate area (MEANAREA) 

in male and female carp and in bass (Micropterus sp.), by station. Shown for each group are points representing individual 

fish and the mean (red horizontal line), median (black horizontal 

line), interquartile range (box), and the 10th and 90th percentiles 

(whiskers). Stations are ordered from upstream to downstream 

and are grouped by sub-basin. See Table 1 for station descriptions 

and collection dates. 

were observed (see MAMM), individual MEANAREA values 


ranged from 462 μm2

 at Station 513 to 12,972 μm2 at Station 


514, and most of the values were between 600 and 8,000 μm2


(Fig. 17). More females than males had large values, however; 


MEANAREA in individual female carp from Stations 63 (n 


= 2), 64 (n = 1), 511 (n = 1), 512 (n = 3), 514 (n = 2) and 516 

66 Environmental Contaminants and their Effects on Fish 

(n = 1) were >8,000 μm2 (Fig. 17). The basin-wide mean for 

MEANAREA in female carp was also larger (4,119 μm2), with 

station means ranging from 1246 μm2 at Station 64 to 8,300 

μm2 at Station 512 (Fig. 17). Overall, MEANAREA values 

tended to be smallest at Station 16 and 513 and greatest at Station 512 (Fig. 17). 

The ANOVA model indicated that age was significant for 

MEANAREA in carp (Table 17), and age-adjusted values were 

computed (Table 21). Age-adjusted geometric station means 

for carp (sexes combined) ranged from 1,430 μm2

 at Station 

514 to 6,393 μm2 at Station 512 and differed significantly 

(Table 21). As noted for MAMM, age-adjusted MEANAREA 

in carp was greatest at Station 512 and smallest at Stations 513 

and 514, but the ordering of the intermediate stations differed. 

Differences among intermediate stations were not statistically 

significant, however (Table 21). In addition, and in contrast 

to MAMM, the range of MEANAREA values in carp was 

slightly greater in the RGB than what was reported for the 

MRB in 1995, but the station means were about the same 

(Blazer and others, 2002). 

In bass, individual MEANAREA values ranged from 

699 μm2

 at Station 513 to 12,262 μm2 at Station 512 (Fig. 

17). Ninety percent of the bass examined had MEANAREA 

values of 1,403-7,747 μm2. MEANAREA was >6,000 μm2 

in nine bass representing all five stations, and three of these 

values were >8,000 μm2

 (Fig. 17). Age-adjusted geometric 

station means (sexes combined) were between 2,477 and 3,685 

μm2 for all stations and did not differ significantly (Table 21). 

Except for the one large value at Station 512, the distribution of MEANAREA in RGB bass was similar to what was 

reported for MRB bass collected in 1995 (Blazer and others, 

2002). 

TISOCC 

Preliminary ANOVA models that included terms for 

station, gender, age, and the interactions of these factors were 

statistically significant and explained 61% of the total variation in log-transformed TISSOC in carp and 40% in bass 

(Table 17). After accounting for all other factors in the model, 

this analysis indicated that differences among stations were 

significant in carp, and that the effect of age was marginally 

significant (P = 0.09). In contrast, neither factor was significant in bass despite the fact that the overall model was significant (Table 17). Further analysis with the genders combined 

indicated a positive association between age and TISOCC in 

both taxa, however, and differences among stations were statistically significant after accounting for age (Table 21). A 1-y 

increase in age was associated with a multiplicative change of 

1.23 (23%) in median TISOCC. In bass, a 1-y increase in age 

was associated with a multiplicative change of 1.25 (24%) in 

median TISOCC. 

In male carp, with the exception of the two fish from 

Station 514 in which no MAs were observed (see MAMM), 

individual TISSOC values ranged from 0.04% at Station 514 

%

%

%

20 Carp-male 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

25 

20 

15 

10 

5 

0 

12 

10 

8 

6 

4 

2 

0 

sp. 

Carp-female 

Upper RG Mid-RG/Pecos Lower RG 

Micropterus

64 63 516 65 515 514 513 16 511 512 

Station 

Figure 18. Percentage of splenic tissue occupied by macrophage aggregates (TISSOC) in male and female carp and in bass 

(Micropterus sp.), by station. Shown for each group are points 

representing individual fish and the mean (red horizontal line), 

median (black horizontal line), interquartile range (box), and the 

10th and 90th percentiles (whiskers). Stations are ordered from 

upstream to downstream and are grouped by sub-basin. See 

Table 1 for station descriptions and collection dates. 

to 18.2 % at Station 512 (Fig. 18). Most values (90%) were 

between 0.06 and 9.5%. Seven male carp had TISOCC values 

>10%; of these, all but one were from Station 512 (Fig. 18). 

In contrast, TISSOC was <4% in all male carp from Station 

513 (Fig. 18, TISSOC). The basin-wide mean for TISSOC in 
67 

male carp was 3.2%, with station means ranging from 0.9% 

at Station 514 to 9.7% at Station 512 (Fig. 18). Overall, the 

pattern was similar to the other MA parameters in male carp, 

tending to be lowest at Stations 513 and 514 and greatest at 

Station 512 (Fig. 18). TISSOC in female carp were similar 

to that in males, but were less variable. Except for the five 

fish from Stations 16, 513, and 514 in which no MAs were 

observed (see MAMM), individual TISSOC values in female 

carp ranged from <0.1% at Station 513 to 20.0% at Station 

512 (Fig. 18). As was also true for the males, most values 

were between 0.6 and 9.5 %; however, only fish from Station 512 exceeded 10% (Fig. 18). The basin-wide mean for 

TISSOC in female carp was 3.5%, station means ranged from 

0.7% at Station 513 to 13.3% at Station 512, and the overall 

pattern was similar to that observed in males (Fig. 18). 

Because the ANOVA model indicated that age was 

also significant for TISSOC in carp (Table 17), age-adjusted 

geometric station means were computed (Table 21). Ageadjusted TISOCC means (genders combined) for carp ranged 

from <1% at Stations 513 and 514 to 8.49% at Station 512 

(Table 21). As was true for the other MA parameters, the largest station means differed significantly from the smallest, but 

there was considerable overlap among the intermediate values 

(Table 21). Most TISSOC values in RGB carp were similar to 

those reported for MRB carp (Blazer and others, 2002) with 

the notable exception of Station 512, where TISSOC in carp 

(raw data and age-adjusted) was the greatest observed in either 

basin. 

Individual TISOCC values in bass ranged from 0.08% 

at Station 513 to 10.1% at Station 512 (Fig. 18). Most values 

(90%) were between 0.25 and 5.5%. TISOCC was >5% 

in a total of eight bass from three stations (16, 63, 512). In 

contrast, all but two values from Stations 512, 513, and 514 

were <3.6%. Except for one fish with a large value (noted 

previously), and in contrast to carp, TISSOC values were 

generally low in bass from Station 512 (Fig. 18). The basinwide mean for TISSOC in bass was 2.3%, with station means 

ranging from 1.5% at Station 514 to 3.4% at Station 16 (Fig. 

18). Age-adjusted mean TISOCC (sexes combined) was 0.89-

1.66% at Stations 63, 512, 513, and 514 but was 2.67% at Station 16; the latter differed significantly from all others (Table 

21). Overall, and also in contrast to results for carp, TISSOC 

in bass was within the range of values reported for bass from 

the MRB (Blazer and others, 2002). 

Fish Health Indicators: Summary 

A high incidence of confirmed tumors or other grossly 

visible indications that fish were exposed to high concentrations of toxic chemicals were not observed at any RGB sites. 

Similarly, histopathological analyses revealed chemicallyinduced microscopic alterations in only a few fish. Of the 

quantitative fish health biomarkers, CF, HSI, and HAI have 

Results and Discussion 

been the most widely used and discussed in the literature. 

CFs of 1.0-2.0 for carp and bass, the range of most values in 

the RGB, are typical for these taxa (Blazer and others, 2002; 

Carlander, 1977; 1969). Liver enlargement has been reported 

in largemouth bass from contaminated sites (Adams and 

McLean, 1985) as well as in other fishes exposed to contaminants in both field and laboratory studies (see review by 

Dethloff and Schmitt, 2000). Most male and female bass collected from the MRB in 1995 had liver weights representing 

0.5-1.5% of body weight, but enlarged livers were detected in 

bass from a few contaminated sites (Blazer and others, 2002). 

According to Gingerich (1982), the liver constitutes 1-2% of 

body weight in most fishes. Most RGB bass had proportionally smaller livers, as did most bass collected from the MRB in 

1995 (Blazer and others, 2002). 

Background information on external lesions, MA parameters, and SSI are either not completely relevant because studies 

conducted to date investigated only marine or estuarine fish or 

because information is not available. For external abnormalities, difficulties in comparing results among studies arise from 

probable systematic error caused by increasing familiarity of 

field personnel as the study progresses (Leonard and Orth, 

1986) and from differences in the anomalies characterized 

and recorded (Fournie and others, 1996; Karr, 1981; Sanders 

and others, 1999). We used criteria modified only slightly 

from the 1995 MRB investigation (Blazer and others, 2002), 

so the results of these surveys are comparable. In general, we 

noted sites at which ≥50% of carp, bass, or all fish had lesions, 

particularly if both carp and bass exceeded 50%. For MA 

parameters a USEPA study conducted in the Gulf of Mexico 

established a value of >40 splenic MAs/mm2

 in at least one 

fish as a threshold for possible effects of hypoxia or sediment 

contamination (Fournie and others, 2001). Although Blazer 

and others (2002) used it as a benchmark for carp and bass in 

the MRB, this value was derived for estuarine fishes and it is 

important to note that additional research on freshwater fishes, 

particularly bass and carp, is necessary to determine a threshold for possible effects. Regardless, it is also important to note 

that using only MA numbers does not take into consideration 

that there can be a few very large aggregates. 

In the RGB, mean CF and SSI in carp were greatest at 

Stations 513 and 514, and individual carp with relatively high 

CFs were captured Stations 16, 64, 511, and 514 (Figs. 13 and 

15). Spleen size was also relatively large in individual carp 

from Station 16, 64, 514, and 516 (Fig. 15). In bass, mean 

SSI was also relatively high at Station 514, and individual fish 

with relatively large spleens were captured from Stations 16, 

513 and 514 (Figs. 14 and 15). Enlarged spleen size is often 

associated with infections. In contrast, both carp and bass 

from Station 512 had relatively small spleens (Fig. 15), a condition that has been associated with exposure to contaminants 

(Blazer and others, 2002). At no stations were HSI values in 

bass indicative of liver enlargement associated with chronic 

exposure to chemical contaminants [See review by (Blazer and 

Dethloff, 2000) and subsequent studies by Sepúlveda and others (2001; 2003)]. This finding is in contrast to the enlarged 
68 Environmental Contaminants and their Effects on Fish 

livers in bass reported for contaminated MRB sites sampled in 

1995 (Blazer and others, 2002). 

The HAI had not been used with carp prior to the 1995 

study (Blazer and others, 2002); however, it was used to assess 

largemouth bass populations in Tennessee Valley Authority reservoirs (Adams and others, 1993) and in the Catawba 

River system (Coughlan and others, 1996). In these studies, 

a positive linear relation between average fish weight or age 

and HAI score was noted, and Coughlan and others (1996) 

suggested that only bass of 250-450 mm (TL) be included in 

comparisons. Approximately 90% of the bass collected in the 

RGB met these criteria. Based on previous studies (Adams 

and others, 1993; Coughlan and others, 1996) and conservative 

precedent (Blazer and others, 2002), we assumed that mean 

HAI values ≤20 were indicative of un-impacted or minimally 

impacted sites, whereas values >50 indicated intermediate 

sites and those >70 indicated a heavily impacted area. These 

values were used to characterize both carp and bass from the 

MRB as well as those from the RGB. Carp and bass from 

many MRB sites sampled in 1995 averaged >70, and most station means were >50 (Blazer and others, 2002). In the RGB, 

HAI scores were similar; the mean for carp from Station 515 

was 70, and the means for Stations 511, 513, and 516 were 

>50 (Fig. 12). The mean for bass from Station 63 was also 70, 

and those from Stations 16, 513, and 514 were >50 (Fig. 12). 

It is also important to note the “causes” of abnormal HAI 

ratings, as determined by histopathological analysis. In bass, 

abnormal ratings in fish from Station 63 were primarily due 

to frayed or marginate gills and abnormal livers. The latter 

were all due to the presence of helminth parasites. At Station 

16, most bass gills were normal and the bulk of the abnormal 

ratings resulted from internal nodules, focal discolorations, 

or spots on the liver, kidney, and spleen. Histologically it 

was determined these were parasitic lesions. At Station 513 

many of the bass kidneys were rated in the field as urolithic. 

However, histologically no calcifications were noted and these 

lesions were also determined to be parasite-induced. Overall, 

most microscopic lesions in bass were diagnosed as inflammatory and often occurred in association with parasites. A 

high HAI rating for carp from Station 515 was primarily due 

to external lesions; the only abnormal internal ratings were for 

tan livers. At Stations 511, 513, and 516 many of the abnormal ratings were also due to tan livers. Histologically, the tan 

livers were found to contain hepatocytes with increased vacuolization, and ceroid/lipofuscin deposits were often contained 

within or replacing hepatocytes (Fig. 19). In addition, in many 

of the carp from these stations ceroid-containing cells were 

present within blood vessel walls (Fig. 19). These lesions 

could represent oxidative damage resulting from contaminant 

exposure. Four of 14 carp from Station 512 had abnormal 

kidneys rated as granular. In these four fish (and in one not 

identified as granular in the field), and in contrast to the bass 

described previously, urolithiasis or nephrocalcinosis (Fig. 19) 

was noted histologically in five of 14 carp. 

No fish from the RGB contained >40 splenic MAs/mm2 

(MAMM), the criterion suggested by Fournie and others 

(1996) as indicative of exposure of marine and estuarine fish 

to sediment contaminants or hypoxia. Moreover, all carp 

except one from Station 512 contained <20 MA/mm2

 and all 

bass had <12 MA/mm2 (Fig. 12). These relatively low values 

were generally about the same as what was reported for carp 

and bass collected from the MRB in 1995 (Blazer and others, 2002). The distributions of the MA parameters in carp 

(age-adjusted as necessary) from all RGB stations sampled 

overlapped, but all three differed significantly among stations 

even after accounting for age effects (Table 17). Although 

low relative to the 40 MA/mm2 criterion of Fournie and others (1996), values of MAMM and the other MA parameters 

tended to be lowest at Stations 513 and 514 and greatest at 

Station 512 (Figs. 16-18; Table 21). In addition, the TISSOC 

values for carp from Station 512 exceeded those for any other 

station sampled in either the RGB or the MRB (Blazer and 

others, 2002). As a group, differences among stations were 

less evident for bass (Table 17); nevertheless, values of all MA 

parameters in bass were also relatively low at Station 514 and 

high at Station 16. Although MA values in bass from Station 512 were generally low, the greatest individual MEANAREA and TISSOC values for bass also occurred at Station 

512 (Figs. 17 and 18). Increased numbers of MAs have been 

associated with exposure of fish to crude oil and to As [see 

review by (Blazer and Dethloff, 2000)]. In contrast to our 

findings, Mora and others (2001) reported that fish of several 

species obtained from resacas along the lower RG contained 

high numbers of MAs. It is important to note that their sample 

numbers were small and their fish were not aged, which makes 

direct comparisons with the 1997 data problematic. 

In addition to the anomalies and parameters that we quantified, we attempted to identify other lesions as they became 

apparent during the histopathological examination of the 

various tissues. Among these was an apparent gradient in the 

proliferation of thyroid follicles within the kidney of carp that 

was not evident during gross examination. Fish from Stations 

63, 64 and 516 contained very few thyroid follicles and those 

that were present tended to be small (Fig. 20). Greater numbers and larger size characterized the follicles in the kidneys 

of carp from Stations 65, 514 and 515 (Fig. 20). The kidneys 

of on carp from Station 514 contained a large area of abnormal-appearing thyroid follicles (Fig. 20). A greater number 

of even larger follicles were evident in carp from Stations 

513, 512, 511 and 16 (Fig. 20). Similar observations were 

made for carp from the lower MRB (Blazer and others, 2002). 

As noted for the MRB, thyroid hyperplasia in fish and other 

organisms has been caused by exposure to contaminants and 

may represent a potential biomarker for contaminant effects 

on thyroid function. The effects of confounding factors such 

as area of kidney sampled, fish age, and so forth have not been 

evaluated, however. 
Results and Discussion 69 

Figure 19. Microscopic pathology observed in carp from the Rio Grande Basin. A. Pigmented cell accumulations within the hepatic 

blood vessel walls (arrows) typical of carp from Stations 515, 511, 513 and 516. H & E stain (X 165). B. Higher magnification (X 412) of A. 

C. The pigmented cells contained Perl’s positive material, hemosiderin, which stained blue; and ceroid/lipofuscin pigment, which stained 

yellowish-brown (arrow). Perl’s stain (X 824). D. Nephrocalcinosis or mineral deposition within tubules (arrow) was observed at Station 

512. H & E stain (X 412). 

Reproductive Biomarkers	 where they were collected (Fig. 21); at Stations 511, 512, and 

513 they were less advanced (mostly stages 0 and 1) than at 

the other sites (mostly stages 1 and 2). These differences are 

probably related to sampling dates; although collections of fish Gonadal Histopathology	 from some stations occurred on multiple dates separated by 

several weeks, fish were collected at Station 511 in late September-early October 1997, 4-8 weeks before the other lower Gonadal Stage	 RGB sites (Table 1). Similarly, the stage-1 female carp from 

Ovary samples representing 113 female carp from all Station 512 were collected on the first sampling visit, in late 

10 stations were examined. Only gonadal stages 0 through October, whereas those collected in late November were all in 

2 were present (Figs. 21 and 22). Stage 2 was predominant stage 2. Conversely, female carp from Station 65, which were 

(81.5% overall) and represented 64-100% of the female carp not collected until late January 1998, were in the same stage 

examined at most sites (Fig. 21). Only 17 fish (15%) were (all stage 3) as those from nearby stations collected months 

in stage 1; these represented 33-62% of the females at Sta- earlier (Fig. 22). Overall, the female carp from the RGB were 

tions 511, 512, and 513 (Fig. 21). Four female carp (3.5%), generally in less advanced gonadal stages than those collected 

all from Station 511, were in stage 0. The distribution of from the MRB in late 1995; in the MRB, 83% of the female 

stages in female carp differed slightly among the ten stations carp were in stage 3, and stage-3 females were predominant 
70 Environmental Contaminants and their Effects on Fish 

Figure 20. Thyroid follicles within the kidneys of carp from the Rio Grande Basin. A. Kidney tissue of a carp from Station 64 illustrating 

solitary (arrow) to small clusters of thyroid follicles commonly found in carp from the upper Rio Grande. H & E stain (X 165). B. Kidney 

tissue of a carp from Station 514 illustrating larger accumulations (arrow), often of eight or more follicles, observed in the middle Rio 

Grande. H & E stain (X 165). C. Kidney tissue of a carp from Station 513 illustrating accumulations of enlarged thyroid follicles (a) often 

observed in carp from the lower Rio Grande. H & E stain (X 165). D. A nodule of abnormal thyroid tissue (arrow) in a carp from Station 

515. H & E stain (X 412). 

at many sites, especially in the southern parts of the basin 

(McDonald and others, 2002). One 3-y old female carp from 

Station 64 could not be staged because no normal gonad tissue was observed; all follicles showed signs of degeneration, 

inflammation, and fibrosis (Fig. 23). This condition was not 

observed in other fish from Station 64, elsewhere in the RGB, 

or in previous studies with carp (Goodbred and others, 1997; 

Lee and others, 2000; McDonald and others, 2002). 

Testes samples representing 92 male carp from all 10 

stations were examined. Gonadal stages 0 through 3 were 

represented (Figs. 21 and 24). Most male carp were in more 

advanced gonadal stages than females; overall, 82 (89%) were 

in stage 3. At all stations except Station 511, 75-100% of the 

male carp examined were in stage 3, and stage frequencies 

did not differ appreciably among these stations (Fig. 21). As 

noted for females, most male carp from Station 511 were also 

in less advanced gonadal stages (predominantly stage 2) than 

at the other stations (Fig. 21), which is consistent with the earlier collection date (Table 1). Stage-2 males (few) were also 

obtained at Stations 16, 64, and 512, and one stage-1 male was 

collected at Station 16 (Fig. 21). The stage-2 male carp from 

Station 512 was collected in late October and was the only one 

obtained. All other male carp from Station 512, which were 

universally in stage 3, were collected in late November. As 

was true for female carp, male carp from Station 65, which 

were not collected until late January 1998, were in the same 

stage (all stage 2) as those from nearby stations that were 

sampled in late 1997 (Fig. 21). In contrast to female carp from 

the RGB, which were generally in earlier gonadal stages (1 

and 2) than those collected from the MRB in 1995 (McDonald 

and others, 2002), male carp from all RGB sites except Station 

511 were slightly more advanced (stage 3) than those from the 
Results and Discussion 71 

% 

100

90

80

70

60

50

40

30

20

10

0

64 63 516 65 515 513 16 511 512 

F(22)M(16) F(15)M(10) F(10) M(8) F(10)M(10) F(6) M(4) F(10) F(11) M(9) F(10) M(10) F(13) M(7) F(6) M(8) 

Stage 3 

Stage 2 

Stage 1 

Stage 0 

514 

Upper RG Mid-RG/Pecos Lower RG 

M(10) 

Station 

M(8) 0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

64 63 516 65 515 514 513 16 511 512 

Upper RG 

% 

F(6) F(5) M(3) F(13) M(9) F(11) M(10) F(7) 

Mid-RG/Pecos Lower RG 

Stage 4 

Stage 3 

Stage 2 

Stage 1 

Stage 0 

Station 

Figure 21. Gonadal stage of male and female carp (upper panel) and bass (lower panel), by station. Shown are the percentages (y-axis) 

and total numbers (in parentheses) of fish in each stage. Stations are ordered from upstream to downstream and are grouped by subbasin. See Table 1 for station descriptions and collection dates. 
72 Environmental Contaminants and their Effects on Fish 

Figure 22. Gonadal stages in female carp and bass ovaries. A. Stage-0 (immature) ovary containing only previtellogenic oocytes. B. 

Stage-1 (early vitellogenic) ovary containing cortical alveolar oocytes (a) as well as previtellogenic stages. C. Stage-2 (mid-vitellogenic) 

ovary containing cortical alveolar (a) and more advanced oocytes (b) in which the yolk vacuoles are being pushed to the periphery and 

yolk globules are filling the central portion of the oocyte. D. Stage 3 (late vitellogenic) ovary in which most oocytes are enlarged, a thin 

layer of yolk vacuoles lines the periphery, yolk globules fill the cytoplasm, and the chorion (arrow) is thickened. E. Stage-4 (mature) 

ovary containing oocytes in which the yolk globules appear condensed. F. Stage-5 (spent) ovary (not encountered in fish from the Rio 

Grande Basin) containing post-ovulatory follicles (shown at the ends of the arrow). H & E stain (X 165). 
73 

Figure 23. Ovarian section of a carp from Station 64. H & E stain. 

A. Lower power (X 40) illustrating a lack of normal ovarian tissue. 

Follicles (a) are degenerate with remnants of yolk. B. At higher 

magnification (X 100) areas of calcification (arrow) and macrophage aggregates (b) are visible. 

MRB, where 86% were in stages 1 and 2. 

Ovary samples representing 42 female bass from five 

stations (16, 63, 512, 513, and 514) were examined. Of these, 

all were largemouth bass except for two smallmouth from 

Station 63. Gonadal stages 0-4 were represented (Figs. 21 and 

22). Overall, 32 (76%) were in stage 1, four (10%) were in 

stage 2, and two (5%) were in each of stages 0, 3, and 4. Most 

(60-92%) of the female bass at all stations sampled were in 

stage 1 (Fig. 21). Stage-0 (immature) female bass were collected only at Station 512; one was obtained in late October, 

the other when the site was re-sampled in late November. 

Stage-2 female bass were obtained from Stations 16 and 513, 

and stage-3 females were found only at Station 63 (Fig. 21); 

the latter were smallmouth bass. Two female bass collected 

at Station 514 in early November (Table 1) were in stage 4 

(Fig. 21). The occurrence of stage-4 female largemouth bass 

in South Texas at this time of the year is unusual; in the fall 

of 1995, stage-4 female bass (all smallmouth) were found 

Results and Discussion 

exclusively at two sites in the northernmost parts of the MRB 

(McDonald and others, 2002). The two fish from Station 514 

were subsequently found to be harboring ovarian parasites 

(see atresia discussion in next section). In all other respects, 

the distribution of female bass among reproductive stages is 

similar to what was reported for female largemouth bass from 

stations in the southern parts of the MRB. 

Testes samples representing 30 male bass from four stations (16, 512, 513, and 514) were examined; no male gonad 

samples were obtained at Station 63. All male bass examined 

were largemouth except for one smallmouth obtained from 

Station 514. Gonadal stages 1-3 were represented (Figs. 21 

and 24). As was true for male carp, male bass were generally 

in more advanced gonadal stages than females. Overall, 20 

(67%) were in stage 2, nine (30%) were in stage 1, and one 

(3%) was in stage 3 (Fig. 21). As was true for female bass, the 

stage-3 male was the smallmouth. Stage 2 male largemouth 

bass were obtained at Stations 16, 513, and 514 (Fig. 21). All 

male bass from Station 512 were in stage 1 as was one from 

Station 16 (Fig. 21). Overall, these results are also similar to 

what was reported for male largemouth bass from the MRB 

(McDonald and others, 2002). 

Atresia 

Statistical analysis indicated that neither age nor stage 

were significant contributors to atresia differences RGB carp 

or bass (Table 17). After accounting for these factors, differences among stations were also not significant in either 

taxon (Table 17). A model that included terms for station, 

stage, age, and the interactions of these factors was nevertheless statistically significant overall and accounted for 49% of 

the variation in carp (Table 17). Although no factors were 

strongly significant, the effect of age approached statistical 

significance (P = 0.11) . In female bass the model was also 

statistically significant and accounted for 95% of the total 

variability. In addition, the interaction of station, age, and 

stage was significant, and one other term that included age 

approached significance (P = 0.11; Table 17). Collectively, 

these results indicate a slight increase in atresia with fish age 

and gonadal stage, and the data are presented accordingly. 

Atresia in Carp 

Overall, atresia in carp tended to be greatest at stations 

in the lower RGB than in the upper (Fig. 25), but there was 

considerable variability and the effects of age and stage were 

confounded (Table 17). As expected, small percentages of 

atretic eggs were observed in stage-0 female carp, which are 

immature. In four stage-0 fish from Station 511 the mean was 

3.0%; three had 0% and one had 12% (Appendix Table 6). For 

those in stages 1 and 2, the overall mean was 6.7% with values 

ranging from 0% to 25%, but most of these fish also had no 

or few (0-15%) atretic eggs (Appendix Table 6). Stage-1 and 

-2 fish from Station 65 contained relatively few atretic eggs 
74 Environmental Contaminants and their Effects on Fish 

Figure 24. Gonadal stages in male carp and bass testes. A. Stage-1 (early spermatogenic) testes containing all stages of spermatogenesis; spermatogonia (arrows), spermatocytes (a), spermatids (b), and spermatozoa (c) may be observed. H & E stain (X 824). B. Stage-1 

testes in which spermatocytes and spermatids predominate. H & E stain (X 412). C. Stage-2 (mid-spermatogenic) testes containing 

approximately equal numbers of spermatocytes, spermatids, and spermatozoa. H & E stain (X 412). D. Stage-3 (late spermatogenic) 

testes containing primarily spermatozoa. H & E stain (X 412). 

(mean = 0.6%) whereas means for the other stations ranged 

from 4.0% at Station 64 to >10% at Station 512 (Fig. 25; 

Appendix Table 6). Individual female carp with >20% atretic 

eggs were obtained from Stations 16, 512, and 515 (Fig. 25). 

All three female carp collected at Station 512 on the first visit 

(late October, stages 1 and 2) had 15-25% atresia whereas 

the three obtained in late November (stages 2 and 3) had only 

3-6% (Appendix Table 6). The low incidence of atresia at 

Station 65, a number of higher values (>10%) at Station 511, 

and the two values >20% at Station 512 together with means 

exceeding 10% at these two stations were noteworthy; however, a large amount of overlap among stations occurred (Fig. 

25) and, as noted, differences among stations were not statistically significant (Table 17). In female carp collected from the 

MRB in 1995 the range of atresia was much greater than in 

the RGB; individual female fish with >30% atretic eggs were 

found at many stations (McDonald and others, 2002). In the 

Hudson River (New York), Baldigo and others (2001) reported 

that atresia in carp was generally greatest (up to 20%) at sites 

least contaminated by PCBs, and tended to be lowest (<10%) 

at the most contaminated sites. They did not identify the fish 

to stage, however. 

Atresia in Bass 

Atresia occurred at comparatively low rates in bass at 

all stations and was generally less variable than in carp. The 

exception was two stage-4 fish with unusually large values 

from Station 514 (Fig. 25). As noted for carp, stage-0 female 

bass (obtained exclusively at Station 512, n = 2; Fig. 21) 

contained small percentages (0-2%) of atretic eggs (Appendix Table 6). Atresia in female bass tended to increase with 
75 

%

%

30 Carp-female 

25 

20 

15 

10 

5 

0 

100 

80 

60 

40 

20 

0 

sp.-female 

Upper RG Mid-RG/Pecos Lower RG 

Micropterus

64 63 516 65 515 514 513 16 511 512 

Station 

Figure 25. Percentage of atretic oocytes in female carp and bass 

(Micropterus sp.), by station. Shown for each group are points 

representing individual fish and the mean (red horizontal line), 

median (black horizontal line), interquartile range (box), and the 

10th and 90th percentiles (whiskers). Stations are ordered from 

upstream to downstream and are grouped by sub-basin. See 

Table 1 for station descriptions and collection dates. 

gonadal stage; the basin-wide atresia mean for stage-1 was 

2.8% (data not shown), with individual fish ranging from 0 to 

18% (Appendix Table 6). Most stage-1 fish had 0-10% atretic 

eggs. Atresia values >10% contributed to elevated stage-1 

means at Stations 16 (5.3%) and 63 (6.5%) compared to the 

other three stations (0.4–3.0%; Appendix Table 6). Fish in 

later gonadal stages had slightly greater percentages; in four 

stage-2 females from Stations 16 and 513 it was 0-10% and 

it was 0-7% in the two stage-3 smallmouth bass from Station 

63 (Fig. 25; Appendix Table 6). In contrast to findings for 

female carp, atresia was universally low (0-2%) in female bass 

from Station 512 regardless of when the fish were collected. 

Atresia was 50-80% in the two stage-4 female bass from 

Station 514 (Fig. 25). As noted in the discussion of gonadal 

stage, the collection of stage-4 female bass in South Texas during November is in itself unusual. Further histopathological 

examination revealed heavy microsporidian parasite infections within the oocytes of these two fish (Fig. 26). Although 

the parasites did not appear to have penetrated the pre-vitellogenic oocytes, the more advance ones were affected; they 

Results and Discussion 

consequently appeared to have progressed to stage 4 but were 

not released. The oocytes observed were in various stages 

of degeneration and necrosis and had begun to be reabsorbed 

(Fig. 26). Except for these two fish, the incidence of atresia 

in RGB bass was similar to what was reported for the MRB 

(McDonald and others, 2002). Although the ovaries of the 

fish were examined histologically, atresia was not reported in 

the female largemouth bass exposed to paper mill effluents in 

Florida by Sepúlveda and others (2003; 2001). As they also 

reported for carp, Baldigo and others (2001) reported that 

atresia rates were greatest (up to 30%) in bass (largemouth and 

smallmouth) from the Hudson River sites least contaminated 

by PCBs, and were lower (10-20%) at more contaminated 

sites; however, these fish were also not identified to stage. As 

noted by McDonald and others (2000), atresia can be induced 

in fish by exposure to a number of contaminants; however, it is 

also important to note that other factors may also be involved 

(June, 1970; 1977). 

Ceroid Pigment 

Pigmented cell accumulations were present to varying 

degrees in the gonads of carp and bass from the RGB (Fig. 

27). Pigment scores ranged from 0 (no ceroid pigment) to 3 

(moderately large amount of pigment) and differed between 

genders (Appendix Table 6). Oxidation products can increase 

over time, so associations between age and ceroid pigment 

were also investigated. Preliminary statistical analysis indicated that pigment scores were correlated with age in female 

carp and bass but not in males of either species (data not 

shown). Except for female carp from Station 511, in which 

ceroid pigment scores tended to decrease with age, scores generally paralleled patterns in average age for both carp and bass 

(data not shown). Most fish of both taxa were scored as 0 or 1 

for pigment accumulation, a total of five received a rating of 3, 

and none received scores of 4 despite the collection of 7- and 

8-y old fish from several stations (Appendix Table 6). Collectively, the results indicate that the fish were under little oxidative stress. In contrast, fish of several species obtained from 

resacas along the lower RG by Mora and others (2001) in 

1996 contained high concentrations of ceroid/lipofuscin pigments; however, as noted with regard to MAs, these fish were 

not aged and it is consequently difficult to compare results of 

the two studies. The pigment data, because of its categorical 

nature and the confounding age factor, did not provide clear 

information with the limited numbers of fish collected in the 

RGB and the data were not investigated further. 

Ovotestis in Male Bass 

Intersex gonads were only observed in male bass, as 

was also true in the 1995 MRB study (McDonald and others, 

2002). Of the 30 male bass from four RGB stations examined 

(all largemouth except one smallmouth from Station 514), 10 

(all largemouth) showed evidence of ovotestis as identified 
76 Environmental Contaminants and their Effects on Fish 

Figure 26. Ovarian section of a largemouth bass from Station 514. 

A. Developing, vitellogenic oocytes in various stages of degeneration (a). Many ceroid/lipofuscin pigment accumulations (b) are 

also present. Previtellogenic oocytes (c) show no signs of infection. H & E stain (X 40). B. Higher magnification (X 200) illustrating 

the presence of numerous microsporidian parasites (arrows). 

by the presence of developing oocytes in an otherwise normal 

male gonad. This intersex condition was detected in two of 

10 fish from Station 16, four of eight from Station 512, and 

four of nine from Station 513. None of three male bass from 

Station 514 (two largemouth, one smallmouth) examined were 

intersex, however. No gonad samples were obtained from the 

Station 63 male bass. All of the intersex fish were identified 

as being in either gonadal stage 1 or 2. 

Overall, relatively high percentages (20-56%) of intersex male largemouth bass were found at stations in the lower 

RGB. In the MRB (McDonald and others, 2002), such high 

percentages were only observed in smallmouth bass from one 

site; the incidence in largemouth and spotted bass was lower 

(<20%). In the Hudson River, Baldigo and others (2001) 

reported high incidences of intersex largemouth and smallmouth bass, but no intersex males were reported among the 

largemouth bass exposed to paper mill effluents in Florida by 

Sepúlveda and others (2001; 2003). 

The distribution of intersex bass in the RGB cannot be 

determined from our data because relatively few male bass 

(or gonad samples) were obtained. At least one intersex male 

bass was present at all the lower RG sites from which bass 

were obtained, but none were detected at Station 514, an 

upstream site; however, the sample size was small (n = 3). In 

addition, and in keeping with previous U.S. reports (Folmar 

and others, 1996; Goodbred and others, 1997; Lee and others, 

2000; McDonald and others, 2002) no intersex male carp were 

found. No intersex fish were reported among the 18 fish (carp 

and other species) from resacas along the lower RG examined 

by Mora and others (2001). In contrast, in Europe intersex 

male carp and other cyprinids have been collected near sewage 

treatment plants (Jobling and others, 1998; Minier and others, 

2002; Solé and others, 2002; 2003). 

Gonadosomatic Index (GSI) 

ANOVA models containing terms for station, sex, stage, 

and the interactions of these effects were significant for both 

carp and bass (Table 17). In carp, the model explained 72% 

of the total variation in GSI; differences among stations, but 

not between genders, were significant after accounting for 

all other factors in the model (Table 17). In bass the model 

accounted for 70% of the total variation in GSI. Differences 

between genders were significant, and among-station differences were marginally significant (P = 0.06; Table 17). As 

expected, stage and interactions containing stage were significant in both taxa (Table 17). Differences between stage-2 and 

-3 male carp, the most prevalent stages, were not significant, 

but few female bass were in any stage other than stage 1 at all 

sites (Fig. 21; Appendix Table 6). 

GSI in Carp 

Generally, female carp from Stations 16, 513, and 514 

had proportionately larger ovaries than those from most other 

stations, but there was wide variation at the former (Fig. 28) 

that only partly corresponded to stage differences among the 

stations (Fig. 21). GSI in female carp ranged from 0.3% in 

a stage-0 fish from Station 511 to 36.4% in a stage-2 fish 

from Station 16 (Fig. 28; Appendix Table 6). All four stage-0 

female carp were from Station 511 (mean GSI = 0.9%, range 

= 0.3-1.3%; Appendix Table 6). As noted previously, 92% of 

the female carp examined were in gonadal stage 2 (Fig. 21). 

Basin-wide the mean GSI for stage-2 females was 14.8% with 

individual fish ranging from 3.3% to 36.4%. Most stage-2 

GSI values were between 6.2% and 22.0%, and stage-2 station 

means ranged from 5.2% at Station 515 to 19.5% at Station 

514 (Appendix Table 6). Other stations with stage-2 means 

<11.5% included 511, 512, 63, and 65; means for all other 
Results and Discussion 77 

Figure 27. Pigmented cell accumulations (arrows) in the gonads of carp from the Rio Grande Basin. The amount of these ceroid-lipofuscin pigments was rated on a scale of 0 (no pigment) to 4 (heavily pigmented). A. Ovary of female carp. H & E stain (X 165). B. Testes of 

male carp. H & E stain (X 412). 

stations were >15.0% (Appendix Table 6). At Stations 513, 

514, and 64, all but one or two stage-2 fish had GSI values that 

exceeded the basin-wide mean. Conversely, individual stage-2 

values >22.0% occurred at Stations 16, 64, and 514. Stage-1 

females were present at Stations 63, 511, 512, 513, and 516 

(Fig. 21). The basin-wide mean for stage-1 female carp was 

5.1% with a range of 0.5%-18.2% (Appendix Table 6). Station means ranged from 1.3% at Station 513 to 8.9% at Station 

512; one stage-1 female carp from Station 516 had a GSI of 

18.2%. Except for the maximum, all GSI values for stage-1 

female carp were <10.0%. 

The distribution of GSI in male carp generally paralleled 

that of female carp and, like the females, was partly related 

to stage differences (Figs. 21 and 28). The one stage-1 male 

carp from Station 16 had a GSI of only 0.2% (Appendix Table 

6). Most male carp were in stage 3 (Fig. 21). The basin-wide 

mean GSI for males in stages 2 and 3 was 8.7%, with individual values ranging from 2.2% to 20.3% (Fig. 28; Appendix 

Table 6). Most values were between 3.0% and 12.5%. Station 

means for stage-2 male carp ranged from 4.2% at Station 511 

to 11.8% at Station 514 (Appendix Table 6). Station means 

for Stations 16, 64, 513, 514, and 516 exceeded the basinwide mean as did that of most individual male carp from these 

stations (Fig. 28). Conversely, relatively low station means 

characterized male carp from Stations 65, 512, and 515, and 

all individual values for these stations were less than the basinwide mean (Fig. 28). Wainwright and others (2001) noted that 

male carp collected from one resaca sampled in July-August 

1997 had comparatively high GSI values (4.68-6.46%) relative 

to other resacas and settling basins, and that plasma androgen 

levels and DDE concentrations were also elevated in these 

fish. These values are similar to most of those we report for 

stage-1 and -2 male carp (Appendix Table 6). Although collected at least a month earlier than most of our fish, the carp 

examined by Wainwright and others (2001) were described as 

being in the “spermatid” gonadal stage, which is equivalent to 

our Stages 1 and 2. Therefore, the GSI values report by both 

studies are similar. 

GSI in Bass 

In female bass all GSI values were between 0.25% and 

1.2% except for three fish—one (of two) stage-3 smallmouth 

bass from Station 63 (1.8%) and one stage-2 largemouth bass 

from each of Stations 16 (1.5%) and 513 (1.5%) (Fig. 28; 

Appendix Table 6). Most GSI values were between 0.35% and 

1.0% (Fig. 28; Appendix Table 6). The basin-wide mean for 

GSI in stage-1 female bass was 0.68, with station means ranging from 0.59 at Station 513 to 0.83 at Station 514, (Appendix 

Table 6). At Station 514, all stage-1 fish exceeded the basinwide mean, but no values were especially high or low. Two 

stage-0 female bass from Station 512 averaged 0.47% (range 

0.43-0.49%); the two stage-3 smallmouth bass from Station 

63 averaged 1.28 (range 0.74-1.80); the mean GSI for three 

stage-2 females from Station 16 was 1.06 (range 0.72-1.46); 

and the GSI of one Stage-2 female from Station 513 was 1.49 

(Fig. 28; Appendix Table 6). The ovaries of the two stage-4 

females from Station 514 were not especially large; GSI averaged 0.64% in these two fish (range 0.61-0.68%; Appendix 

Table 6). These were the same fish identified previously as 

having high percentages of atretic eggs and heavy microsporidian infections within the oocytes (Fig. 26). Although in 

later gonadal stages than all other female bass from the RGB 

examined, the gonads of these two fish were proportionally 

smaller (lower GSI) than many fish in earlier gonadal stages. 

Overall, most GSI values in female bass from the RGB were 

lower than those observed in female largemouth bass exposed 
78 Environmental Contaminants and their Effects on Fish 

25 

20 

15 

10 

5 

0

40

30 

20 

10 

0 

1.0 

0.8 

0.6 

0.4 

0.2 

0 

2.0 

1.8 

1.6 

1.4 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

64 63 516 65 515 514 513 16 511 512 

Carp-male 

Carp-female 

sp.-male 

sp.-female 

Micropterus

Upper RG Mid-RG/Pecos Lower RG 

Micropterus

Station 

Figure 28. Gonadosomatic index (GSI) values of male and female 

carp and bass (Micropterus sp.), by station. Shown for each 

group are points representing individual fish and the mean (red 

horizontal line), median (black horizontal line), interquartile range 

(box), and the 10th and 90th percentiles (whiskers). Stations are 

ordered from upstream to downstream and are grouped by subbasin. See Table 1 for station descriptions and collection dates. % 

%

%

%

to paper mill effluents in Florida by Sepúlveda and others 

(2001; 2003), but as noted previously the Florida study was 

conducted in the Spring. 

GSI values in male bass were all between 0.1% and 0.4% 

except for two fish with unusually large gonads relative to all 

others irrespective of stage: a stage-2 largemouth bass from 

Station 513 (0.9%) and the stage-3 smallmouth bass from Station 514 (0.8%) (Fig. 28; Appendix Table 6). Station means 

and ranges for GSI in male bass did not differ greatly among 

stations, however. At Stations 16, 512, 513, and 514 the station means for stage-2 male bass ranged from 0.14% to 0.22% 

(Appendix Table 6). No station-stage combination represented 

by more than one fish differed notably from the others. As 

described for females, all GSI values for male RGB bass 

except the two values noted were smaller than those (including controls) reported for male largemouth bass from Florida 

examined by Sepúlveda and others (2001; 2003). And, as also 

noted, the Florida study was conducted in the Spring. 

Vitellogenin (Vtg) 

Concentrations of vtg in most male carp and bass were 

below the LOD (<0.005 mg/mL in carp, <0.002 mg/mL in 

bass; Appendix Table 6), so these were not analyzed statistically. In female carp, ANOVA indicated that age and gonadal 

stage, as well as several interaction terms involving these 

factors, significantly influenced vtg concentrations (Table 

17). The model accounted for 87% of the total variation in vtg 

concentrations, but differences among stations were not significant after the other factors (stage, age, and so forth) were 

accounted for (Table 17). In female bass this model was also 

significant and accounted for 80% of the total variation, but 

no individual factors or their interactions were significant and, 

as was true for female carp, differences among stations were 

not significant after accounting for all other factors (Table 17). 

However, large numbers of bass scales were regernerated and 

could not be read, and the statistical analysis incorporated relatively few fish. Based on the results of previous studies (for 

example, McDonald and others, 2002) it is likely that stage 

would have been significant had more fish been included in 

the analysis. For this reason, results for vtg in female carp and 

bass are presented and discussed by gonadal stage. 

Vtg in Female Carp 

Overall, vtg in female carp ranged from <0.005 mg/mL 

in a few fish from several stations to 4.3 mg/mL in a stage2 fish from Station 514 (Fig. 29; Appendix Table 6). As 

expected, vtg concentrations in female carp increased with 

stage. Concentrations in the four stage-0 fish from Station 511 

were all <0.005 mg/mL whereas in the other stages present (1 

and 2) they averaged 1.2 mg/mL and 2.8 mg/mL, respectively 

(Appendix Table 6). The basin-wide mean for stage-1 females 

was 1.1 mg/mL, with concentrations ranging from below 

detection (<0.005 mg/mL) to 3.1 mg/mL (Appendix Table 
79 

mg/mL mg/mL

5

 Carp-female 

4 

3 

2 

1 

0 

12 

10 

8 

6 

4 

2 

0 

Micropterus sp.-female

Upper RG Mid-RG/Pecos Lower RG 

64 63 516 65 515 514 513 16 511 512 

Station 

Figure 29. Plasma vitellogenin (Vtg) concentrations in female carp 

and bass (Micropterus sp.), by station. Shown for each group are 

points representing individual fish and the mean (red horizontal 

line), median (black horizontal line), interquartile range (box), and 

the 10th and 90th percentiles (whiskers). Stations are ordered 

from upstream to downstream and are grouped by sub-basin. See 

Table 1 for station descriptions and collection dates. 

6). The mean for stage-1 female carp from Station 513 was 

low (0.1 mg/mL) relative to the other four stations from which 

stage 1 females were obtained (means = 1.2 – 2.0 mg/mL; 

Appendix Table 6). Concentrations in stage-2 female carp 

ranged from 0.6 to 4.3 mg/mL with a mean of 1.9 mg/mL. 

Most values were between 1.0 and 3.0 mg/mL (Appendix 

Table 6), and station means ranged from 1.3 (Station 65) to 2.4 

mg/mL (Station 514; Appendix Table 6). Except for the low 

concentrations in stage-1 females from Station 513, no station 

appeared to differ greatly from the others and overall differences among stations were not statistically significant (Fig. 29; 

Table 17; Appendix Table 6). 

Vtg in Male Carp 

Vtg concentrations in 77 of 91 male carp (85%) were 

below the LOD (<0.005 mg/mL; Appendix Table 6). Concentrations exceeded the LOD in at least one fish from Stations 

16, 63, 64, 511, 514, and 515 (Appendix Table 6). Except for 

Results and Discussion 

one stage-2 fish from Station 511, all male carp with detected 

concentrations were in stage 3 (Appendix Table 6), and most 

of the measured concentrations were low (<0.02 mg/mL). 

Only three values exceeded 0.02 mg/mL; these were in fish 

from Stations 64 (0.08 mg/mL) and 511 (0.8 and 1.6 mg/mL; 

Appendix Table 6). The greater of these concentrations (1.6 

mg/mL, Station 511) is within the normal range of early to 

mid-vitellogenic (stage-1 or –2) female carp as identified in 

this and other studies (Goodbred and others, 1997; McDonald 

and others, 2002). This value is also within the range of concentrations induced in male carp exposed to sewage (Folmar 

and others, 1996) and to 17-β estradiol in laboratory studies 

(Folmar and others, 2001). Similar sporadic occurrences of 

male carp with comparatively high vtg concentrations have 

been noted in previous field studies (McDonald and others, 

2002; Smith and others, 2002). 

Vtg in Female Bass 

Vtg Concentrations in female bass also increased with 

gonadal stage. Concentrations were <0.002 mg/mL in two 

stage-0 fish from Station 512 vtg (Appendix Table 6). In 

stage-1 females, which were obtained at all five stations from 

which bass were collected, the overall mean concentration was 

1.5 mg/mL, with values for individual fish ranging from below 

detection (<0.002 mg/mL) to 9.9 mg/mL (Appendix Table 6). 

Mean vtg concentration was 0.03 mg/mL in the two stage-3 

female smallmouth bass from Station 63 and 3.1 mg/mL in 

the two stage-4 females from Station 514. The ovaries of the 

latter were infected with microsporidian parasites (Fig. 26). 

The overall mean for stage-2 females, which were obtained at 

Stations 16 and 513, was 3.8 mg/mL (range 1.9-7.2 mg/mL). 

Concentrations in most fish ranged from below detection to 

3.5 mg/mL (Fig. 29). The station mean was lowest at Station 

512 (0.01 mg/mL) and greatest at Station 513 (3.1 mg/mL) 

(Fig. 29; Appendix Table 6). Overall, differences among stations were not statistically significant, nor were any other factors or interactions (Table 17). Except for the consistently low 

values at Station 512 and the one fish from Station 513 with 

a relatively high concentration (7.2 mg/mL), concentrations 

in most stage-1 female bass were similar. The two highest 

concentrations in female bass, regardless of stage, were in fish 

from Station 513 (Fig. 29). 

Vtg in Male Bass 

Vtg was not detected (<0.002 mg/mL) in 26 of the 27 

male bass analyzed (96%; Appendix Table 6). The one exception was a stage-2 fish from Station 513 with a concentration 

of 3.15 mg/mL, which is well into the range of mid-vitellogenic female bass collected in this and other studies (for example McDonald and others, 2002). Concentrations in this range 

have been detected in male fish of other species exposed to 

sewage (Folmar and others, 1996) and to 17-β estradiol in the 

laboratory (Folmar and others, 2001), but not in largemouth 
80 Environmental Contaminants and their Effects on Fish 

bass exposed to paper mill effluents; in the latter plasma vtg 

concentrations were reduced relative to controls (Sepúlveda 

and others, 2001; 2003). Liver size is also influenced by vtg 

synthesis. The HSI of the male bass from Station 513 with the 

elevated vtg concentration was 1.03%, which was within the 

range of bass from other RGB sites (Fig. 14). It was also one 

of the four intersex males collected at Station 513 (Appendix 

Table 6). In contrast, one male bass from this site had a very 

large liver (2.3%; Fig. 14), but its vtg concentration was low 

(<0.002 mg/mL) and it was histopathologically unremarkable. 

As noted for male carp, sporadic occurrences of male bass 

with comparatively high vtg concentrations have been noted 

in previous field studies (Baldigo and others, 2001; McDonald 

and others, 2002). 

Reproductive Biomarkers: Summary 

Both male and female carp from Station 511 (AC at Harlingen) were in earlier gonadal stages than those at all other 

stations. However, fish were collected at Station 511 during 

late September-early October, 4-8 weeks before all other stations in the lower RGB. Therefore, some of the stage difference may reflect the sampling date, and these stage differences 

probably also influenced other reproductive biomarkers. Bass 

from Station 512 were also in slightly earlier stages than those 

from the other three stations from which bass gonads were 

obtained for examination (females only from Station 63). Fish 

were collected in the RGB over a 4-month period; although 

most were obtained between late September and early December 1997, Station 65 was not sampled until late January 1998 

(Table 1). Nevertheless, carp from Station 65 were in the 

same stages (females all stage 2, males all stage 3) as nearby 

stations that were sampled earlier (Fig. 21). Thus, it does not 

appear that reproductive biomarker results at sites other than 

Station 511 were confounded by sampling period differences. 

Reproductive biomarker measurements of note were 

determined primarily through comparison with the 1995 MRB 

study (McDonald and others, 2002) and the literature. For 

atresia, McDonald and others (2002) defined high percentages as ≥25% for female carp and >10% for female bass. In 

the RGB, most female carp had <20% atresia whereas most 

female bass had <10%. High individual values were found 

in both carp and bass from Station 16, but only in bass from 

Stations 512 and 514. Values in female bass from Station 514 

were greater than any reported in bass previously, and were 

related to a parasite infection (Fig. 26). The ovaries of one 

female carp from Station 64 contained no normal gonad tissue, 

only necrotic, degenerated follicles (Fig. 23). 

To our knowledge, the background occurrence of intersex 

male bass has not been established. We therefore recorded 

where they were found and at what frequency, but have no 

criteria for comparison. Intersex male bass were detected at 

three of the four RGB sites from which gonads were available 

for examination; none (of only three examined) were observed 

in fish from Station 514, and gonad samples were not obtained 

from Station 63 male bass. Ovotestis was detected in four of 

eight male bass from Station 512, four of nine from Station 

513, and two of 10 from Station 16. Ovotestis was detected 

exclusively in largemouth bass, but only one male smallmouth 

bass (from Station 514) was examined. In contrast to bass, 

ovotestis was not detected in carp from any of the sites investigated. 

Differences between carp and bass were obvious for GSI; 

as noted in previous studies (for example, McDonald and 

others, 2002), gonads constituted a substantially greater proportion of the total body mass in carp than in bass. Amongstation differences were statistically significant in carp; GSI 

values were higher than most in male carp and stage-2 female 

carp from Station 514. Although not significantly different, 

GSI was also greater in stage-1 female bass from Station 514 

than at other stations. Other stations at which GSI values 

in carp were comparatively high (mean or individual fish) 

included Stations 64 and 513. Wainwright and others (2001) 

noted that elevated GSI in male carp from resacas and settling 

basins in the lower RG valley were positively correlated with 

plasma androgen concentrations and with carcass concentrations of p,p’-DDE. Conversely, GSI in carp was lower than 

most at Stations 511, 512, and 515, as it also was in female 

bass from Stations 512 and 513 (Fig. 28). The ovaries of the 

parasite-infected stage-4 female bass from Station 514 (Fig. 

26) were proportionately smaller than many fish in earlier 

gonadal stages. 

Vtg concentrations in female carp did not appear abnormal at any station. In contrast, vtg concentrations in two male 

carp from Station 511 were within the range of early- to midvitellogenic females, a condition that has been associated with 

the exposure of male fish to exogenous estrogens in both field 

and laboratory studies. In addition, and keeping with the findings of most U.S. field studies (Goodbred and others, 1997; 

Lee and others, 2000; McDonald and others, 2002; Solé and 

others, 2002), none of the vitellogenic male carp were intersex. These findings contrast with those of a study of reproductive biomarkers in another European cyprinid (roach, Rutilus 

rutilus) collected near sewage treatment plants in the U.K.; in 

this study, the proportion of intersex fish and the severity of 

the intersex condition were greater, and the mean vtg concentration of the intersex males was intermediate between that of 

males and female controls (Jobling and others, 1998). In the 

RGB, relatively high vtg concentrations in stage-1 female bass 

and an elevated concentration in one male bass were detected 

at Station 513. The vtg concentration in this latter fish as well 

as those in the two male carp from Station 511 were within 

the range of concentrations shown to induce pathological 

changes in the liver of male fish exposed to 17-β estradiol in 

laboratory studies (Folmar and others, 2001). In contrast to 

male bass from Station 513, no male carp from this station had 

vtg concentrations greater than the detection limit. However, 

Stage-1 female carp from this site had very low vtg concentrations relative to all others. At Station 512, vtg concentrations 
in female carp appeared normal but concentrations in female 

bass were uniformly low. 

Spatial Patterns in Contaminant Concentrations 

and Biomarker Responses

Upper Rio Grande (Stations 64, 63, and 516)

Contaminant concentrations in fish from the upper basin 

were generally low, and most biomarker responses were 

unremarkable (Figs. 30 and 31; Table 22). Contaminant 

concentrations in carp and northern pike from Station 64 (RG 

at Alamosa, CO) were universally low; however, the ovaries 

of one female carp contained only necrotic follicles (Fig. 23). 

EROD rates in several carp were also minimally indicative 

of exposure to exogenous AhR ligands; however, PCB and 

TCDD-EQ concentrations were relatively low (Figs. 9, 10, and 

32), indicating that the fish had probably been exposed to PAH 

(Table 4). Station 64 is situated downstream from the influence of the Creede mining district where degraded water quality and elevated metals concentrations in fish and other matrices have been documented in the past (Levings and others, 

1998). Station 63 (Elephant Butte Reservoir) was the most 

contaminated of the three upper basin sites. Concentrations 

of Hg were elevated in predatory fishes (Figs. 3 and 31; Table 

22), as they have been in the past, and elevated EROD rates in 

carp and female bass (no males were obtained) were consistent 

with exposure of the fish to exogenous AhR ligands (Figs. 

10 and 32; Table 22). Because concentrations of PCBs and 

ÚÊ

#S

#S

%U

#S

$T

#S $T

$T

#S

64

63

516 65

515

514

513

16

511

512

Total DDT (μg/g) 

 <0.14 

 0.14-0.5 

 0.5-1.0 

>1.0 

$T

#S

#S

%U

#S

#S

#S #S

#S

#S

64

63

516 65

515

514

513

16

511

512

Toxaphene (μg/g) 

 <0.04 

 0.04-0.2 

>0.2

$T

#S

#S

%U

#S

#S

#S #S

#S

#S

64

63

516 65

515

514

513

16

511

512

Total Chlordanes (μg/g) 

 <0.05 

 0.05-0.1 

>0.1 

Figure 30. Maximum concentrations of total DDT (p,p’-homologs), toxaphene, and total chlordanes (sum of six components) in composite samples of whole fish. Total DDT concentrations of 0.15 μg/g are potentially harmful to the brown pelican (Pelicanus occidentalis), 

the most sensitive avian species (Anderson and others, 1975), and 1–3 μg/g is potentially harmful to other piscivorous birds (Blus, 1996). 

For toxaphene, reviews by Jarvinen and Ankley (1999) and Eisler and Jacknow (1985) noted acute and chronic effects on freshwater fish 

at whole-body concentration ≥0.04 μg/g. For chlordane, concentrations >0.1 μg/g may affect the health of predatory fish and piscivorous birds (Eisler, 1990). See Table 1 for station descriptions and collection dates and text for explanation of chlordane components.

Results and Discussion 81
TCDD-EQ were low (Figs. 9 and 32), these EROD rates suggest exposure to PAH at this site (Table 4). The HAI scores 

of a few individual carp and bass from Station 63 were also 

comparatively high (Fig. 12) and CF was lower in carp than 

at some other RGB sites (Fig. 13); however, all values were 

within the range of those reported for carp in other studies. 

Vtg concentrations in the two stage-3 female smallmouth bass 

from Station 63 were lower than stage-2 females largemouth 

bass from other stations; however, female smallmouth bass 

were not obtained elsewhere in the RGB. In addition, male 

gonads were not obtained from Station 63 bass, so the incidence of ovotestis at this site could not be determined. At Station 516 (RG at El Paso) concentrations of DDT were elevated 

(total p,p’ homologs 0.21 μg/g, mostly as DDE) in carp (Figs. 

7 and 30). Slightly elevated EROD rates in carp from this site 

also indicated exposure to exogenous AhR ligands (Fig. 10); 

however, as was true for most RGB sites, PCB and TCDD-EQ 

levels were low (Figs. 9, 10, and 30). Nevertheless, it should 

be noted that in contrast to others sites in the upper basin and 

despite the lack of detectable PCB residues in the fish, PCBs 

were detected in sediments from this site by previous investigations (Appendix Table 1). 

Except for Station 63, where Hg contamination has been 

well documented, relatively little extant information was available for the stations upstream of El Paso other than the NCBP 

(Schmitt and others, 1999b), NAWQA (Carter and Anderholm, 1997; Healy, 1997; Levings and others, 1998), and the 

studies by Ong and others (1991) and Roy and others (1992). 

Concentrations of organochlorine chemical residues were 

historically low in NCBP fish from these stations and declined 

$T

$T

$T

$T

$T

$T

%U $T

#S

%U

64

63

516 65

515

514

513

16

511

512

Hg (μg/g) 

 <0.1 

 0.1-0.3 

>0.3 $T

#S

$T

$T

$T

$T

$T#S

$T

$T

64

63

516 65

515

514

513

16

511

512

As (μg/g) 

 <0.2 

≥0.2 

$T

$T

%U

$T

$T

$T

%U %U

#S

#S

64

63

516 65

515

514

513

16

511

512

Se (μg/g) 

 <0.6 

 0.6-0.8 

>0.8 

Figure 31. Maximum concentrations of mercury (Hg), arsenic (As), and selenium (Se) in composite samples of whole fish. An Hg 

concentration of Hg 0.1 μg/g in fish has been suggested as a guideline for the protection of piscivorous mammals (Yeardley and others, 

1998), and concentrations of 0.3 μg/g cause reproductive impairment in the common loon (Gavia immer) (Wiener and Spry, 1996; Wiener 

and others, 2002). For Se, concentration should be <0.6 μg/g to avoid toxicity to piscivorous wildlife and <0.8 μg/g to avoid toxicity to 

fish (Lemly, 1996). Arsenic is not considered harmful to fish or wildlife at the concentrations present in RGB fish; the indicated thresholds are arbitrary and were selected for illustration only. See Table 1 for station descriptions and collection dates.

82 Environmental Contaminants and their Effects on Fish
Results and Discussion 83 

Table 22. Summary of chemical and biological indicator results, by sub-basin and station. Within each column, colors 

indicate the severity, incidence, or both of the indicated condition or conditions at each station 

(green<yellow<orange<red). These designations are relative; see text for explanations. Male (m) and female (f) bassa

 and a carp were collected from all sites unless otherwise indicated. If gender is not specified, then the indicated condition was 

present in both. See footnotea

 for additional abbreviations and Table 1 for station locations and collection dates. 

Station Contaminants and EROD activity 

Upper Rio Grande basin 

64 EROD (c) 

63 Cr (mc), Hg (b, stb), EROD (c, b) 

516 DDE (c), EROD (c) 

Middle Rio Grande basin 

65 As (mwb, mc), Se (mwb, mc, fc), EROD (c, b) 

515 Se (c) 

514 As (c), Se (c, b), Hg (b) 

Lower Rio Grande basin 

513 Ni (fb), Cr (fb), EROD (b) 

16 Pb (fb), DDE (mc) 

511 DDE (mc, ccf), tox (c, ccf), chlordane (ccf), 

dieldrin (ccf), EROD (c, ccf) 

512 As (fb), Ni (mc), TCDD-EQ (mb), EROD (c, b) 

Fish health 

indicators 

CF (c-), HAI (c, b) 

HAI (c) 

CF (c-) 

EL (c), HAI (c) 

SSI (b) 

HAI (c, b) 

HAI (c) 

CF (c-), HAI (b-), 

MA (mc, fc) 

Reproductive biomarkers 

vtg (mc), ovarian degeneration (fc, n=1) 

vtg (fb-) 

stage (fb); atresia (fb); ceroid (fb); GSI 

(fb); ovarian parasites (fb, n=2) 

vtg (mb), ovt (mb) 

ovt (mb) 

stage (c), vtg (mc) 

stage (b), atresia (fc), ovt (mb), vtg (fb-) 

a DDE, p,p’-DDE; tox, toxaphene; chlordanes, sum of cis- and trans chlordanes and nonachlors; oxychlordane; and hepatochlor epoxide; TCDDEQ, dioxin-like activity as determined by H4IIE bioassay; As, arsenic; Cr, chromium; Hg, mercury; Pb, lead; Ni, nickel; Zn, zinc; EROD, 

ethoxyresorufin O-deethylase activity; EL, external lesions; CF, condition factor; vtg, vitellogenein; HAI, health assessment index; SSI, 

splenosomatic index; HSI, hepatosomatic index; GSI, gonadosomatic index; ovt, ovotestis; MA, macrophage aggregates (one or more 

parameters); b, bass (Micropterus spp.); c, carp (Cyprinus carpio); ccf, channel catfish (Ictalurus punctatus); wb, white bass (Morone chrysops); 

stb, striped bass (Morone saxatilis); - indicates that the response or condition was smaller or lower than most; all others larger or greater. 

further through 1997. NAWQA also sampled water, bed sediments, and fish tissues in 1992-1993 from a site near Station 

64; minimal organic contamination was detected in fish tissues 

and sediments, and concentrations of dissolved constituents 

in water were low. Although trace elements emanating from 

the Creede, CO mining district were elevated in carp livers in 

the NAWQA study, elemental concentrations in 1997 samples 

of whole fish from Station 64 were universally low. Ong 

and others (1991) also reported low concentrations of most 

analytes in fish collected from the RG upstream of Elephant 

Butte Reservoir during the 1980s. A review of local influences found few sources of direct releases of contaminants to 

waterways around these upper-basin sites (Appendix Table 1). 

Few altered biomarkers (only elevated EROD and HAI 

in several carp; Figs. 10 and 12) were noted in the fish from 

Station 516 (RG at El Paso), and only total DDT (mostly 

DDE) exceeded threshold concentrations for toxicity to fish 

and wildlife (Figs. 6, 7, 30, and 32; Table 22). These findings 

generally followed the overall upstream-downstream trend 

evident in the 1997 results. NASQAN analyses of water during 1995-97 detected traces of several water-soluble pesticides 

at this site (Appendix Table 1). The NAWQA program also 

reported low levels of water-soluble pesticides at El Paso 

as well as traces of p,p’-DDE in bed sediments (Carter and 

Anderholm, 1997; Healy, 1997; Levings and others, 1998). 

The latter finding agrees with the 1997 fish data for Station 

516 (Figs. 6 and 7; Table 22), but NAWQA fish tissue data 

were not available for comparison. Compared to Stations 63 

and 64, greater numbers of regulated and reporting facilities 

were situated around the El Paso site (Appendix Table 1). Previous studies had indicated that sediment toxicity and a large 

number of contaminants in water, fish tissue, and sediment 

were of concern; that water quality was generally impaired 

downstream of El Paso; and that the benthic and fish communities were affected (Appendix Table 1). 

Middle Rio Grande and Pecos River (Stations 65, 

515, and 514) 

The most evident contaminants in fish from the central 

parts of the RGB were As and Se (Figs. 2 and 31; Table 22); 

concentrations of one or both of these elements were greater 

than most in at least one sample from all three stations (65, PR 

at Red Bluff; 515, RG at Langtry; and 514, RG below Amistad 

Dam). Hg concentrations were also comparatively high in 

bass from Station 514 (Figs. 3 and 31; Table 22). Concentrations of Se in fish from Station 65 exceeded threshold criteria 

for toxicity to fish and wildlife (Table 22). The comparatively 

high As concentrations in fish from sites in the central parts 

of the RGB reflect previously documented natural sources 

exacerbated by irrigation activities; concentrations of both As 
and Se were lower than in the past, however. The fish health 

biomarkers from the three central RGB sites were largely 

unremarkable; the only noteworthy findings were comparatively low CF in carp at Station 65 (Fig. 13), relatively large 

spleen size (SSI) in bass from Station 514 (Fig. 15), and a 

greater than average percentage of the fish from Station 515 

(carp only) with external lesions, the latter also reflected in 

elevated HAI (Fig. 12; Tables 19 and 22). In contrast, reproductive biomarker anomalies were apparent in female bass 

from Station 514; however, these were all associated with a 

microsporidian parasite infection in the ovaries (Fig. 26). Station 514 was the only RGB site of the four from which male 

bass gonads were obtained where ovotestis was not detected, 

but the sample size was small (n = 3; Fig. 33). Little extant 

information was available for comparison with 1997 findings 

for Station 65. In the past, NCBP fish from this site contained 

comparatively high concentrations of As and Se (Schmitt and 

other, 1999b), as they did in 1997. 

In contrast to Station 65, the RG below El Paso has been 

heavily investigated. Earlier studies by the TNRCC (1994b; 

Davis and others, 1995) identified As, Hg, Cd, DDE, and HCB 

in sediments as contaminants of concern from Presidio downstream to Amistad Dam, as were Se, Hg, and other metals in 

fish (Appendix Table 1). Additional data for Stations 514 and 

515 were available from NASQAN, which detected watersoluble pesticides and trace elements in water during 1995-97 

at both sites; however, water quality criteria were not exceeded 

(Appendix Table 1). Previous studies also noted concentrations of several metals, As, Se, and DDT in fish as contaminants of concern in this reach. 

#S

#S

#S

$T

$T

#S

#S #S

#S

#S

64

63

516 65

515

514

513

16

511

512

TCDD-EQ (pg/g)

 <5 

≥5 #S

%U

%U

%U%U

%U

%U #S

%U

%U

64

63

516 65

515

514

513

16

511

512

EROD (pmol/min/mg protein): Common carp

 Males <6 or Females <4 

 Males ≥6 or Females ≥4

%U

S

S

S%U

%U

%U S

S

%U

64

63

516 65

515

514

513

16

511

512 EROD (pmol/min/mg protein): Micropterus sp. 

 Males <22 or Females <16 

 Males ≥22 or Females ≥16 

 No Data 

Figure 32. Maximum concentrations of TCDD-EQ in composite samples of whole fish and mean hepatic ethoxyresorufin O-deethylase 

(EROD) activity (pmol/min/mg protein) in carp and bass (Micropterus sp.). TCDD-EQ concentrations >5 pg/g are potentially toxic to 

piscivorous avian and mammalian wildlife, but the threshold for toxicity to fish is about 35 pg/g. For EROD activity, the thresholds indicated are levels identified in previous studies as indicative of exposure to exogenous Ah-R agonists (Schmitt and others, 2002c). See 

Table 1 for station descriptions and collection dates.

84 Environmental Contaminants and their Effects on Fish
85 

$T

S

S

S%U $T

#S S

S

S

64 

63 

65 

16 ≥50%

516 

515 

514 

513 511 

512 

Intersex: Micropterus sp. (males)

 <20%

 20-50% 

 No Data 

Figure 33. Percentages of male bass from the indicated stations in which ovotestis was detected through histopathological 

examination of the gonads. See Table 1 for station descriptions 

and collection dates. 

Lower Rio Grande and Arroyo Colorado (Stations 

16, 511, 512, and 513) 

Greatest organochlorine chemical burdens and prevalence 

of anomalous biomarkers occurred in the lower RGB (Figs. 

7, 9, 10, 30, and 32; Table 22). Some samples also contained 

high concentrations of metals (Table 22). Female bass from 

Station 513 (RG at Amistad) contained particularly high 

concentrations of Cr and Ni (Fig. 5), and EROD rates in bass 

were elevated (Figs. 10 and 32). Concentrations of PCBs and 

TCDD-EQ in fish from this site were low (Fig. 9); the elevated 

EROD rates may therefore have resulted from exposure to 

PAH (Table 4). Plasma vtg concentrations were elevated in 

one male bass from Station 513, and four of nine male bass 

were intersex as identified by the presence of ovotestis (Fig. 

33; Table 22). In addition, HAI was elevated in both carp and 

bass from Station 513 (Fig. 12; Table 22). Slightly elevated 

concentrations of DDE characterized fish from Station 16 (RG 

at Mission, TX; Figs. 6, 7, and 30), and male bass from this 

site had elevated MA parameters (Figs. 16-18; Table 22). In 

addition, the Pb concentrations in one sample (female bass) 

from this site was very high (Fig. 4; Table 22). Ovotestis was 

detected in two of the 10 male bass from Station 16 examined, 

Results and Discussion 

but vtg concentrations were low in all male fish and all other 

biomarkers were unremarkable (Fig. 33). Fish from Station 

511 (AC at Harlingen) contained elevated concentrations of 

several organochlorine pesticides (Figs. 7 and 30) and EROD 

activity was elevated (Figs. 10 and 32; Table 22), indicating 

exposure to exogenous AhR ligands. The lack of PCBs and 

low levels of TCDD-EQ (Figs. 9 and 32) suggest exposure to 

PAH. HAI scores in carp from this site were elevated (Fig. 

12) and elevated plasma vtg was detected in one male carp 

(Fig. 33; Table 22). Fish from Station 511 were collected 

earlier than at other sites in the lower RGB and were in correspondingly less advance gonadal stages (Fig. 21; Table 1). 

Fish from Station 512 (RG at Brownsville) contained slightly 

elevated concentrations of As, Ni, and TCDD-EQ (Figs. 2, 9, 

and 31), and EROD activity indicated exposure to exogenous 

AhR ligands (Figs. 10 and 32; Table 22). Several fish health 

biomarkers (CF, HAI, MA) were also consistent with exposure 

to contaminants at Station 512 (Table 22). In addition, ovotestis was detected in four of the eight male bass examined, but 

no male carp or bass from Station 512 contained high concentrations of vtg (Fig. 33). A comparatively large percentage of 

atretic eggs were also present in the ovaries of the female carp 

collected from Station 512, however (Fig. 25; Table 22). 

The 1997 findings corroborated those of many other 

programs and studies indicating that the lower RGB is 

contaminated to the extent that fish and wildlife may be at 

risk. Historically, NCBP fish from Station 16 contained high 

organochlorine chemical concentrations relative to those 

from Stations 63, 64, and 65 (Schmitt and others, 1999b). 

NASQAN data for the lower RGB also revealed the presence 

of atrazine, malathion, and chlorpyrifos in water; at Station 

511, 90th percentile concentrations for atrazine and chlorpyrifos exceeded water quality criteria for the protection of aquatic 

life during the period preceding the collection of the fish, and 

the 90th percentile concentration of malathion was 84% of 

the criterion value (Appendix Table 1). An existing advisory 

for the AC (including Station 511) recommends that no fish 

be consumed due to contamination by DDE, chlordanes, and 

toxaphene (Appendix Table 1). Earlier studies by USEPA 

(1992b) also indicated than fish from the AC at Harlingen 

were contaminated with organochlorine chemical residues. 

River segments identified by TNRCC (1994a; 1994b) with 

contaminant concentrations in water ‘of possible concern’ and 

sediment and fish tissue contaminant concentrations exceeding screening levels included: Falcon International Reservoir 

to the head of the tidal reach (Hg, Pb, and chlordane in water); 

below Falcon Dam (DDE in sediment); RG tidal (DDE in sediment, PCBs in fish); and the AC above tidal (Se and DDE in 

sediment; toxaphene, dieldrin, and Pb in fish). Another recent 

TNRCC study (Davis and others, 1995) found that Se concentrations in water and Hg, Se, and chlordane concentrations in 

fish exceeded screening levels at Brownsville. Contamination 

was more evident at the Port of Harlingen, where screening 

levels were exceeded for Se in water, DDE in sediment, and 

Hg, Se, chlordane, total DDT, and γ-HCH in fish (Davis and 

others, 1995). As a result, Brownsville was rated as having a 
86 Environmental Contaminants and their Effects on Fish 

slight potential for toxic chemical impact whereas Harlingen 

was rated with a moderate potential (Davis and others, 1995). 

Brownsville and Harlingen were also identified by Mora and 

Wainwright (1997) because of organic chemical residues in 

fish during the period of record (1960s-1980s). The residues 

included organochlorine pesticides, PCBs, PAH, phthalates, 

and phenolic compounds. In the past, concentrations of DDE 

were considered elevated near the Port of Harlingen (Mora and 

Wainwright, 1998) and Hg contamination was reported near 

Station 16 (TNRCC, 1994a) and Harlingen (Davis and others, 1995). Searches of online databases located fewer mines 

in the lower RGB than upstream; however, there were more 

landfills and TRI, NPDES, CERCLIS, and TNRCC Superfund 

sites (Appendix Table 1). 

Overall, it is important to note that although the lower 

RGB was contaminated relative to other RG sites investigated, 

contaminant concentrations have declined greatly over the last 

two decades. Although still high enough to represent a threat 

to human and wildlife fish consumers, maximum 1997 concentrations of residues derived from DDT were 20-fold lower 

than those reported in the late 1970s. Concentrations of other 

obsolete pesticides have also declined. These trends and most 

of the biomarker responses noted for the lower RGB were 

similar to those found in the southernmost parts of the MRB 

investigated during 1995 (Schmitt, 2002a). 

Correlations Between Contaminant 

Concentrations and Biological Endpoints 

Rank correlations between selected biological endpoints 

(fish size and age; biomarkers) and contaminant concentrations in fish indicated statistically significant relations between 

many pairs of variables (Table 23). Correlation coefficients 

ranged from marginally (P = 0.10) to highly (P < 0.001) 

significant, and most statistically significant correlations were 

consistent across taxa and genders. That is, nearly all significantly correlated pairs varied in the same direction (positive or 

negative) irrespective of species and gender (Table 23). 

Concentrations of several contaminants were correlated 

with fish size (Table 23). Although there was some variation, 

p,p’-DDE and Se in carp and Hg in bass tended to increase 

with fish size, age, or both whereas concentrations of TCDDEQ, Cu, and Ni tended to decline (Table 23). Among biomarkers, EROD was positively correlated with chlordane in 

carp and catfish, TCDD-EQ and Cr in carp, Zn in male bass, 

and with As in all taxa (Table 23). In contrast, EROD was 

negatively correlated with Cu in all taxa, DDE and Se in bass, 

and Cd in female carp. SSI was negatively correlated with 

many contaminants and positively correlated with only Cu in 

female carp and Zn in catfish (Table 23). HSI, which was not 

analyzed in carp, was positively correlated with DDE, TCDDEQ, Cr, Cu, and Hg in bass of either or both genders, but was 

negatively correlated with Cd in catfish and Pb in female bass. 

The MA parameters were variably correlated with a number 

of contaminants, some consistently and some not (Table 23). 

Negative relationships were documented between the MA 

variables and concentrations of As, Cu, and Se whereas correlations with TCDD-EQ, Cd, Cr, Hg, Ni, and Zn were positive 

and others were neutral or varied in direction and strength. 

Although HAI and external lesions are functionally related 

variables, they were correlated with difference contaminants 

(Table 23). HAI was positively related to DDE, Cr, Hg, Pb, 

and Se in all taxon-gender combinations and both positively 

and negatively correlated in others. Significant correlations 

with external lesions were universally positive for TCDD-EQ, 

Cd, Cu, and Zn and universally negative for Pb (Table 23). 

Among the reproductive biomarkers, GSI was negatively 

correlated with DDE, chlordane, As, and Pb in catfish, but correlations with Cu, Ni, and Zn in catfish were positive (Table 

23). Overall, correlations between GSI and contaminants were 

highly variable in direction and magnitude and, in contrast to 

the findings of Wainwright and others (2001), GSI was not 

significantly correlated with DDE in male carp (Table 23). 

Atresia was correlated with relatively few variables, but all 

correlations were positive; atresia was correlated with DDE 

and Se in carp and with Hg in bass (Table 23). Stage was 

negatively correlated with chlordane and Zn in carp and bass 

and with Pb in male carp; however, stage was positively correlated with Cu in both male and female bass and with Hg in 

female carp (Table 23). Ceroid pigment scores were positively 

correlated with DDE in male bass and in carp, with chlordane 

and Cr in female carp, with Hg in female and all bass, and 

with Zn in all carp (Table 23). In contrast, ceroid pigment 

was negatively correlated with chlordane in female bass, As in 

male carp, Cu in male bass and female carp, Pb in female bass, 

and Se in male bass. Vtg concentrations (female carp and bass 

only) were positively correlated with Cd in carp and bass and 

with Cu in carp, but were negatively correlated with As in bass 

(Table 23). 

Many of the statistically significant correlations confirmed observations already made about the data; for example, 

HAI, HSI, atresia, and ceroid pigment scores increased with 

p,p’-DDE concentrations, reflecting the generally poorer overall health of fish from the lowermost parts of the RG, where 

pesticide residue concentrations were greatest. Concentrations of several contaminants, most notably DDE and Hg, also 

tended to increase with size, however, indicating that some 

of the association between these contaminants and biomarkers may be related to fish size or age differences between the 

sites. Some correlations were also counter-intuitive; that is, 

they were the opposite of what was anticipated based on extant 

knowledge of the biomarkers. Although there may be many 

explanations for these fidnings, they may represent relations 

that have not been documented in laboratory studies. Such 

counter-intuitive results are common in post-hoc analyses, 

especially where the range of contaminant concentrations is 

narrow, as was true for most analytes in the RGB. In addition, 

the sample sizes were small (n = 6 for catfish and within-gender analyses of bass and carp). For these reasons, and as noted 
87 

Table 23. Statistically significant correlations a between biological and chemical endpoints in bass (Micropterus sp.), carp, and catfish (Ictalurus sp.) from the Rio Grande 

Basin.

Biological endpoints 

Chemical endpoints 

p,p’-DDE Chlordanes e TCDD­ EQ As Cd Cr Cu Hg Ni Pb Se Zn 

Total Length fc, mc, 

AC 

FB, FC mb ab fc, ac MB, FC, 

AC 

mb 

Weight fc, mc, 

AC 

mb, ac fb, FC MB, AB fc FC, AC 

Age fb FB, ab fc, mc, 

ac 

fc, mc, ac MB,

AB 

EROD ab, cf fc, AC, cf mc, AC fb, mb, ab, 

cf 

fc mc, ac mb, fc, AC, 

cf 

CF MB, ab MB 

SSI mb fc, fb, ab, cf ab ab, cf ac,

cf 

ab FC ac, cf 

HSI b MB mb cf ab FB, ab fb fb CF fc, AC fc fc, AC fc, ac MAMM c ac MB, ab fc, ac mb mb, ab MEANAREA c mb, mc mc, ac mb, AB ab MC, 

ac 

MB, mc, ab 

mc, ac, mb, ab 

TISSOC c fc mb, mc, ac mb fc, ac AB ab AC ab, MC, AC HAI fc, AC, cf ab, CF FB, fc, cf mb fc, mc, 

AC 

fb, ab CF fc mc, zn 

Ext. Lesions mb, ab fc,

ac

 mc mb,

ab 

fb, ab 

GSI cf mb, fc, ac, cf ac CF ac AC, cf cf cf ab, cf % Atresia d fc FB fc Stage fc, mc, MB, 

AB 

mb, ab fc mc MB, ab, AC 

Ceroid MB, ac fc, fb mc fc mb, fc fb, ab fb mb AC Vtg d fb fc, fb fc a Blue text, positive correlations; Red text, negative correlations; plain text, P < 0.10; bold italics, P < 0.05; BOLD SMALL CAPS, P < 0.01; BOLD ALL CAPS, P < 0.001; mb, male bass (n = 6); fb, 

female bass (n = 6); ab, all bass (n = 12); mc, male carp ( n= 11); fc, female carp (n = 11); ac, all carp (n = 22); cf, catfish (n=6). 

b HSI in bass and catfish only; no livers from Station 63 bass 

MA parameters in carp and bass only 

d % Atresia and vtg in female carp and bass only. e Total chlordanes (sum of cis- and trans-chlordanes and nonachlors, oxychlordane, and heptachlor epoxide). 

Results and Discussion c 
88 Environmental Contaminants and their Effects on Fish 

in the report summarizing the 1995 MRB study (Schmitt, 

2002a), simple correlations between pairs of variables should 

only be considered a first step in an exploratory statistical 

analysis. Subsequent analyses using more robust methods 

focused on the 1997 data combined with that from other studies should seek to better define relations between contaminants 

and biomarkers. These analyses should also incorporate data 

characterizing contaminant concentrations in water and sediment, such as the data from NAWQA and NASQAN. 

Summary and Conclusions 

Overall, fish from stations in the lower RGB contained 

greater concentrations of contaminants and appeared to be less 

healthy than those from sites in the central and upper parts of 

the basin. As noted by other studies, and with the exceptions 

indicated, there was a general gradient of concentrations and 

biomarker responses from upstream to downstream. In the 

upper parts of the basin, a minimal number of altered biomarkers and few or no elevated contaminant concentration were 

noted. The exception in the upper basin was elevated concentrations of Hg (up to 0.46 μg/g) in predatory fishes from 

Station 63 (Elephant Butte Reservoir), a condition noted in the 

past and for which a consumption advisory remains in effect. 

Elemental contaminants were also prevalent in fish from 

the central RGB sites. Fish from Station 514 (Amistad International Reservoir) contained slightly elevated concentrations 

of Se (1.1-1.5 μg/g) and were characterized by comparatively 

high organosomatic indices, proportion of fish with external 

lesions, and HAI scores. In addition, bass from Station 514 

contained elevated concentrations of Hg (up to 0.40 μg/g). Se 

concentrations were also elevated at Stations 65 (up to 1.9 μg/ 

g in white bass) and 515 (>1.0 μg/g in carp). A comparatively 

large percentage of the carp from Station 515 had high HAI 

scores caused primarily by the presence of parasite-induced 

external lesions; and some samples from Stations 65 and 514 

contained comparatively high concentrations of As (>0.3 

μg/g). Concentrations of As and Se in fish from Station 65 

were lower than in the past, but Se concentrations at this site 

as well as others in the central RGB were sufficiently high to 

represent a threat to fish and wildlife. 

In the lower RGB, organochlorine pesticide residues were 

evident in fish from most sites, and concentrations exceeded 

toxicity thresholds for fish and wildlife at Stations 16, 511, 

and 512. At Station 513, concentrations of DDE and Se were 

high relative to other sites and at Station 511 (AC at Harlingen) organochlorine concentrations were similar to those 

found in fish from the cotton-growing regions of the MRB 

sampled in 1995 (Schmitt and others, 2002c); a consumption 

advisory is in effect for the AC and nearby waters because 

of DDE, chlordane, and toxaphene contamination. Although 

high by contemporary standards, residue concentrations were 

nevertheless lower than those reported in the past, indicating 

the continued weathering of obsolete pesticides. This conclusion was further supported by the lack of p,p’-DDT and the 

increasing proportions of p,p’-DDE in 1997 samples relative to the past, both of which indicate the weathering of old 

material rather than the influx of new pesticides. However, 

concentrations of cis-chlordane remained high relative to those 

of trans-nonachlor and other chlordane components at Station 511, indicating more recent inputs of these compounds. 

Slightly elevated HAI values characterized carp from Station 

511 and a comparatively high frequency of external lesions 

was noted on bass at Station 512; histopathological examination revealed the latter to be parasite-induced. Comparatively 

high individual HAI values were also recorded at Stations 

16, 512, and 513, and one or more macrophage aggregate 

parameters were elevated in carp, bass, or both at all lower 

RGB stations. Histopathological examination of liver, spleen, 

and kidney tissues of fish from the lower RGB sites revealed 

conditions that are consistent with contaminant exposure. The 

reproductive biomarkers also suggested that fish from the 

lower RGB sites had been exposed to contaminants; intersex 

male bass, relatively low GSI, and elevated vtg concentrations 

in male fish were noted at three of the four stations, and large 

percentages of atretic eggs were present in female carp from 

Station 512. 

Concentrations of PCBs and TCDD-EQ were comparatively low in most RGB samples, but EROD rates in at least 

some carp, bass, channel catfish, or multiple species were 

indicative of exposure to exogenous AhR ligands at all sites. 

Collectively, and as suggested by the information in Table 

4, these findings suggest that fish from most RGB sites had 

been exposed to PAH, which is consistent with the widespread 

energy extraction and transportation activities that occur 

throughout much of the basin. Some PAH are highly toxic at 

relatively low concentrations; however it is also important to 

note that although elevated relative to basal activity in the species investigated, EROD rates in RGB fish were generally low 

relative to those reported for these species from other locations 

(Schmitt and others, 2002c; Whyte and others, 2000). 

Overall, and as reported for the MRB (Schmitt and others, 1999b) we saw no evidence indicating that fish in the 

RGB had been exposed to extremely high concentrations of 

toxic chemicals. Rather, and as reported for some MRB sites 

sampled in 1995 (Schmitt, 2002a), the biomarker data for the 

lower basin stations are consistent with subtle responses to 

contaminants. In addition, EROD rates were slightly elevated 

at most sites. Absent elevated PCB residues and TCDD-EQ, 

these EROD rates indicate that the fish were exposed to PAHs 

(Whyte and others, 2000). Previous studies (Davis and others, 

1995; TNRCC, 1997) indicated the potential for toxic chemical impacts on biota in the AC at Harlingen and in the RG 

at Brownsville. The USFWS also recognized contaminants 
References 89 

from agriculture and energy extraction as threats to Laguna 

Atascosa NWR and other refuges in the lower RGB (USFWS, 

1986). Our findings for sites in the lower RGB support these 

conclusions and concerns; concentrations of several organochlorine and elemental contaminants were great enough 

to represent a potential hazard to populations of the most 

sensitive fish-eating wildlife species. In addition, the human 

population of the RGB is growing rapidly, which will expand 

both the magnitude and scope of potential contaminant threats 

in the region and the demand for increasingly limited amounts 

of water. Although our data and that from other programs, 

including NCBP, have documented declining concentrations 

of persistent organochlorine chemical residues and of As and 

Se at one site, the continuing growth of irrigated agriculture 

in the RGB may further exacerbate the leaching of these 

elements into ground and surface waters. Concentrations of 

Hg remain at potentially problematic concentrations in the 

mainstem impoundments, which are inhabited by susceptible 

DOI trust species including the bald eagle and the interior 

least tern. Contemporary-use agricultural pesticides such 

as atrazine, chlorpyrifos, and diazinon were also detected at 

potentially problematic concentrations at some sites in the 

lower RGB by other programs. Although concentrations of 

only a few of these compounds exceeded current standards and 

criteria, levels may rise in the future due to changing agricultural practices and declining water availability. In addition, 

subtle responses to these and other chemicals may occur at 

concentrations lower than current standards, and their cumulative effects are largely unknown (McDonald and others, 2000; 

Scholz and others, 2000). Overall, results from the suite of 

biological endpoints measured were consistent with exposure 

to chemicals at the lower basin sites. These responses would 

also be expected to increase in magnitude with increasing 

pesticide concentrations, which might ultimately threaten fish 

populations. 

Mora and Wainwright (1998) urged further monitoring 

of the RGB due to limited historical data, increasing development, and a dearth of information on activities and contaminant releases in northern Mexico. Our findings also support 

this recommendation. Continued monitoring provides the 

basis for identifying consistently degraded sites as well as 

those with emerging problems, and for evaluating the success 

of remedial activities. Focused investigations are also necessary to document chemical sources and processes, cause-effect 

relationships, and possible roles of factors other than contaminants. 

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Appendix 1 101


Appendix 1.

d

Appendix Table 1. Summary of contaminant sources and extant data collected by other major assessment programs within a 25-mile (40-km) radius of the sampling stations, listed 

from upstream to downstream. TRI, Toxic Release Inventory; NPDES, National Pollutant Discharge Elimination System; CERCLA, Comprehensive Environmental Responsibility and Liability Act; NCBP, National Contaminant Biomonitoring Program; NAWQA, National Water Quality Assessment Program; NASQAN, National Stream Quality Accounting Network; NEC, not elsewhere classified; USEPA, U.S. Environmental Protection Agency. See footnote (**) for description of search methods and criteria and Table 1, Stations for station locations and collection dates.—Continue 

Station Location / Program (specific site designations); TRI/NPDES/CERCLA facilities and sites (n) 5 Medium Other Water Sediment Fish tissue 64 Rio Grande at Alamosa, CO 

NCBP (Station 64) i

NAWQA (Station 08240000) j TRI e (none)

NPDES e

Sewerage systems (n = 1) 

CERCLA y (none) 

No exceedances No exceedances No exceedances No further information 

63 Rio Grande at Elephant Butte Reservoir, NM 

NCBP (Station 63) i 

TRI e (none)

NPDES e

Sewerage systems (n = 1) 

CERCLA y (none) 

ChemistryMercury k, v, w 

ChemistryMercury v, w 

ChemistryMercury i Arsenic iHuman HealthElephant Butte Reservoir: 

fish consumption advisory due to mercury (NM) g 

Contaminants in other Biota Mercury

Phytoplankton v, w 

Zooplankton v, wAvian wildlife l 

516 Rio Grande at El Paso, TX 

NASQAN (Station 08364000) d

Binational Phase I (Stations 1, 2) a, b Binational Phase II (Stations 1.1, 2) c USEPA (Station 3072) h 

TRI e

Food and related products (n = 6)Finished fabric apparel products (n = 1) 

Chemicals and allied products (n = 1) 

Petroleum refining/related industries (n = 2) 

Leather and leather products (n = 1) Stone, clay, glass, concrete products (n = 1) 

Primary metal industries (copper) (n = 2)

Electronic/electrical equip., components, not 

computers (n = 3) 

Measuring, analyzing, controlling instruments; 

photographic, medical, optical goods; watches, clocks (n = 1) 

NPDES e

Sewerage systems (n = 3) 

CERCLA y

Metal plating (n = 2) Soil contamination by 

chromium, nickel arsenic, cadmium, and zinc; cleanup completed at one site, ongoing at the other 

ChemistryChloride c

Unionized ammonia c 

Arsenic c 

Copper c

Residual chlorine a NASQAN 90th pctl 

(μg/L) d, z

Atrazine 0.004 Chlorpyrifos 0.004 Dacthal 0.004 Diazinon 0.005 Metolachlor 0.006 Simazine 0.012 Arsenic 4.0 Chromium 3.0Copper 2.0Selenium <1.0 Zinc 5.0

ToxicityC. dubia – survival a 

C. dubia – survival and 

reproduction b 

ChemistryMethylene chloride aPhenolics (recoverable) c 

Toluene a

Chromium a Nickel a, cCadmium c Copper a, c 

Lead a, c Zinc c

Arsenic a, cMercury a ToxicityC. dubia – reprod. b P. promelas – toxicity c 

ChemistryCadmium a, c Copper a, c 

Zinc a, c

Selenium a Mercury h Lead a p,p'-DDE n, cc 

Water Quality Impairment Rio Grande below Riverside 

Diversion designated as “impaired” due to chloride (TX) and salinity, TDS, and chlorides (EPA) f 

Biological Assessments 

Benthic Community–potential 

concern c

Fish–concern c

Contaminants in Other Biota (Big Bend area)Avian wildlife 

Total DDT m, o, p, q, s 

Mercury o, p, q

Selenium o, p, q 

102 Environmental Contaminants and their Effects on Fish 
Appendix Table 1. Summary of contaminant sources and extant data collected by other major assessment programs within a 25-mile (40-km) radius of the sampling stations, listed 

from upstream to downstream. TRI, Toxic Release Inventory; NPDES, National Pollutant Discharge Elimination System; CERCLA, Comprehensive Environmental Responsibility and Liability Act; NCBP, National Contaminant Biomonitoring Program; NAWQA, National Water Quality Assessment Program; NASQAN, National Stream Quality Accounting Network; NEC, not elsewhere classified; USEPA, U.S. Environmental Protection Agency. See footnote (**) for description of search methods and criteria and Table 1, Stations for station locations and collection dates.—Continued 

Station Location / Program (specific site designations); TRI/NPDES/CERCLA facilities and sites (n) 5 Medium Other Water Sediment Fish tissue 65 Pecos River at Red Bluff Lake, TX 

NCBP (Station 65) h 

TRI e (none)

NPDES e

Refuse systems (n = 1)Natural gas liquids (n = 1) 

CERCLA y (none) 

No exceedances No exceedances Chemistry

Arsenic i

Selenium i 

Water Quality Impairment Pecos River approx 20 miles

upstream from site designated as “impaired” due to stream bottom deposits (NM and EPA) f 

515 Rio Grande at Foster Ranch, Langtry, TX 

NASQAN (Station 08377200)Binational Phase I (Stations 5b, 6) a, b 

TRI e (none)

NPDES e (none)

CERCLA y (none) 

NASQAN 90th pctl 

(μg/L) d, z

Atrazine 0.003 Dacthal 0.002 Diazinon 0.008 Simazine 0.005 Arsenic 3.0 Chromium 2.0Copper 2.0Selenium 2.0Zinc 3.0 

ChemistryArsenic a

Chromium a 

Nickel a 

ChemistryMercury a

Selenium aTotal DDT m, o, q, u, s 

Contaminants in Other Biota (Big Bend area)Avian wildlife 

Total DDT m, o, p, q, s 

Mercury o, p, qSelenium o, p, q 

514 Rio Grande below Amistad Reservoir, TX 

NASQAN (Station 08450900) d

Binational Phase I (Stations 7, 8) a, b Binational Phase II (Stations 6.1, 6.2) c TRI e

Footwear, except rubber, NEC (n = 1) 

NPDES e

National security [USAF (n = 1)] 

Sewerage systems n = (2) 

CERCLA y (none) 

ChemistryArsenic c

NASQAN 90th pctl 

(μg/L) d, z

Atrazine 0.004 Diazinon 0.010 Simazine 0.005 Arsenic 5.0 Chromium 3.0Copper 2.0Selenium <1.0 Zinc 6.0 

ChemistryArsenic c

Chromium a, c Nickel a, cCopper c 

Lead c Zinc c 

ChemistryCopper aMercury a, c

Selenium a 

Zinc aTotal DDT a, s, t 

Dieldrin a

PCBs (total) a Arsenic c 

No information 

513 Below Falcon Dam, TX

NASQAN (Station 08461300)Binational Phase II (Station 12.3) c 

TRI (none) e NPDES e

Sewerage systems (n = 2) 

CERCLA y (none) 

ChemistryArsenic c

Chlorpyrifos, max. = 

0.061 μg/L (97/01/09) 

d ; criterion = 0.041 μg/L 

No exceedances No exceedances Water Quality Impairment 

Rio Grande below Falcon Dam 

designated “impaired” due to pathogens (TX, EPA) f Appendix 1 103 
Appendix Table 1. Summary of contaminant sources and extant data collected by other major assessment programs within a 25-mile (40-km) radius of the sampling stations, listed 

from upstream to downstream. TRI, Toxic Release Inventory; NPDES, National Pollutant Discharge Elimination System; CERCLA, Comprehensive Environmental Responsibility and Liability Act; NCBP, National Contaminant Biomonitoring Program; NAWQA, National Water Quality Assessment Program; NASQAN, National Stream Quality Accounting Network; NEC, not elsewhere classified; USEPA, U.S. Environmental Protection Agency. See footnote (**) for description of search methods and criteria and Table 1, Stations for station locations and collection dates.—Continued 

Station Location / Program (specific site designations); TRI/NPDES/CERCLA facilities and sites (n) 5 Medium Other Water Sediment Fish tissue 16 Rio Grande at Mission, TX 

NCBP (Station 16) i

Binational Phase I (Stations 13, 14, 15, 16) a, b TRI e

Manufacturing - agricultural chemicals, NEC (n 

= 1)

Manufacturing - frozen fruits and vegetables (n = 

2)

Electronic components, NEC (n = 1) 

Manufacturing - aircraft engines /engine parts (n 

= 1)

Manufacturing – surgical / medical instruments 

(n = 1) 

Steel springs, except wire (n = 1) Bottled and canned soft drinks (n = 1) 

Manufactured ice (n = 1) Engine electrical equipment (n = 1) Plating and polishing (n = 1) 

NPDES e

Sewerage systems (n = 8) Electric services (n = 1)Manufacturing - canned fruits and vegetables (n 

= 1)

CERCLA y

Pesticide storage (n = 1) Groundwater 

contamination; cleanup completed 

ChemistryCyanide a 

ChemistryChromium a 

Nickel a

Arsenic a 

ChemistryCopper aMercury a, aaSelenium a p,p'-DDE n, aaTotal DDT i, m, g, s, aa Zinc aArsenic iChlordane i, p, q, r, aa Dieldrin i, q, aaToxaphene i, n, aa Dacthal ® i, aa 

Water Quality Impairment Rio Grande below Falcon Dam 

designated as impaired due to pathogens (TX, EPA) f 

511 Arroyo Colorado at Harlingen, TX 

NASQAN (Station 08470400) d 

USEPA (Station 3084) h TRI e

Manufacturing - sausages, other prepared meats 

(n = 2)

Ice cream and frozen deserts (n = 3) Manufactured ice (n = 1) Knit underwear mills (n = 1)Manufacturing - paints and allied products (n = 

1)

Manufacturing - fertilizers, mixing only (n = 1) 

Minerals, ground or treated (n = 1)Manufacturing - industrial valves (n = 1)

Manufacturing - fabricated metal products, NEC 

ChemistryChlorpyrifos 

max = 0.078 μg/L (96/06/12) d ; criterion = 

0.041 μg/L

Malathion

max = 0.840 μg/L (96/06/26) d ; criterion = 0.1 μg/L

Atrazine

max=15.10 μg/L(97/10/12) d ; criterion 

= 1.8 μg/L 

No exceedances Chemistry

p,p'-DDE h, m, n, t, u, bb, cc, dd 

Chlordane h, r Dieldrin h Endrin h, gTCDD-EQ h Toxaphene a Dacthal ® aaHuman HealthArroyo Colorado and nearby waters: fish consumption advisory due to DDE, chlordane r , and 

toxaphene 

Water Quality Impairment Arroyo Colorado above site designated as impaired due to pathogens (TX, USEPA) f ; below site

designated as “impaired” due to organic enrichment and low dissolved oxygen (TX, EPA) f; body contact 

advisory due to pathogens (TX) f 

104 Environmental Contaminants and their Effects on Fish 
Appendix Table 1. Summary of contaminant sources and extant data collected by other major assessment programs within a 25-mile (40-km) radius of the sampling stations, listed 

from upstream to downstream. TRI, Toxic Release Inventory; NPDES, National Pollutant Discharge Elimination System; CERCLA, Comprehensive Environmental Responsibility and Liability Act; NCBP, National Contaminant Biomonitoring Program; NAWQA, National Water Quality Assessment Program; NASQAN, National Stream Quality Accounting Network; NEC, not elsewhere classified; USEPA, U.S. Environmental Protection Agency. See footnote (**) for description of search methods and criteria and Table 1, Stations for station locations and collection dates.—Continued 

Station Location / Program (specific site designations); TRI/NPDES/CERCLA facilities and sites (n) 5 Medium Other Water Sediment Fish tissue 

(n = 1) 

Boat building and repairing (n = 3)Switching and terminal services (n = 1) 

NPDES e

Manufacturing - canned fruits and vegetables (n 

= 1)

Manufacturing - raw cane sugar (n = 1) 

Electric services (n = 1) Sewerage systems (n = 13) Refuse systems (n = 1)Wholesale trade - fresh fruits and vegetables (n = 

1)

Wholesale trade - petroleum bulk stations & 

terminals (n = 1)

Land, mineral, and wildlife conservation (n = 1) 

CERCLA y

Pesticide manufacture (n = 1) Groundwater 

contamination; cleanup completed. 

512 Brownsville, TX

NASQAN (Station 08475000)Binational Phase I (Stations 17, 18) a, b 

TRI e

Manufacturing - Fresh or frozen prepared fish (n 

= 1)

Abrasive products (n = 1)

Manufacturing - non-clay refractory (n = 1) 

Manufacturing - nonferrous wire drawing & 

insulating (n = 1) 

Motors and generators (n = 2) 

Engine electrical equipment (n = 1)

Motor vehicles and car bodies (n = 1) 

Truck trailers (n = 1) Environmental controls (n = 1) 

NPDES e

Electric services (n = 1) 

Sewerage systems (n = 5)

Wholesale trade - petroleum bulk stations and 

terminals (n = 2) 

CERCLA (none) 

No exceedances Chemistry

Chromium a 

Nickel a 

ChemistryCopper a

Mercury aSelenium a

Chlordane q, r 

Dacthal ® aa 

Contaminants in Other Biota Mercury in avian wildlife x

Water Quality Impairment Rio Grande below Falcon Dam designated as “impaired” due to pathogens (TX, EPA) f 

Appendix 1 105 
106 Environmental Contaminants and their Effects on Fish **Information on local sources of contaminants to and in biota, such as permitted discharges, fish consumption advisories, and hazardous waste sites, was also available from the web-sites of the New Mexico Environment Department and the Texas Department of Health (TDH). Databases of the Texas Natural Resources Conservation Commission (TNRCC), the Toxic Release Inventory (TRI) of the U.S. Environmental Protection Agency (USEPA), the USEPA Permit Compliance System (PCS), the Minerals Availability System (MAS), and the USEPA Comprehensive Environmental Response, Compensation, and Liability Information System (CERCLIS) were accessed and queried for information on documented permits, releases, and facilities that might be influencing water quality in the RGB. Such information was only available from U.S. facilities and sites, however. In addition, these queries investigated potential releases to water; air releases and possible deposition were not considered. Data from TNRCC on Texas Superfund sites and landfills released in 1996 and 1997 were obtained at <http://www.tnrcc.state.tx.us/gis/sites.html>. Data on surface water releases, transfers to public water treatment facilities, and non-production releases from TRI sites were available for 1987- 1999 from the USEPA’s [Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) GIS database, USEPA Envirofacts]. Location data for the PCS, an information management system for the National Pollutant Discharge Elimination System (NPDES), were obtained from BASINS. Discharge information was obtained for 1996. Known mining operations, mineral deposits and occurrences, and processing plants are listed by MAS; these data were also retrieved through BASINS. Data on CERCLIS locations were compiled in BASINS for 1997. The CERCLIS ID was used to query the Envirofacts CERCLIS database for Records of Decision or for site documentation. Facilities listed by TRI, NPDES, CERCLIS, and TNRCC Superfund within a 20-mi radius around each sampling station were summed. (None were located within 20 mi of Stations 63 or 64). Detailed information was obtained for facilities located 10-20 mi upstream and 10 miles downstream; mines were included in this grouping. Texas landfills were noted if they were within a 5-mi radius of the sampling site. TDH fish tissue data within a 25-mi radius of each stations were obtained from the database compiled by Mora (2001). 

a Exceedances and concerns identified in the Binational Study, Phase 1 (TNRCC, 1994b) and the 1994 Regional Assessment of Water Quality in the Rio Grande Basin (TNRCC, 1994a) b USEPA/TNRCC TOXNET results cited in the Binational Study, Phase 1 (TNRCC, 1994b) c Exceedances identified in the Binational Study, Phase 2 (TNRCC, 1997) d NASQAN data e USEPA TRI/PDES website f USEPA 303d websiteg USEPA Fish Advisory website h USEPA, 1992b i Schmitt and others (1999) j Levings and others (1998) k Canavan and others (2000)l Caldwell and others (1999)m Mora and Wainright (1997) n White and others (1983) o Mora and others (2000) p Irwin and others (1989) q Mora and Wainright (1997) r Sum of six-, seven- and eight-chlorine components; see text for explanation s p,p'-homologs t Gamble and others (1988) u Davis and others (1995) v Kidd and others (1974) w Garcia and Kidd (1979) x Mora and others (1997) y USEPA CERCLA web-site (see **) z 90th percentile concentrations for the NASQAN full period of record (generally 1995-97) preceding the capture of the fish at stations 512, 513, 514, and 515.aa TDH fish data from Mora (2001) (see **) bb Wainwright and others (2001) cc Mora and others (2001) dd Mahler and Van Metre (2002) 
Appendix 1 107 

Appendix Table 2. Numbers of fish collected from stations in the Rio Grande Basin, by station, species, and gender (M, 

male; F, female; J, juvenile; NG, no gonad sample obtained). See Table 1 for station locations and collection dates. 

Species and Gender Species Station Station Species and gender Gender Species Station Station gender count count count count count count 

16 -- -- --

-- Carp -- 20 

-- F 10 --

-- M 10 --

-- Largemouth bass -- 21 

-- F 11 --

-- M 10 --

-- -- -- --

63 -- -- --

-- Carp -- 25 

-- F 15 --

-- M 10 --

-- Largemouth bass -- 6 

-- F 4 --

-- NG 2 --

-- Smallmouth bass -- 2 

-- F 2 --

-- Striped bass -- 1 

-- F 1 --

-- -- -- --

64 -- -- --

-- Carp -- 40 

-- F 23 --

-- M 16 --

-- NG 1 --

-- Northern pike -- 16 

-- F 6 --

-- M 10 --

-- -- -- --

65 -- -- --

-- Carp -- 20 

-- F 10 --

-- M 10 --

-- White bass -- 21 

-- F 18 --

-- M 3 --

-- -- -- --

511 -- -- --

-- Carp -- 20 

-- F 13 --

-- M 7 --

-- Channel catfish -- 20 

-- F 8 --

-- M 3 --

-- J 9 --

41 512 -- -- -- 30 

-- -- Carp -- 14 --

-- -- F 6 -- --

-- -- M 8 -- --

-- -- Largemouth bass -- 16 --

-- -- F 7 -- --

-- -- M 8 -- --

-- -- NG 1 -- --

34 -- -- -- -- --

-- 513 -- -- -- 42 

-- -- Carp -- 20 --

-- -- F 11 -- --

-- -- M 9 -- --

-- -- Largemouth bass -- 22 --

-- -- F 13 -- --

-- -- M 9 -- --

-- -- -- -- -- --

-- 514 -- -- -- 28 

-- -- Carp -- 20 --

-- -- F 10 -- --

56 -- M 10 -- --

-- -- Largemouth bass -- 7 --

-- -- F 5 -- --

-- -- M 2 -- --

-- -- Smallmouth bass -- 1 --

-- -- M 1 -- --

-- -- -- -- -- --

-- 515 -- -- -- 16 

-- -- Blue catfish -- 6 --

41 -- F 3 -- --

-- -- NG 3 -- --

-- -- Carp -- 10 --

-- -- F 6 -- --

-- -- M 4 -- --

-- -- -- -- -- --

-- 516 -- -- -- 40 

-- -- Carp -- 18 --

40 -- F 10 -- --

-- -- M 8 -- --

-- -- Channel catfish -- 22 --

-- -- F 11 -- --

-- -- M 2 -- --

-- -- J 5 -- --

-- -- NG 4 -- --

-- -- -- -- -- --
108 Environmental Contaminants and their Effects on Fish 

Appendix Table 3. Total lengths, weights, and ages of fishes other than common carp and bass (Micropterus sp.) collected in the 

Rio Grande Basin, by station and gender (M, male; F, female; J, juvenile; NG, no gonad sample obtained). See Table 1 for station 

locations and collection dates. 

Weight (g) Age (y) Station, taxon, and gender Length (mm) 

No. fish Mean Range No. fish Mean Range No. fish Mean Range 

All stations 

Catfish a, all 

Catfish a, F 

Catfish a, M 

Catfish a, NG/J 

Morone spp., all 

Morone spp., F 

Morone spp., M 

Northern pike, all 

Northern pike, F 

Northern pike, M 

Station 63 

Striped bass, F 

Station 64 

Northern pike, F 

Northern pike, M 

Station 65 

White bass, F 

White bass, M 

Station 511 

Channel catfish, F 

Channel catfish, M 

Channel catfish, J 

Station 515 

Blue catfish, F 

Blue catfish, NG

Station 516 

Channel catfish, F 

Channel catfish, M 

Channel catfish, NG/J 

48 

22 

5 

21 

22 

19 

3 

16 

6 

10 

1 

6 

10 

18 

3 

8 

3 

9 

3 

3 

11 

2 

9 

381 

367 

439 

382 

262 

265 

243 

497 

575 

450 

719 

575 

450 

239 

243 

353 

464 

414 

383 

295 

372 

402 

380 

246-650 

246-650 

352-464 

264-539 

202-719 

202-719 

221-256 

216-738 

292-738 

216-652 

719-719 

292-738 

216-652 

202-268 

221-256 

305-433 

463-464 

309-539 

246-650 

264-327 

271-445 

352-451 

266-523 

48 

22 

5 

21 

22 

19 

3 

16 

6 

10 

1 

6 

10 

18 

3 

8 

3 

9 

3 

3 

11 

2 

9 

567 

505 

816 

573 

262 

276 

173 

1115 

1460 

908 

2300 

1460 

908 

164 

173 

406 

959 

720 

868 

169 

478 

600 

561 

97-2400 

97-2400 

400-1033 

126-1512 

85-2300 

85-2300 

120-210 

61-3000 

160-3000 

61-1800 

2300-2300 

160-3000 

61-1800 

85-335 

120-210 

213-733 

891-1033 

196-1512 

97-2400 

126-231 

137-800 

400-800 

200-1450 

48 

22 

5 

21 

22 

19 

3 

15 

5 

10 

1 

5 

10 

18 

3 

8 

3 

9 

3 

3 

11 

2 

9 

2.4 

2.5 

2.6 

2.3 

0.2 

0.2 

0.0 

1.8 

2.2 

1.6 

4.0 

2.2 

1.6 

0.0 

0.0 

2.0 

2.7 

1.9 

1.3 

1.0 

3.1 

2.5 

3.1 

0-7 

0-7 

2-3 

1-5 

0-4 

0-4 

0-0 

0-3 

0-3 

0-3 

4-4 

0-3 

0-3 

0-0 

0-0 

1-5 

2-3 

1-4 

0-3 

1-1 

1-7 

2-3 

2-5 a Channel catfish (Ictalurus punctatus) and blue catfish (I. furctatus) 
ontinued

Appendix Table 4. Mean, minimum (min.), and maximum (max.) values of ethoxyresorufin O-deethylase (EROD) activity), health assessment index (HAI), 

condition factor (CF), and hepatosomatic index (HSI) in carp and bass (all largemouth bass, Micropterus salmoides, except as noted) from the Rio Grande 

Basin, by station, gender, and reproductive stage. Also shown for each category are the total number of fish collected (N) and the number included in the 

computations (n). ND, no data/not determined. See Table 1 for station locations and collection dates.—C 

Taxon, EROD (pmol/min/mg protein) HAI CF HSI (%)

gender, Stage N n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. station 

Carp, F

166363646465

511511511512512513513514515516516

Carp, M

16161663646465

511511512512513 

2

1

2

ND

2

2

0121

2

1

2

2

2

12

123

3

2

33

23

2

3

3 

10

2

13

1

22

10

4812

4

4

7

10

6

19

118

10

1

1510

61

1

7

9 

10

2

6

0

19

10

48124

4

7

10

6

19

118

9

1

1310

61179 

1.4

3.7

2.9

ND

0.5

7.7

61.027.812.711.4

9.9

5.0

1.2

1.8

1.6

0.81.7

4.03.43.1

11.8

0.2

1.32.9

36.324.4

18.8

8.9

4.2 

0.8

3.5

0.4

ND

0.1

0.1

18.2

0.1

12.710.8

3.2

2.6

0.5

0.1

0

0.80.9

4.03.40.7

0.4

0.2

0.10.1

20.124.4

18.8

2.0

2.6 

3.4

4.0

4.9

ND

2.7

55.5

161.1

84.0

12.712.0

15.1

8.0

2.8

3.4

3.0 

0.82.8

4.03.44.5

27.2

0.2

7.87.4

54.224.4

18.8

14.1

6.9 

10 

2

13 

1

22 

10

4

8

12

4

4

7

10 

6

19

118

10

1

15 10 

611

7

9 

39.0

80.0

28.5

40.0

32.3

24.0

87.5 

66.3

30.065.0

42.5 

82.5

51.4

54.0

75.0

90.053.3

70.040.051.3

58.0

30.0

33.332.0

56.7

70.0 

100.0

42.9

47.8 

0

80

0

40

0

10

60 

40

30

40

10

30

10

0

40

9030

704040

10

30

00

40

70 

100

30

30 

70 

80

70 

40

80 

40

100 

120

30

90

70

130

70

130

130

90

110

704070

110

30

90 40 

70

70 

100

70

70 

10 

2

13 

1

22 

10

4

8

1

2

4

4

7

10

6

1

9

118

10

1

15 10 

6

1

1

7

9 

1.3

1.1

1.1

1.8 

1.5

1.3

1.3 

1.3 

1.3 

1.2 

1.3 

1.7 

1.6 

1.7

1.3

1.6 

1.4 

1.11.11.4

1.2

1.4

1.41.2

1.3 

1.2

1.0 

1.2 

1.5 

0.9

1.1

0.7

1.8

1.1

1.2

0.8

1.1

1.3

1.1

1.2

1.6

1.4

1.2

1.2

1.6

1.3

1.11.11.2

1.0

1.4

1.21.0

1.1

1.2

1.0

1.0

1.3 

1.5

1.2

1.3

1.8

2.6

1.5

2.2

2.1

1.3

1.3

1.4

1.7

1.8

2.1

1.5

1.6

1.5

1.11.1

2.0

1.3

1.4

1.51.3

1.6

1.2

1.0

1.3

1.8 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

00

0

0

0

00

0

0

0

0

0 

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

NDND

ND

ND

ND

NDND

ND

ND

ND

ND

ND 

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

NDND

ND

ND

ND

NDND

ND

ND

ND

ND

ND 

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

NDND

ND

ND

ND

NDND

ND

ND

ND

ND

ND 

Appendix 1 109

Appendix Table 4. Mean, minimum (min.), and maximum (max.) values of ethoxyresorufin 

O-deethylase (EROD) activity), health assessment index (HAI), 

condition factor (CF), and hepatosomatic index (HSI) in carp and bass (all largemouth bass, Micropterus salmoides, except as noted) from the Rio Grande 

Basin, by station, gender, and reproductive stage. Also shown for each category are the total number of fish collected (

N) and the number included in the 

computations (n). ND, no data/not determined. See Table 1 for station locations and collection dates.—Continued 

Taxon, EROD (pmol/min/mg protein) HAI CF HSI (%)

gender, Stage N n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. station 

514515516

Carp, ND

64

Bass, F 

161663

63 a

512512513513514514

Bass, M 

1616

512513

514 a

514

Bass, ND 

63

512 

3

3

3

ND

12

1

3

01

1

2

14

1

2

1

2

2

3

ND

ND 

10

4

8

1

83

4

2

25

12

1

32

1

9

8

9

2

1

2

1 

10

4

8

1

8

3

4

2

25

12

1

3

2

1

9

8

9

2

1

2

1 

5.2

2.7

11.3

0.1

33.6

24.7

59.7

113.4

61.377.1

44.5

15.5

18.0

41.2

33.5

62.3

78.2

56.7

17.6

52.6

96.3

111.2 

0.1

0.6

2.3

0.1

21.2

9.1

38.5

79.6

55.128.1

17.9

15.5

5.0

38.1

33.5

34.1

46.2

19.7

13.0

52.6

62.9

111.2 

8.7

4.5

28.0

0.1

58.5

37.5

81.0

147.2

67.5

107.4

96.6

15.5

33.5

44.3

33.5

131.9

107.1

83.5

22.3

52.6

129.8

111.2 

10 

4

8

1

8

3

4

2

2

5

12

1

3

2

1

9

8

9

2

1

2

1 

40.0 

62.5

47.5

10.0

70.0

10.0

72.5

40.0

50.0

18.0

55.8

100.0 

56.7

100.0

30.0

64.4

38.8

38.9

65.0

40.0

95.0

30.0 

0

10

30

10

30

0

40

40

10

10

30

100

40

70

30

0

10

0

60

40

60

30 

100 

100

80

10

120

30 

100

40

90

30

130

100

70

130

30

100

60

90 

70

40

130

30 

10

4

8

1

8

3

4

2

2

5

12

1

3

2

1

9

8

9

2

1

2

1 

1.5

1.4 

1.3 

1.4

1.3

1.5

1.4

1.4 

1.7

1.7

1.5

2.0 

1.6 

1.5

1.4

1.4 

1.7

1.4

1.4 

3.5

1.5

1.2 

1.2

1.1

0.8

1.4

1.1

1.4

1.1

1.4

1.5

1.4

1.2

2.0

1.5

1.5

1.4

1.2

1.3

0.9

1.4

3.5

1.5

1.2 

1.7

1.6

1.5

1.4

1.5

1.7

1.5

1.4

1.8

2.0

1.8

2.0

1.7

1.6 

1.4

1.6

2.1

1.6

1.5

3.5

1.5

1.2 

0

0

0

0

8

3

0

0

2

5

12 

1

3

2

1

8

8

9

2

1

0

1 

ND

ND

ND

ND

0.8

1.1

ND

ND

0.9

1.1

1.0

1.1

1.4

0.7 

0.5

0.7

1.0

1.0

1.0

1.0

ND

0.9 

ND

ND

ND

ND

0.6

0.8

ND

ND

0.7

0.9

0.6

1.1

1.0 

0.6

0.5

0.5

0.8

0.7

0.9

1.0

ND

0.9 

ND

ND

ND

ND

1.5

1.4

ND

ND

1.0

1.4

1.8

1.1

2.0

0.7

0.5

0.9

1.4

2.3

1.1

1.0

ND

0.9 

110 Environmental Contaminants and their Effects on Fish 

a Smallmouth bass, Micropterus dolomieui 
ontinued

Appendix Table 5. Mean, minimum (min.), and maximum (max.) values of the splenosomatic index (SSI), splenic macrophage aggregate (MA) density (MAMM), 

mean MA area (MEANAREA), and MA percent tissue occupied (TISSOC) in carp and bass (Micropterus sp.) from the Rio Grande Basin, by station, gender, and age (y). Also shown for each category are the total number of fish collected (N) and the number included in the computations (n). ND, no data/not determined. 

See Table 1 for station locations and collection dates.—C 

Taxon, SSI (%) MAMM (No./mm2) MEANAREA (μm2) TISSOC (%) 

station, Age N 

gender n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. 

Carp, F

161616161663636363636464646464656565

511511511511511512512512512512512 

12345

ND

3456

ND

1234134

ND

2345

1

2

3

4

5

6 

1

2

322

3

35

1

3

1

2

11

8

1154

3

1531

1

1

1

1

1

1 

1

2

322

3

3513

1

2

11

71144

3

1531

1

1

1

1

1

1 

0.2

0.2

0.20.40.1

0.2

0.20.10.20.1

0.1

0.3

0.2

0.30.30.10.10.2

0.2

0.20.10.10.1

<0.0

0.1

0.2

0.1

0.2

0.2 

0.2

0.1

0.10.30.1

0.1

0.10.10.20.1

0.1

0.2

0.2

0.20.30.10.10.1

0.1

0.20.10.10.1

<0.0

0.1

0.2

0.1

0.2

0.2 

0.2

0.2

0.20.60.1

0.2

0.20.2

0.2

0.2

0.1

0.3

0.2

0.80.30.10.20.3

0.3

0.20.10.20.1

<0.0

0.1

0.2

0.1

0.2

0.2 

1

2

322

3

25

1

3

1

2

11 

81154

3

1531

1

1

1

1

1

1 

4.7

1.8

4.9

10.310.9

10.8

10.6

6.8

13.5

10.6

10.0

10.3

9.1

8.26.59.48.26.2

9.2

3.58.09.68.2

17.6

14.1

17.6

15.9

19.4

12.4 

4.7

0.0

1.2

8.84.1

5.9

9.4

4.7

13.5

7.1

10.0 

8.2

2.4

4.1

6.59.45.34.7

7.1

3.52.97.68.2

17.6

14.1

17.6

15.9

19.4

12.4 

4.7

3.5

10.0

11.817.6

15.9

11.8

8.8

13.5

14.1

10.0

12.4

14.1

17.6

6.59.4

12.4

7.1

11.8

3.5

12.912.4

8.2

17.6

14.1

17.6

15.9

19.4

12.4 

1

2

3

22

3

2

5

1

3

1

2

11

8

11

5

4

3

1

53

11

1

1

1

1

1 

1055.4

2012.5

1305.2

3077.82851.1

3802.7

4973.0

7721.2

2614.6

3673.5

3641.2

5168.6

2974.4

4701.3

4316.51119.1

2017.3

4036.3

3989.8

8372.0

4029.34634.9

2289.0

4500.7

5259.6

7628.7

12542.4

8106.2

11760.2 

1055.4

0.0

617.0

2658.71090.5

3348.8

4090.0

5135.6

2614.6

2947.6

3641.2

3729.1

1779.7

2003.6

4316.51119.1

1484.5

3009.7

3040.8

8372.0

2416.13090.6

2289.0

4500.7

5259.6

7628.7

12542.4

8106.2

11760.2 

1055.4

4024.9

2447.7

3496.84611.6

4549.9

5855.9

10709.2

2614.6

4705.8

3641.2

6608.1

3831.1

10825.3

4316.51119.1

2809.7

5980.8

4705.3

8372.0

7795.67023.5

2289.0

4500.7

5259.6

7628.7

12542.4

8106.2

11760.2 

1

2

3

22

3

2

5

1

3

1

2

11

8

11

5

4

3

1

53

1

1

1

1

1

1

1 

0.5

0.7

0.4 

3.1

1.9 

4.1

5.2

5.3

3.5

3.9

3.6

5.0

2.8

3.5

2.81.1

1.7

2.5

3.7

3.0 

3.4 4.8 

1.9

7.9

7.6

13.5 

19.9 

15.8

14.5 

0.5

0.0

0.3

3.1

1.9

2.1

4.8

3.4

3.5

2.1

3.6

4.6

0.6

0.9

2.81.1

0.9

1.8

2.7

3.0

1.22.7

1.9

7.9

7.6

13.5

19.9

15.8

14.5 

0.5

1.4

0.6

3.1

1.9 

5.3

5.5

9.5

3.5

5.0

3.6

5.4

5.4

6.6

2.81.1

2.6

3.9

5.0

3.0

8.78.6

1.9

7.9

7.6

13.5

19.9

15.8

14.5 

Appendix 1 111

Appendix Table 5. Mean, minimum (min.), and maximum (max.) values of the splenosomatic index (SSI), splenic macrophage aggregate (MA) density (MAMM), 

mean MA area (MEANAREA), and MA percent tissue occupied (TISSOC) in carp and bass (Micropterus sp.) from the Rio Grande Basin, by station, gender, and age (y). Also shown for each category are the total number of fish collected (N) and the number included in the computations (n). ND, no data/not determined. 

See Table 1 for station locations and collection dates.—Continued 

Taxon, SSI (%) MAMM (No./mm2) MEANAREA (μm2) TISSOC (%)

station, Age N 

gender n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. 

513513513513514514514514514514515515516516516516516

Carp, M

161616161663636363646464 

124

5

ND

1

2458

ND

4

ND

1235

1

2345

ND

34

7

ND

23 

513

2

2

2

2211

4

2

3

14

1

1

1

2

313

2

43

1

1

10

4 

5

13

2

2

2

2211

4

2

3

14

1

1

1

2

312

2

43

1

1

10

4 

0.2

0.30.3

0.2

0.2

0.3

0.20.40.30.2

0.2

0.2

0.2

0.20.3

0.3

0.2

0.1

0.2

0.20.30.2

0.2

0.20.3

0.2

0.4

0.4

0.3 

0.2

0.30.2

0.2

0.2

0.2

0.20.30.30.2

<0.0

0.2

0.2

0.20.2

0.3

0.2

0.1

0.2

0.20.30.2

0.2

0.10.2

0.2

0.4

0.2

0.2 

0.3

0.30.3

0.3

0.3

0.3

0.30.50.30.2

0.3

0.2

0.3

0.20.3

0.3

0.2

0.1

0.30.20.30.2

0.2

0.40.3

0.2

0.4

1.5

0.4 

3

12

2

2

2

2211

4

1

2

14

1

1

1

2

313

2

43

0

1

10 

4 

0.8

1.84.7

0.3

2.4

0.0

2.14.41.85.3

11.5

10.6

9.1

2.46.2

11.2

5.9

2.9

11.8

11.8

7.1

11.2

8.2

10.3

8.4

ND

4.1

8.1

9.9 

0.0

1.84.7

0.0

1.8

0.0

1.24.11.85.3

7.6

10.6

7.1

2.42.9

11.2

5.9

2.9

11.8

7.6

7.1

4.7

7.6

7.1

5.9

ND

4.1

1.8

4.7 

1.8

1.84.7

0.6

2.9

0.0

2.94.71.85.3

16.5

10.6 

11.2

2.48.2

11.2

5.9

2.9

11.8

16.5

7.1

14.7

8.8

14.1

10.0

ND

4.1

19.4

12.9 

3

12

2

2

2

2211

4

1

2

14

1

1

1

2

3

1

3

2

4

3

0

1

10

4 

847.4

2160.15335.8

231.1

7542.1

0.0

2070.15978.8

13080.0

4572.9

4414.6

4380.8

5060.6

2679.23530.0

8868.2

5486.7

903.6

2184.9

2009.6

6746.3

3107.8

7993.3

3958.5

4749.0

ND 

2893.8

3011.1

2576.0 

0.0

2160.14840.4

0.0

2112.6

0.0

1900.13495.6

13080.0

4572.9

3007.6

4380.8

2713.1

2679.22782.7

8868.2

5486.7

903.6

1033.5

1926.9

6746.3

1116.6

4166.2

2317.7

3256.6

ND

2893.8

1210.9

1139.1 

1422.2

2160.15831.1

462.2

12971.5

0.0

2240.18462.0

13080.0

4572.9

6517.6

4380.8

7408.0

2679.24922.6

8868.2

5486.7

903.6

3336.3

2086.7

6746.3

5415.8

11820.3

5376.0

6795.5

ND

2893.8

5018.5

3540.6 

3

12

2

2

2

22

1

1

4

1

2

1

4

1

1

1

2

3

1

3

2

4

3

0

1

10

4 

0.1 

0.4 2.5

0.0 

1.5

0.0

0.42.6

2.3

2.4

5.0

4.6 

5.1

0.62.3

9.9

3.2

0.3

2.6

2.4

4.8

4.1

6.8

4.3 

3.7

ND 

1.2

2.3

2.7 

0.0

0.4

2.3

0.0

0.6

0.0 

0.21.6

2.3

2.4

2.3

4.6

1.9

0.60.8

9.9

3.2

0.3

1.2

1.5

4.8

0.5

3.2

1.6

3.3

ND 

1.2

0.8 

1.1 

0.3

0.4

2.7

0.0 

2.3

0.0

0.73.5

2.3

2.4

6.5

4.6

8.3

0.64.1

9.9

3.2

0.3

3.9

3.3

4.8

7.6

10.4 

7.6

4.0

ND 

1.2

4.7

4.6 

112 Environmental Contaminants and their Effects on Fish 
Appendix Table 5. Mean, minimum (min.), and maximum (max.) values of the splenosomatic index (SSI), splenic macrophage aggregate (MA) density (MAMM), 

mean MA area (MEANAREA), and MA percent tissue occupied (TISSOC) in carp and bass (Micropterus sp.) from the Rio Grande Basin, by station, gender, and age (y). Also shown for each category are the total number of fish collected (N) and the number included in the computations (n). ND, no data/not determined. 

See Table 1 for station locations and collection dates.—Continued 

Taxon, SSI (%) MAMM (No./mm2) MEANAREA (μm2) TISSOC (%)

station, Age N 

gender n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. 

6465656565

511511511512512512512512513513513513513513514514514514515515515516516516516 

4234

5

3

452

3

4

68

ND

1234812

35

ND

24

ND

234 

11

5

3

1

2

323

1

2

11

1

1212243

21

2

11

1

321 

11

5

3

1

2

323

1

2

11

1

1212243

21

2

11

1

321 

0.30.2

0.2

0.2

0.2

0.2

0.20.10.1

0.2

0.2

0.30.3

0.3

0.20.30.30.30.50.40.3

0.30.4

0.3

<0.0<0.0

0.2

0.40.50.4 

0.30.2

0.1

0.1

0.2

0.1

0.10.10.0

0.2

0.1

0.3

0.3

0.3

0.20.30.30.20.50.30.3

0.1

0.4

0.2

<0.0<0.0

0.2

0.30.30.4 

0.30.20.30.2

0.2

0.2

0.3

0.1

0.2

0.2

0.2

0.3

0.3

0.3

0.20.30.30.30.50.50.4

0.50.4

0.4

<0.0<0.0

0.2

0.50.80.4 

11

5

3

1

2

3

2

3

1

2

1

1

1

1212243

1

1

2

11

1

321 

17.114.1

14.0

12.4

6.5

3.8

14.3

2.4

15.5

15.3

11.5

11.8

12.4

11.8

1.21.55.34.48.25.34.1

0.6

1.8

6.2

13.512.9

10.6

6.15.08.2 

17.114.1

5.3

10.0

6.5

0.6

10.0

0.6

10.0

15.3

8.2

11.8

12.4

11.8

1.21.25.32.47.10.00.6

0.6

1.8

3.5

13.512.9

10.6

3.54.78.2 

17.114.1

18.8

15.3

6.5

7.1

17.1

4.1

24.1 

15.3

14.7

11.8

12.4

11.8

1.21.85.36.59.4

19.4

7.6

0.6

1.8

8.8

13.512.9

10.6

8.25.38.2 

11

5

3

1

2

3

2

3

1

2

1

1

1

12122

4

3

1

1

2

11

1

321 

2183.93967.0

1774.5

1679.9

876.7

4192.7

4033.4

1985.1

9709.4

7179.9

3254.4

7947.1 

8030.4

1890.9 

1210.31498.92954.43142.94337.4

1221.3

1337.7

591.7

3586.9

2067.7

2461.96235.6

2128.9

5033.92277.53410.5 

2183.93967.0

351.0

1307.6

876.7

2132.1

3776.7

748.5

5252.8

7179.9

2484.6

7947.18030.4

1890.9

1210.31023.02954.42059.83979.5

0.0

1105.2

591.7

3586.9

1710.9

2461.96235.6

2128.9

3268.72085.63410.5 

2183.93967.0

3249.8

2204.1

876.7

6253.3

4492.0

3221.7

14708.8

7179.9

4024.2

7947.1

8030.4

1890.9

1210.31974.82954.44225.94695.2

2899.8

1513.7

591.7

3586.9

2424.5

2461.96235.6

2128.9

6300.12469.43410.5 

11

5

3

1

2

3

2

3

1

2

1

1

1

12

1

22

4

3

1

1

2

1

1

1

3

2

1 

3.75.6

2.9

2.0

0.6

0.9

5.8

0.7

13.3

11.0 

3.5

9.4

9.9

2.2

0.10.21.61.23.5

1.5

0.6

0.0

0.6

1.2

3.3

8.1 

2.3

3.31.12.8 

3.75.6

0.2

1.5

0.6

0.4

3.8

0.0

9.2

11.0

3.3

9.4

9.9

2.2

0.10.11.61.03.3

0.0

0.1

0.0

0.6

0.9

3.3

8.1

2.3

1.21.12.8 

3.75.6

5.7

2.3

0.6 

1.5

7.1

1.3

18.2

11.0

3.7

9.4

9.9

2.2

0.10.31.61.33.7

5.6 

1.2

0.0

0.6

1.5

3.3

8.1

2.3

4.6

1.22.8 

Appendix 1 113

Appendix Table 5. Mean, minimum (min.), and maximum (max.) values of the splenosomatic index (SSI), splenic macrophage aggregate (MA) density (MAMM), 

mean MA area (MEANAREA), and MA percent tissue occupied (TISSOC) in carp and bass (Micropterus sp.) from the Rio Grande Basin, by station, gender, and age (y). Also shown for each category are the total number of fish collected (N) and the number included in the computations (n). ND, no data/not determined. 

See Table 1 for station locations and collection dates.—Continued 

Taxon, SSI (%) MAMM (No./mm2) MEANAREA (μm2) TISSOC (%)

station, Age N 

gender n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. 

516

Carp, ND

64

Bass, F 

161616636363

512512512512513513513513513513513514514514

Bass, M 

16161616

512 

5

3123235

0

1

2

3

ND

01

2

345123

1234

0 

1

1263321

2

2

2

1

1

24

3

111113

3412

3 

1

1263321

2

2

2

1

1

24

3

111113

3412

3 

0.6

0.40.10.10.10.10.10.1

<0.0

0.1

0.1

0.1

<0.0

0.10.1

0.1

0.10.20.10.20.10.3

0.30.10.20.1

0.1 

0.6

0.40.1

<0.0

0.10.10.10.1

<0.0

0.1

<0.0

0.1

<0.0

0.10.1

<0.0

0.10.20.10.20.10.2

<0.0

0.10.2

<0.0

0.1 

0.6

0.40.1

0.1 

0.10.10.20.1

<0.0

0.1

0.1

0.1

<0.0

0.10.1

0.1

0.10.20.10.20.10.4

0.8 

0.10.2

0.1 

0.1 

1

12

6

3321

2

2

2

1

1

24

3

111113

3

41

2

3 

8.8

8.27.4

7.8

7.36.15.64.7

5.3 

2.6

3.5

8.2

7.1

5.04.4

4.3

2.47.67.66.55.34.3

8.4

7.97.1

8.2

3.7 

8.8

8.26.5

2.9

5.94.72.44.7

3.5

1.8

1.8

8.2

7.1

1.81.2

3.5

2.47.67.66.55.31.8

5.3

5.97.1

5.3

3.5 

8.8

8.28.2

11.8

8.28.28.84.7

7.1 

3.5

5.3

8.2

7.1

8.27.1

5.9

2.47.67.66.55.38.2

11.8

11.2

7.1

11.2

4.1 

1

12

6

3321

2

2

2

1

1

24

3

111113

3

4

1

2

3 

2652.5

1245.83637.3

3213.1

5098.74009.33883.44301.1

1508.4

2216.6

2869.5

12262.1

4769.9

2022.32723.7

3244.1

7276.03683.33128.72824.93820.33440.6

3328.2

5405.3

7747.1

4512.9

2983.3 

2652.5

1245.83222.8

1233.9

2616.02390.22957.44301.1

1463.1

1797.7

1443.5

12262.1

4769.9

1643.7

699.2

2212.2

7276.03683.33128.72824.93820.31129.3

2518.6

2823.6

7747.1

4485.4

2808.6 

2652.5

1245.84051.8

6315.2

8125.36648.24809.34301.1

1553.6

2635.4

4295.5

12262.1

4769.9

2400.9

3884.7

4868.8

7276.03683.33128.72824.93820.36019.0

4078.5

7919.6

7747.1

4540.3

3187.2 

1

12

6

3321

2

2

2

1

1

2

4

3

111113

3

4

1

2

3 

2.3 

1.02.7

2.7

3.82.72.5

2.0 

0.8

0.5

1.3 

10.1

3.4 

1.1

1.4

1.5 

1.7

2.8 

2.41.8

2.0 1.9 

2.9

4.4

5.5

3.7

1.1 

2.3

1.02.1

0.4

2.01.10.7

2.0

0.5

0.5

0.3

10.1

3.4

0.3

0.1

0.8

1.72.82.41.82.00.3

1.3

1.7

5.5

2.4

1.0 

2.3

1.03.3

4.8

6.75.54.2

2.0 

1.0

0.6

2.3

10.1

3.4

2.0

2.4

2.9

1.72.82.41.82.05.0

4.0

6.5

5.5

5.0

1.2 

114 Environmental Contaminants and their Effects on Fish 
Appendix Table 5. Mean, minimum (min.), and maximum (max.) values of the splenosomatic index (SSI), splenic macrophage aggregate (MA) density (MAMM), 

mean MA area (MEANAREA), and MA percent tissue occupied (TISSOC) in carp and bass (Micropterus sp.) from the Rio Grande Basin, by station, gender, and age (y). Also shown for each category are the total number of fish collected (N) and the number included in the computations (n). ND, no data/not determined. 

See Table 1 for station locations and collection dates.—Continued 

Taxon, SSI (%) MAMM (No./mm2) MEANAREA (μm2) TISSOC (%) 

station, Age N 

gender n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. 

512512512513513513514514

Bass, ND 

6363

512 

1

2

3

0

1

2

03

34

3 

1

3

1

7

1

1

21

11

1 

1

3

1

7

1

1

21

11

1 

0.1

0.1

0.1

0.1

0.1

<0.0

0.10.4

0.10.2

0.1 

0.1

<0.0

0.1

<0.0

0.1

<0.0

0.10.4

0.10.2

0.1 

0.1

0.1

0.1

0.4

0.1

<0.0

0.20.4

0.10.2

0.1 

1

3

1

7

1

1

21

11

1 

6.5

6.1

4.7

3.6

7.6

8.2 

4.11.8

5.36.5

5.9 

6.5

4.7

4.7

1.2

7.6

8.2

1.81.8

5.36.5

5.9 

6.5

7.1

4.7

6.5

7.6

8.2

6.51.8

5.36.5

5.9 

1

3

1

7

1

1

21

11

1 

4930.7

3153.1

5001.9

2573.6

4728.2

3642.4

1852.82035.0

3090.08287.0

2156.6 

4930.7

2557.2

5001.9

975.0

4728.2

3642.4

1669.12035.0

3090.08287.0

2156.6 

4930.7

3799.9

5001.9

4786.9

4728.2

3642.4

2036.42035.0

3090.08287.0

2156.6 

1

3

1

7

1

1

21

11

1 

3.2 3.2 3.2 

2.0 1.2 2.7 

2.4 2.4 2.4 

1.0 0.1 1.7 

3.6 3.6 3.6 

3.0 3.0 3.0 

0.8 0.3 1.3 

0.4 0.4 0.4 

1.6 1.6 1.6 

5.4 5.4 5.4 

1.3 1.3 1.3 

Appendix 1 115

nued

Appendix Table 6. Mean, minimum (min.), and maximum (max.) values of percent atresia, ceroid pigment scores, gonadosomatic index (GSI), and plasma 

vitellogenin (vtg) concentrations in carp and bass (all largemouth bass, Micropterus salmoides except as noted) from the Rio Grande Basin, by station, gender, and 

reproductive stage. Also shown for each category are the total number of fish collected (N) and the number included in the computations (n). ND, no data/not 

determined. See Table 1 for station locations and collection dates.—Conti 

Taxon, Atresia (%) Ceroid pigment GSI (%) Vtg (mg/mL)

gender, Stage N n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. station 

Carp, F

166363646465

511511511512512513513514515516516

Carp, M

16161663646465

511511512512513514515516

Carp, ND

64 

2

1

2

ND

2

2

0

1

2

12

1

2

2

2

1

2

1

2

3

323

3

2

3

2

33

3

33

ND 

10

2

13

1

22

10

4

8

1

2

4

4

7

10

6

1

9

1

1

8

10

1

15

10

6

1

1

79

10

48

1 

10

2

13

0

22

10

4

8

1

2

4

4

7

10

6

1

9

0

0

0

0

0

0

0

0

0

0

00

0

00

0 

8.6

5.5

6.8

ND

4

0.6

3

11.3

5

20

9.3

4.8

7.4

7.7

9.3

4

7.7

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

NDND

ND

NDND

ND 

1

3

2

ND

0

0

0

3

5

15

3

0

0

3

2

4

4

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

NDND

ND

NDND

ND 

21

8

15

ND

12

4

12

19

5

25

23

9

18

15

25

4

12

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

NDND

ND

NDND

ND 

10

2

13

1

22

10 

4

8

1

2

4

4

7

10

6

1

9

1

1

8

10

1

15

10

6

1

1

79

10

48

0 

1.1

2

1.3

1

0.4

0.1 

2.8 

1.6 

1

1

1.3 

0.3

0.7 

0.8

1.8 

1

0.9 

2

0

0.9 

0.5

0

0.5

0.4

0.2 

0

1

0.4 0.6 

0.1

0.8 0.6 

ND 

0

2

1

1

0

0

2

1

1

1

1

0

0

0

1

1

0

2

0

0

0

0

0

0

0

0

1

00

0

00

ND 

2

2

3

1

2

1

3

3

1

1

2

1

2

1

2

1

2

2

0

1

1

0

2

1

1

0

1

11

1

21

ND 

10 

2

13

1

22 

10

4

8

1

2

4

4

7

10

6

1

9

1

1

7

10 

1

15

10

6

1

1

79

10

48

0 

17

4.7

10.1

21.1 

19.3

10.4

0.9 

4.5 

11.3

8.9 

9.3 

1.3

16.9

19.5

5.2 

18.2

15.5

0.2 

10.3

9.8 

7.8 

10.8

10.5

6.1

4.3 

6.7 

5.3 

6.4 

11.3

11.8 

5.4 9.4 

ND 

5.2

3.2

6.7

21.1

12.2

5.9

0.3

1.1

11.3

8.2

6.1

0.5

3.3

12

3.6

18.2

12.1

0.2

10.3

3.5

5.9

10.8

7.9

3.9

2.2

6.7

5.3

4.5

2.8

7.8

2.66.4

ND 

36.4

6.3

13.1

21.1

26.6

15.4

1.3

7.2 

11.3

9.7

12.2

2.4

21.7

28.9

7.5

18.2

20.6

0.2

10.3

12.7

10.1

10.8

13.4

7.7

6.6 

6.7

5.3

7.9 

20.3

14.6

8.2 

12.2

ND 

10

2

13

1

22

10

4

8

1

2

4

4

7

10

6

1

9

1

1

8

10

1

15

10 

6

11

7

9

10

4

8

1 

2.2

2

1.6

3.4

2.2

1.3

<0.005

1.2 

2.8

1.2

1.9

0.1

1.8

2.4

1.6

1.1

1.4

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005 

<0.005 

1.6

<0.005

<0.005 

<0.005

<0.005

<0.005 

<0.005

<0.005 

1.3 

1.6

<0.005

3.4

1.2

0.6

<0.005

<0.005

2.8

0.7 

1

<0.005

1.1

1.3 

1

1.1

0.7

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

1.6

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005 

3.7

2.3

2.7

3.4

3.4

2.3

<0.005

3.1

2.8

1.7

3.1

0.3

2.3

4.3

2.6

1.1

2.3

<0.005

<0.005

<0.005

<0.005

<0.005

0.1

<0.005

0.2

1.6

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005 

116 Environmental Contaminants and their Effects on Fish 
Appendix Table 6. Mean, minimum (min.), and maximum (max.) values of percent atresia, ceroid pigment scores, gonadosomatic index (GSI), and plasma 

vitellogenin (vtg) concentrations in carp and bass (all largemouth bass, Micropterus salmoides except as noted) from the Rio Grande Basin, by station, gender, and 

reproductive stage. Also shown for each category are the total number of fish collected (N) and the number included in the computations (n). ND, no data/not 

determined. See Table 1 for station locations and collection dates.—Continued 

Taxon, Atresia (%) Ceroid pigment GSI (%) Vtg (mg/mL)

gender, Stage N n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. n Mean Min. Max. station 

Bass, F 

161663

63 a

512512513513514514

Bass, M 

1616

512513514

514 a

Bass, ND 

63

512 

1

2

1

3

0

1

1

2

1

4

1

2

1

2

2

3

ND

ND 

8

3

4

2

2

5

12

1

3

2

1

9

8

9

2

1

2

1 

8

3

4

2

2

5

12

1

3

2

0

0

0

0

0

0

0

0 

5.3

4

6.5

3.5

1

0.4

0.8

4

3

65

ND

ND

ND

ND

ND

ND

ND

ND 

0

0

1

0

0

0

0

4

0

50

ND

ND

ND

ND

ND

ND

ND

ND 

18

10

11

7

2

2

3

4

7

80

ND

ND

ND

ND

ND

ND

ND

ND 

8

3

4

2

2

5

12 

1

3

2

1

9

8

9

2

1

0 ND 

0 ND 

1

0.7 

1.5 

1

1.5

0

0.9 

1

1.3

2

1

0.3 

0.1 

0.2 

0

0 

ND

ND 

0

0

1

1

1

0

0

1

1

1

1

0

0

0

0

0 

ND

ND 

2

1

2

1

2

0

2

1

2

3

1

1

1

1

0

0 

8

3

4

2

2

5

12

1

3

2

1

9

8

9

2

1

0

0 

0.7 

1.1 

0.8 

1.3

0.5

0.7

0.6

1.5

0.8

0.6 

0.1 

0.2 

0.2 

0.2 

0.1 

0.8 

ND 

ND 

0.5

0.7

0.5

0.7

0.4

0.4

0.3

1.5

0.7

0.6

0.1

0.1

0.1

0.1

0.1

0.8

ND

ND 

0.9

1.5 

1.1

1.8

0.5

1

1.2

1.5

1

0.7

0.1

0.3 

0.3

0.9 

0.2

0.8

ND

ND 

8

3

4

2

2

5

12 

1

3

2

1

9

8

9

2

1

2

1 

0.4

2.7 

0.7

<0.005

<0.005

<0.005

3.1

7.2

1.9

3.1

<0.005

<0.005 

<0.005

0.4 

<0.005

<0.005

<0.005 

<0.005 

<0.005

1.9

<0.005 

<0.005

<0.005

<0.005

<0.005

7.2

0.1

2.9

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005

<0.005 

1.5

3.8

2.1

0.1

<0.005

<0.005

9.9

7.2

3

3.3

<0.005

<0.005

<0.005

3.1

<0.005

<0.005

<0.005

<0.005 

a Smallmouth bass, Micropterus dolomieui 

Appendix 1 117

118 Environmental Contaminants and their Effects on Fish 
(BEST) Program: Environmental Contaminants and their Effects on Fish in the Rio Grande Basin—USGS/SIR 2004—5108

i l

Schmitt, C.J.; Dethloff, G.M.; Hinck. J.E.; Bartish, T.M.; Blazer, V.S.; Coyle, J.J.; Denslow, N.D.; Tillitt, D.E.—Biomonitoring of Environmental Status and Trends 

Pr nted on recyc ed paper