Bureau of Reclamation Southern Ute Indian Tribe 

Animas River 

Fisheries Database 

Synthesis and Analysis

Final Report - July 2008 
Animas River Fisheries Database July 25, 2008 

Recommended Citation: 

Valdez, R.A. 2008. Animas River fisheries database synthesis and analysis. Final Report. 

SWCA, Environmental Consultants, Broomfield, Colorado. 

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Animas River Fisheries Database July 25, 2008 

ANIMAS RIVER FISHERIES DATABASE 

SYNTHESIS AND ANALYSIS 

(Phase I) 

by 

Richard A. Valdez, Ph.D. 

SWCA, Inc. Environmental Consultants 

Broomfield, Colorado 

for 

Bureau of Reclamation 

and 

Southern Ute Indian Tribe 

Final Report 

July 25, 2008 

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Animas River Fisheries Database July 25, 2008 

ACKNOWLEDGEMENTS 

This Animas River fisheries database synthesis and analysis was conducted by 

SWCA, Inc. with assistance from the Southern Ute Indian Tribe (SUIT) and Bureau of 

Reclamation (Reclamation), Durango Office. This document provides a description and 

characterization of this fisheries database, as well as preliminary analyses that help describe 

fish populations of the Animas River. 

Thanks to Steve Whiteman and Ben Zimmerman of the SUIT for providing the 

database and for assisting with synthesis and analysis. Thanks also to Michael Francis of 

Reclamation for administering the work and for helping to identify issues of importance for 

data analysis. Kirk Lashmett, formerly of Reclamation coordinated most of the field work 

and was instrumental in collecting much of the field data that went into this database. Jim 

White and Mike Japhet of the Colorado Division of Wildlife provided stocking data and 

mark-recapture data from the Animas River. 

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Animas River Fisheries Database July 25, 2008 

TABLE OF CONTENTS 

Page

 EXECUTIVE SUMMARY ......................................................................................... xi 

1.0 INTRODUCTION .........................................................................................................1 

1.1 Background.....................................................................................................1 

1.2 Purpose and Objectives...................................................................................2 

1.3 Study Area ......................................................................................................2 

2.0 METHODS ....................................................................................................................4 

2.1 Database Development and Quality Control ..................................................4 

2.2 Estimates of Abundance .................................................................................4 

2.3 Length-Weight and Condition ........................................................................5 

2.4 Age and Growth..............................................................................................6 

2.5 Survival...........................................................................................................7 

2.6 Data Evaluation...............................................................................................7 

3.0 RESULTS ......................................................................................................................8 

3.1 Schedule of Studies.........................................................................................8 

3.2 Database Development ...................................................................................8 

3.3 Fish Species Composition.............................................................................12 

3.4 Trout Population Size ...................................................................................15 

 3.4.1 Population Estimates—SUIT Waters ................................................15 

 3.4.1 Population Estimates—CDOW Waters .............................................19 

 3.4.3 Proportional Stock Density................................................................21 

3.5 Catch Rates of Trout and Suckers.................................................................22 

3.6 Length-Frequency Analysis..........................................................................27 

 3.6.1 Length Distributions of Trout and Suckers........................................27 

 3.6.2 Length Discrepancies in Native Suckers ...........................................29 

3.7 Length-Weight and Age-Growth of Trout....................................................32 

 3.7.1 Length-Weight and Condition ...........................................................32 

 3.7.2 Age and Growth.................................................................................34 

3.8 Length-Weight and Age-Growth of Suckers ................................................38 

 3.8.1 Length-Weight and Condition ...........................................................38 

 3.8.2 Age and Growth.................................................................................42 

3.9 Survival of Trout...........................................................................................45 

3.10 Trout Stocking Protocols and Patterns..........................................................46 

4.0 DISCUSSION..............................................................................................................54 

4.1 Data Suitability and Utility for Fish Demographic Parameters....................54 

4.2 Suitability and Utility of Fish Abundance Estimates....................................54 

4.3 Effectiveness of Trout Stocking Program.....................................................55 

4.4 Population Projections for Native Suckers ...................................................56 

4.5 Drift of Young Native Suckers .....................................................................57 

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Animas River Fisheries Database July 25, 2008 

TABLE OF CONTENTS (continued) 

Page

4.6 Insight into Developing a Fish Monitoring Program....................................57 

5.0 RECOMMENDATIONS.............................................................................................59 

LITERATURE CITED ................................................................................................61 

APPENDIX A: Photographic Record....................................................................... A-1 

Photo Set 1. Electrofishing raft in operation on the Animas River and navigating 

past the Farmer’s Ditch diversion....................................................................... A-1 

Photo Set 2. Bluehead sucker, flannelmouth sucker, and white sucker (bottom) 

from the Animas River........................................................................................ A-2 

Photo Set 3. Brown trout, rainbow trout, and Snake River cutthroat trout from the 

Animas River ...................................................................................................... A-3 

Photo Set 4. Farmer’s Ditch diversion during low flow, during medium flow, and 

Farmington Lake diversion................................................................................. A-4 

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Animas River Fisheries Database July 25, 2008 

LIST OF TABLES

Table Page 

1. Key locations along the Animas River indicated by river miles upstream from the 

confluence of the San Juan River near Farmington, New Mexico ................................3 

2. Schedule of primary studies for the Animas River fisheries .........................................8 

3. Description of each of the data files of the Animas River Fisheries Database..............9 

4. Number of records (lines of data) in each worksheet of “MASTER-Animas Fish 

Data” ..............................................................................................................................9 

5. Column designations and field names for MASTER-Animas Fish Data ....................10 

6. Number and proportions of fish species captured in the Animas River between Purple 

Cliffs and the confluence of the San Juan River, 1997-2007 ......................................13 

7. Number and percentage of fish species captured in the Animas River, 1997-2007 ....14 

8. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from SUIT mark-recapture data................................................16 

9. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from CDOW mark-recapture data ............................................19 

10. Kolmogorov-Smirnov test comparing distributions above and below the Farmer’s 

Ditch Diversion at RM 21.9 on the Animas River.......................................................29 

11. Length-weight relationships and condition factors for brown trout, rainbow trout, and 

Snake River cutthroat trout of the Animas River within SUIT lands (RM 37-56)......34 

12. The von Bertalanffy growth function (VBGF), maximum predicted size of fish (L∞) 

and rate of growth (K) for the six principle fish species of the Animas River............35 

13. Total length at age and annual percent increase in length for brown trout (BNT), 

rainbow trout (RBT), and Snake River cutthroat trout (SRC) in the Animas River....36 

14. Length-weight relationships and condition factors for bluehead sucker, flannelmouth 

sucker, and white sucker of the Animas River within SUIT lands (RM 37-56)..........38 

15. Survival of brown trout (BNT), rainbow trout (RBT), and Snake River cutthroat trout 

(SRC) from the Animas River .....................................................................................45 

16. Dates, sizes, strains, and sources of fish stocked in the Animas River by the SUIT 

within SUIT lands (RM 37.1-56.2) during 1996-2007................................................47 

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Animas River Fisheries Database July 25, 2008 

LIST OF TABLES (continued)

Table Page 

17. Numbers of trout stocked in the Animas River by the CDOW in the gold medal and 

standard regulations waters near Durango, Colorado (RM 56.4-60.2)........................48 

18. Summary of trout stocked annually by the Southern Ute Indian Tribe (SUIT) and the 

Colorado Division of Wildlife (CDOW) in the Animas River....................................48 

 

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Animas River Fisheries Database July 25, 2008 

LIST OF FIGURES

Figure Page 

1. Principal locations along the Animas River in Colorado and New Mexico...................1 

2. Fish composition of the Animas River between Purple Cliffs and the confluence of the 

San Juan River, 1997-2007..........................................................................................12 

 

3. Average number of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) per mile for all years in three reaches of the Animas River 

within SUIT lands........................................................................................................15 

4. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from SUIT mark-recapture data in Reaches 1, 2, and 3 within 

SUIT lands ...................................................................................................................18 

5. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from CDOW mark-recapture data in Animas 1 and 2 ..............20 

6. Proportional stock density of brown trout (BNT), rainbow trout (RBT), and Snake 

River cutthroat trout (SRC) in the Animas River within SUIT lands..........................21 

7. Annual electrofishing catch rate estimates for brown, rainbow, and Snake River 

cutthroat trout from the Animas River within SUIT lands (RM 37-56)......................23 

8. Annual electrofishing catch rate estimates for bluehead, flannelmouth, and white 

suckers from the Animas River within SUIT lands (RM 37-56).................................24 

9. Catch rate indices regressed against simultaneous population estimates for brown 

trout, rainbow trout, and Snake River cutthroat trout from the Animas River within 

SUIT lands (RM 37-56)...............................................................................................25 

10. Total numbers of white sucker (top), bluehead x white crosses, flannelmouth x white 

crosses captured by river mile in the Animas River within SUIT lands (RM 37-56), 

1997-2007 ....................................................................................................................26 

11. Modal separation of lengths showing five cohorts for brown trout, six for rainbow 

trout, and two for Snake River cutthroat trout using Bhattachayra’s modal separation 

and the routine NORMSEP found in FiSAT II............................................................27 

12. Modal separation of lengths showing two apparent cohorts for bluehead sucker, six for 

flannelmouth sucker, and seven for Snake River cutthroat trout using Bhattachayra’s 

modal separation and the routine NORMSEP found in FiSAT II ...............................28 

13. Length-frequency distributions for flannelmouth sucker in the Animas River above 

(left column) and below (right column) the two irrigation diversions.........................30 

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Animas River Fisheries Database July 25, 2008 

LIST OF FIGURES (continued)

Figure Page 

14. Length-frequency distributions for bluehead sucker in the Animas River above (left 

column) and below (right column) the two irrigation diversions ................................31 

15. Length-weight relationships for brown trout, rainbow trout, and Snake River 

cutthroat trout from the Animas River in 1999 within SUIT lands (RM 37-56).........32 

16. Relative condition of brown trout, rainbow trout, and Snake River cutthroat trout of 

the Animas River within SUIT lands (RM 37-56).......................................................33 

17. Growth rates of brown trout, rainbow trout, and Snake River cutthroat trout using 

the routine ELEFAN in program FiSAT II to generate von Bertalanffy growth 

functions.......................................................................................................................35 

18. Growth rates of brown trout, rainbow trout, and Snake River cutthroat trout from the 

Animas River based on von Bertalanffy’s growth function derived with ELEFAN in 

the program FiSAT II and compared with growth rates from other populations ........37 

19. Length-weight relationships for bluehead sucker, flannelmouth sucker, and white 

sucker from the Animas River in 1999 within SUIT lands (RM 37-56) .....................39 

20. Relative condition of bluehead sucker, flannelmouth sucker, and white sucker of the 

Animas River within SUIT lands (RM 37-56) ............................................................40 

21. Relative condition of brown trout, rainbow trout, and Snake River cutthroat trout of 

the Animas River within SUIT lands (RM 37-56).......................................................41 

22. Relationship of opercular radius to total length and length-at-age relationship for 

bluehead sucker in the Animas River ..........................................................................43 

23. Relationship of opercular radius to total length and length-at-age relationship for 

flannelmouth sucker in the Animas River ...................................................................44 

24. Survival of brown trout (BNT), rainbow trout (RBT), and Snake River cutthroat trout 

(SRC) from the Animas River .....................................................................................46 

25. Numbers of brown trout (BNT), rainbow trout (RBT), and Snake River cutthroat trout 

(SRC) stocked by the SUIT during 1996-2007 and CDOW during 1998-2007..........49 

26. Numbers of brown trout stocked in the Animas River compared to population 

estimates for CDOW waters and SUIT waters. ...........................................................51 

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Animas River Fisheries Database July 25, 2008 

LIST OF FIGURES (continued)

Figure Page 

27. Numbers of rainbow trout stocked in the Animas River compared to population 

estimates for CDOW waters and SUIT waters. ...........................................................52 

28. Numbers of Snake River cutthroat trout stocked in the Animas River compared to 

population estimates for CDOW waters and SUIT waters. .........................................53 

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Animas River Fisheries Database July 25, 2008 

EXECUTIVE SUMMARY 

 The U.S. Bureau of Reclamation and the Southern Ute Indian Tribe (SUIT) have 

jointly conducted fisheries studies of the Animas River since 1997. The data collected from 

these studies provide a valuable baseline of information to characterize fish populations and 

to assess the impact of the Animas-La Plata Project (ALP). The ALP will withdraw water 

from the Animas River, and understanding the response by the fish populations to that action 

must rely on precise and accurate data with sufficient sensitivity to reasonably detect changes 

in fish numbers and health. The most common fish species of the Animas River include 

bluehead sucker (Catostomus discobolus, 37%), flannelmouth sucker (Catostomus latipinnis

21%), brown trout (Salmo trutta, 14%), rainbow trout (Oncorhynchus mykiss, 14%), Snake 

River cutthroat trout (Oncorhynchus clarkii sp., 7%), mottled sculpin (Cottus bairdi, 3%), 

and white sucker (Catostomus commersonii, 1%). 

 The Animas River Fisheries Database consists of over 43,000 records stored in 11 

worksheets in a Microsoft Excel platform. Each record (i.e., line of information) contains 30 

column-specific data entries with numeric and alphanumeric information specific to each fish 

captured. This data structure allows for easy access to data and for sorting of data partitions 

and analyses. The data contained in the database were collected through four principal study 

types including trout population estimates, sucker tagging, longitudinal sampling, and larval 

surveys. Data for the first three study types consist of individual fish information (e.g., date 

and location of capture, fish length, weight, tag numbers, etc.) from large-bodied fish 

captured with raft electrofishing. The larval data were collected with beach seines and small 

nets. 

 Trout population estimates were conducted in each of three reaches within the SUIT 

lands. These estimates were based on marking brown trout, rainbow trout, and Snake River 

cutthroat trout with fins punches and resampling the area within a period of 2 weeks to 

recapture marked fish. Petersen-Lincoln estimates showed that rainbow trout had the highest 

densities in all reaches with over 298 fish per mile for fish greater than about 135 mm total 

length (TL). Brown trout density was up to 165 fish per mile for fish greater than about 150 

mm TL, and Snake River cutthroat trout were up to 200 fish per mile for fish greater than 

about 200 mm TL. The precision of these estimates was moderate to good, as measured by 

coefficient of variation (CV = standard deviation/mean). A suitable CV should be less than 

about 0.15. Coefficient of variation ranged from 0.05 to 0.47 for brown trout, 0.13 to 0.43 

for rainbow trout, and 0.05 to 0.88 for Snake River cutthroat trout where numbers of marks 

and recaptures were sufficient for an estimate. The precision of this statistic can be improved 

through a more robust sampling design that focuses on smaller sub-reaches of river. 

 A preliminary analysis was conducted of all the data to determine precision, accuracy, 

and sensitivity to detecting change. Catch rate estimators were computed as the numbers of 

large-bodied fish captured with raft electrofishing per hour of effort. This statistic is often 

used to characterize the status and trends of fish populations, but requires a comprehensive 

analysis of precision and sensitivity to insure that it can reliably be used to document change 

in fish populations with a high degree of confidence (generally 95%). Preliminary analyses 

showed that these catch rate data have a moderate to low level of precision, as measured by 

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Animas River Fisheries Database July 25, 2008 

coefficient of variation (CV = 0.34 to 4.90). This level of precision only allowed for 

detecting a catch rate change of 75% or greater; detection level should be less than about 

40%. These data provide the opportunity to develop a sampling design that will reduce 

variability and yield sufficient precision to detect change. This preliminary analysis also 

showed a low level of accuracy for catch rate data when regressed against simultaneous 

mark-recapture estimators (R2 = 0.05-0.52), indicating that the current catch rate data do not 

reflect true fish abundance. 

 Growth rate and condition of the three trout species were comparable to other healthy 

populations. Relative condition for total length ranged 0.98-1.21 for brown trout, 0.95-1.02 

for rainbow trout, and 0.97-1.00 for Snake River cutthroat trout. This metric of condition 

had high precision and length-weight relationships generally showed an R2

 of >0.90. This 

metric may be one of several that can be used to assess health and condition of the fish under 

different management actions. The variability in weight measurements in the field should be 

reduced through more stringent measurement techniques and reliable weighing procedures. 

 The von Bertalanffy growth parameter L∞ predicted maximum total lengths for 

brown trout, rainbow trout, and Snake River cutthroat trout in the Animas River of 720 mm 

(28 in.), 678 mm (27 in.), and 667 mm (26 in.), respectively. Based on length-weight 

relationships, predicted weights for these lengths are 4,049 g (9 lb.), 2,996 g (7 lb.), and 

3,674 g (8 lb.), respectively. These estimated sizes were reasonable, given that actual 

maximum sizes of brown trout, rainbow trout, and Snake River cutthroat trout from the 

Animas River fisheries database were 680, 646, and 631 mm TL, respectively. 

 The Animas River supports a quality trout fishery with brown trout and rainbow trout 

that reach 5 to 6 years of age. The majority of Snake River cutthroat trout that are stocked in 

the Animas River apparently survive only 1 to 2 years. Survival of brown trout after 1 to 6 

years in the river was 73%, 53%, 38%, 28%, 20%, and 15%, respectively, and survival of 

rainbow trout was 62%, 38%, 24%, 15%, 9%, and 6%, respectively. These are good survival 

rates, but it is noted that few fish live past about 7 years in the river. Proportional stock 

density (PSD, proportion of quality fish >406 mm TL to stock fish 252 mm TL) was high for 

brown trout with a PSD >50 in all years. The PSD for rainbow trout was more variable with 

a maximum of 66, and the PSD for Snake River cutthroat trout was generally below 40. 

Available data do not indicate a negative impact from stocking of trout in the Animas 

River on the riverine populations. Fish length, weight, condition, growth rate, and PSD show 

that the population is healthy and provides a quality fishery. Overall, the SUIT stocked an 

average of 10,956 trout per year over the 12-year period (1996-2007), and the CDOW 

stocked an average of 64,389 trout per year over the 10-year period (1998-2007). Brown 

trout were stocked in 2003 and 2006 as small fish by the SUIT and annually by the CDOW 

from 2000 to 2007. These stockings appear to have had little effect on the natural 

reproducing population in the river which remained relatively stable. Conversely, rainbow 

trout were stocked in nearly all years by the SUIT and every year by the CDOW, and these 

stockings apparently helped to maintain and augment the riverine population which has low 

natural reproduction. Based on changes in abundance after stockings, it appears that 

sufficient numbers of rainbow trout stocked by CDOW moved downstream into SUIT waters 

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Animas River Fisheries Database July 25, 2008 

to augment that population. The Snake River cutthroat trout population declined to zero after 

stocking stopped in the SUIT section in 2001, despite continued stocking in the CDOW 

section through 2004. This species is sustained entirely by stocking, does not appear to 

disperse downstream, and stocked fish live for only 1-2 years after stocking. 

The viability of the flannelmouth sucker and bluehead sucker populations is not well 

understood. The fish appear to be reproducing but levels of survival and recruitment are 

unknown. Furthermore, two irrigation diversion dams impede movement of fish and may be 

affecting viability of these upstream populations. These diversion dams have been in place 

since the 1950’s and the persistence of native sucker populations upstream is strong evidence 

that successful reproduction and recruitment continues. However, the effect of these dams—

as well as future management of river flows—to the long-term viability of these populations 

is inconclusive. An age-specific recruitment model is necessary to assess the viability of 

these populations. This will provide an opportunity to simulate population trajectories under 

different survival, recruitment, and movement scenarios. 

 A modification of the current sampling design is recommended that will reflect the 

information needs of managers when evaluating the impacts of the ALP. Discussions should 

be held among managers, fish biologists, and statisticians to identify desired precision and 

accuracy of data, as well as the sensitivity of the data to detect change, and the level of 

acceptable risk. The appropriate monitoring plan should be structured to address the 

information needs at the specified levels of precision, accuracy, sensitivity, and risk. 

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Animas River Fisheries Database July 25, 2008 

1.0 INTRODUCTION 

1.1 Background 

 The U.S. Bureau of Reclamation (Reclamation) is presently constructing the 

Animas-La Plata Project (ALP), a water project that will withdraw water from the 

Animas River in southwestern Colorado (Figure 1). The Southern Ute Indian Tribe 

(SUIT) manages a valuable trout fishery on the Animas River through tribal lands 

downstream of the ALP pumps near Durango, Colorado. This fishery is comprised of 

rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), and Snake River 

cutthroat trout (Oncorhynchus clarkii sp.). 

The Animas River upstream of SUIT lands is also managed as a gold medal trout 

fishery by the Colorado Division of Wildlife (CDOW). The Animas River downstream 

of Durango also contains populations of bluehead sucker (Catostomus discobolus), 

flannelmouth sucker (Catostomus latipinnis), and roundtail chub (Gila robusta). These 

native fish are conservation species under a 2004 Range-Wide Conservation Agreement 

among the states of Arizona, Colorado, Nevada, New Mexico, Utah, and Wyoming. 

Information on fish of the Animas River is a necessary and valuable baseline for 

assessing impacts of water depletions by the Animas-La Plata Project. 

Figure 1. Principal locations along the Animas River in Colorado and New Mexico. 

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Animas River Fisheries Database July 25, 2008 

1.2 Purpose and Objectives 

 

 This is Phase I of a synthesis and analysis of the Animas River Fisheries Database 

which was compiled by the SUIT and Reclamation since 1997 (Whiteman 1999, 2000, 

2002; Zimmerman 2003, 2005). The purpose of this phase is to assimilate and synthesize 

the fisheries data for the Animas River in electronic format, and to provide a preliminary 

analysis of those data. Phase II will be conducted to provide more detailed and directed 

analyses that will help the SUIT and Reclamation develop a reliable, precise and 

sufficiently sensitive monitoring program for fish populations of the Animas River. 

The objectives of Phase I are to: 

1. Develop an accurate and reliable electronic database for fish in the Animas 

River from data collected by the SUIT and Reclamation for use as baseline 

data for future impacts evaluation of the ALP; 

2. Estimate annual population size of rainbow trout, brown trout, and Snake 

River cutthroat trout using mark-recapture estimators and compare across the 

years of data collection to assess trends in fish populations of the Animas 

River through SUIT lands; 

3. Estimate annual catch rates of brown trout, rainbow trout, Snake River 

cutthroat trout, bluehead sucker, and flannelmouth sucker and compare years 

to assess trends in fish populations of the Animas River through SUIT lands; 

4. Characterize length-weight and age-growth of rainbow trout, brown trout, and 

Snake River cutthroat trout using length-frequency analysis; 

5. Characterize length-weight and age-growth of flannelmouth sucker and 

bluehead sucker using length-frequency analysis and aging of opercles; 

6. Evaluate the effect of stocking on trout populations in the Animas River and 

evaluate species and numbers of fish stocked and their survival. 

7. Evaluate data suitability, utility, strengths and weaknesses, and data gaps for 

the Animas River Fisheries Database; 

8. Provide insight into a fish monitoring strategy for the Animas River to 

evaluate effects of the ALP. 

1.3 Study Area 

The Animas River originates in the San Juan Mountains of southwestern 

Colorado and flows into the San Juan River in northwestern New Mexico. Fish 

collections for this database were taken from a 56-mile reach of the Animas River from 

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Animas River Fisheries Database July 25, 2008 

the northern boundary of the Southern Ute Indian Reservation to the confluence of the 

San Juan River near Farmington, New Mexico (Figure 1). 

Locations along the Animas River are designated by river miles upstream of the 

San Juan River confluence (Table 1). The Animas River flows for 19.1 miles within 

SUIT lands from just downstream of Purple Cliffs (RM 56.2) to the Colorado-New 

Mexico state line (RM 37.1). 

The Animas River within the SUIT lands was divided into four reaches for the 

purposes of sampling fish. These reaches include: 

1. Reach 1: Purple Cliffs (RM 56.4) to Basin Creek (RM 52.1), 

2. Reach 2: Basin Creek (RM 52.1) to Weaselskin Bridge (49.5), 

3. Reach 3: Weaselskin Bridge (RM 49.5) to Bondad (RM 41.9), and 

4. Reach 4: Bondad (RM 41.9) to Colorado Stateline (RM 37.1). 

Purple Cliffs is located just upstream of the northern boundary of the SUIT lands, 

and Basin Creek is the tributary that drains Ridges Basin and will be used to return water 

released from Nighthorse Reservoir, the major storage unit for ALP. The Animas River 

flows beneath Weaselskin Bridge at 214 Road and Highway 550 crosses the river at 

Bondad. The southern boundary of the SUIT lands is the same location on the Animas 

River as the Colorado-New Mexico state line. 

Table 1. Key locations along the Animas River indicated by river miles upstream from the 

confluence of the San Juan River near Farmington, New Mexico. 

Location River Mile 

CDOW “Animas 1” Reach (Durango HS footbridge)—upper end 62.2 

CDOW “Animas 1” Reach (9th Street Bridge)—lower end 60.6 

CDOW “Animas 2” Reach (BMX Park) —upper end 58.5 

CDOW “Animas 2” Reach (High Bridge) —lower end 57.4 

Purple Cliffs 56.4 

Northern Boundary—Southern Ute Indian Reservation 56.2 

Basin Creek 52.1 

Weasel Skin Bridge 49.5 

Spring Access 45.7 

Bondad 41.9 

Southern Boundary—Southern Ute Indian Reservation 37.1 

Farmer’s Ditch Diversion 21.9 

Aztec—Highway 550 Bridge (Riverside Park) 17.7 

Farmington Lake Diversion 11.9 

Flora Vista 9.2 

San Juan River confluence 0.0 

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Animas River Fisheries Database July 25, 2008 

2.0 METHODS 

2.1 Database Development and Quality Control 

An electronic database was transferred from the SUIT to SWCA that included 

data collected from 1997 to 2005 and 2007. This electronic database was quality checked 

for data accuracy and consistency. All codes were evaluated and numerical data were 

screened for aberrant entries. Aberrancies were cross checked against field data sheet to 

insure that the correct data had been entered. A clean master fishery database was 

provided to the SUIT and Reclamation for use in a baseline evaluation of the ALP. 

2.2 Estimates of Abundance 

Numbers of brown trout, rainbow trout, and Snake River cutthroat trout were 

estimated using the following Chapman modification of the Petersen-Lincoln estimator 

(Seber 1982; Hayes et al. 2007): 

Ñ = (n1 + 1) (n2 + 1) – 1 

(m2 + 1) 

Where: Ñ = population estimate, 

 n1 = number of fish caught, marked, and released in pass 1, 

n2 = total number of fish caught in pass 2, and 

m2 = number of marked fish that are recaptured in pass 1. 

The variance of this estimator was approximated as (Seber 1982; Hayes et al. 

2007): 

(n1 + 1) (n2 + 1) (n1 – m2) (n2 – m2) V(Ñ) = – 1 

(m2 + 1)2

 (m2 + 2) 

 All trout mark-recapture data were short-term (less than 2 weeks between mark 

and recapture events). Variances estimators and 95% confidence intervals were 

computed for each estimate. Mark-recapture data provided by the CDOW were also used 

in the same estimator model to standardize fish population estimates and to make all 

estimates comparable. Stocking records were procured from the SUIT and CDOW to 

evaluate the effect of stocking on trout populations. 

Mark-recapture data from bluehead sucker and flannelmouth sucker were not 

reconciled for this phase of the project but will be used for population estimates in Phase 

II. Catch rates of large-bodied bluehead sucker and flannelmouth sucker were estimated 

as numbers of fish per hour of electrofishing. Variances estimators and 95% confidence 

intervals were computed for each average catch rate. Numbers and percentage 

composition of flannelmouth sucker, bluehead sucker, and roundtail chub were computed 

for each year using the catch rate index (i.e., number of fish per hour of electrofishing). 

Coefficient of variation (CV) was computed for catch rate indices and for population 

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Animas River Fisheries Database July 25, 2008 

estimators as measures of precision for each statistic. The CV for catch rate indices was 

computed as standard deviation divided by the mean, whereas CV for population 

estimators was computed as standard error divided by the estimate. 

Proportional stock density (PSD) was assessed for brown trout, rainbow trout, and 

Snake River cutthroat trout according to the following (Neumann and Allen 2007): 

PSD = (Pq/Ps) x 100 

 Where: Pq = number of fish ≥ quality length; and 

 Ps = number of fish ≥ stock length. 

 Quality length was considered as fish ≥ 406 mm TL (16 in.) as used by the SUIT 

in their angler regulations (i.e., bag and possession limit is 2 trout ≥ 16 in.; all other fish 

must be released alive). Stock length was considered as ≥ 252 mm TL, or 10 in. which 

was the most common size of fish stocked. 

2.3 Length-Weight and Condition 

The relationship of length to weight of fish is important for assessing the health 

and condition of individuals and the population. The following power function was used 

to express the relationship of fish length to weight (Pope and Kruse 2007): 

W = aLb

 Where: W = predicted weight, 

 L = fish total length, 

 a = coefficient that describes the Y intercept, 

 b = rate of increase. 

 The above power function was log transformed to the following to facilitate 

computation of predicted weights and to express the relationship in the standard format: 

log W = log a + b log L 

An important metric for assessing condition or health of a fish is condition factor 

taken as the relationship of total length to weight (KTL). The coefficients ‘a’ and ‘b’ 

derived from the above length-weight relationship for each species were used to express 

the metric of “relative condition” as follows: 

KTL = W/aLb

 Where: (KTL) = relative condition based on total length, 

 W = fish weight, 

 a = coefficient ‘a’ derived from length-weight relationship, and 

 b = coefficient ‘b’ derived from length-weight relationship. 

5
Animas River Fisheries Database July 25, 2008 

 A relative condition of 1.00 indicates that the fish included in the analysis comply 

with a fundamental principle of fish growth, the “cube law,” in which the weight of a fish 

is the cube of its length; e.g., if a fish 1 foot long weighs 1 pound, a fish 2 feet long is 

expected to weigh 8 pounds (23

 = 8). Clearly, adherence to this principle is speciesspecific and varies considerably with body shape and growth characteristics. 

Nevertheless, most trout species conform to the “cube law” and trout in good condition 

are expected to have a condition index of 1.00 or better (Carlander 1969). Adherence to 

this principle is also reflected as a ‘b’ coefficient of 3.00 in the length-weight 

relationship. 

2.4 Age and Growth 

Age information was not available from scales or otoliths of trout from the 

Animas River and age of trout could only be surmised from length-frequency analysis. 

Length-frequency analysis and modal progression were used to estimate growth with the 

aid of the routine ELEFAN (Electronic Frequency Analysis) in the program FiSAT II 

(Gayanilo et al. 2005). ELEFAN uses subsequent length modes to develop the following 

von Bertalanffy growth function (VBGF): 

L(t) = L∞ * [1 - exp (-K*(t-to))] 

 Where: L∞ = the maximum predicted size of fish, 

 K = rate of growth at which fish approach L∞, 

 to = time in years at which fish length in zero, and 

 t = time in years to for length L(t). 

The VBGF provides an estimate of the rate of growth (K) and the maximum 

possible length of a fish from the population sampled (L∞). Understanding this 

relationship over time is valuable for assessing the effect of management actions on the 

rate of growth of individuals in the population. 

Unlike trout species, length-frequency analysis could not be used to characterize 

age composition of flannelmouth sucker and bluehead sucker because of overlapping 

lengths of same age fish. Opercles from flannelmouth sucker and bluehead sucker were 

examined to develop length-at-age relationships and estimated mean length and growth 

of each cohort (Scoppettone et al. 1986). Carlander’s third degree polynomial was used 

to describe the relationship of fish length to opercular radius (Lagler 1956; Chugunova 

1963; Bagenal and Tesch 1978) as: 

TL = Ax + Bx2

 + Cx3 + a 

 Where: 

 TL = total length, 

 A, B, C = coefficients, 

 x = opercular radius, and 

 a = y-intercept. 

6
Animas River Fisheries Database July 25, 2008 

 The above relationship assumes that the y-intercept (a) represents fish size at time 

of formation of the opercular bone. The y-intercept was specified for bluehead sucker at 

14 mm TL and for flannelmouth sucker at 15 mm TL, based on the size of the fish at the 

mesolarval stage of development (Snyder and Muth 1990). 

2.5 Survival 

Tag data were insufficient for computing surviving of either the trout species or 

the sucker species. Therefore, Bhattachayra’s modal progression in FiSAT II (Gayanilo 

et al. 2005) was used to estimate survival by tracking mean length of age group modes 

from the same cohort over several years. The survival function applied was the Beverton 

and Holt model (1957) that generates Ricker ‘Z’ from progression of modal mean 

lengths. The basic Ricker survival model (Ricker 1975) is expressed as: 

Nt = Noe

-Zt

 Where: Nt = individuals in the population at time ‘t’; 

 No = estimated number of individuals at start of time ‘t’; and 

 Z = instantaneous mortality rate, slope of line. 

2.6 Data Evaluation 

An evaluation was provided of how suitable the data are for evaluation of project 

effects, its usefulness, strengths and weaknesses, as well as data or information gaps that 

are necessary for a more effective project evaluation. Insight was provided for a longterm fish monitoring strategy of the Animas River to evaluate effects of the Animas-La 

Plata Project on native and sport fish. 

7
Animas River Fisheries Database July 25, 2008 

3.0 RESULTS 

3.1 Schedule of Studies 

The Animas River fisheries database consists of data collected primarily for four 

study types, including (1) trout population estimates, (2) larval surveys, (3) sucker 

tagging, and (4) longitudinal sampling (Table 2). Trout population estimates were done 

every other year in 2001, 2003, 2005, and 2007. Larval sampling was done annually in 

2000, 2001, and 2002. Sucker tagging was started in 2001 and done annually to 2005, 

and longitudinal sampling was done in 1997, 1998, and 1999. 

Table 2. Schedule of primary studies for the Animas River fisheries. 

Year Trout 

Population Larval Survey Sucker 

Tagging Longitudinal 

1997 X 

1998 X 

1999 X 

2000 X 

2001 X X X 

2002 X X 

2003 X X 

2004 X 

2005 X X 

2006 No sampling 

2007 X 

3.2 Database Development 

All field data were electronically stored in Excel worksheets and preliminarily 

quality controlled and checked by the SUIT. Additional quality control was performed 

during this phase and data errors were reconciled, where possible. 

The Animas River Fisheries Database is stored in an Excel platform in columnspecific format consisting of unique numeric and alphanumeric codes. The database file 

is entitled “MASTER-Animas Fish Data.” Three additional files are provided that 

contain the analyses performed for this data evaluation. These files are entitled 

“ANIMAS DATA 1 of 2,” “ANIMAS DATA 2 of 2,” and “ANIMAS ANALYSIS.” 

Table 3 provides a description of each of these files. 

The Master database contains all of the cleaned data without any analyses. 

Animas data files 1 and 2 are for a portion of the years with basic data summaries, lengthfrequency analyses, and length-weight analyses for each year. The Animas analysis file 

contains more advanced and comprehensive analyses that span across years. 

8
Animas River Fisheries Database July 25, 2008

9

Table 3. Description of each of the data files of the Animas River Fisheries Database. 

Data File Description 

MASTER-Animas Fish Data Master database, 11 worksheets: data for 1997, 1998, 1999, 2001, 

2002, 2003, 2004, 2005, 2007, 2000-Larva, and Floy Tags 

ANIMAS DATA 1 of 2 7 worksheets: data and analyses for 1997, 1998, 1999, 2001, 

2002, 2000-Larva, and Floy Tags (duplicate of Master database) 

ANIMAS DATA 2 of 2 4 worksheets: data analyses for 2003, 2004, 2005, 2007 (duplicate 

of Master database) 

ANIMAS ANALYSIS Summary analyses for all datasets; 11 worksheets 

The master database file contains separate worksheets for each year of data 

collection, including 1997 to 2005 and 2007. No fisheries data were collected in 2006. 

The numbers of records (i.e., lines of data) in each worksheet are shown in Table 4. Each 

record is populated with column specific numeric or alphanumeric information consistent 

with pre-determined codes or measurement ranges. Missing data are designated by blank 

cells. 

Table 4. Number of records (lines of data) in each worksheet of “MASTER-Animas Fish Data.” 

Spreadsheet Number of Records 

1997 3,005 

1998 2,479 

1999 6,095 

2000 153 

2001 3,410 

2002 1,526 

2003 10,082 

2004 7,247 

2005 6,275 

2006 No Sampling 

2007 2,949 

Total: 43,221 

Column designations and field names for MASTER-Animas Fish Data are 

provided in Table 5. Each record consists of 30 columns of data, designated by field 

names that relate to the type of data entered (e.g., Date, Type, etc.). The descriptions 

provided in Table 5 help to explain the type of information provided in the particular 

column. Each data entry is a unique code or measurement. Codes are predetermined so 

that files can be easily searched, and measurements are entered within specified ranges. 

Codes and measurements were originally recorded in the field on paper data sheets and 

entered electronically into the specified data files. 

Each data entry was quality controlled by checking against the field data sheets 

and then through code and range checks which searched each file for unacceptable codes 

or data entries outside of the specified range. Additional quality control was conducted 

during data analysis when aberrant observations were checked against data sheets.
Animas River Fisheries Database July 25, 2008 

Table 5. Column designations and field names for MASTER-Animas Fish Data. 

Column Field Names Description Codes or Measureme

A Date Month, day, and year of sample. MM/DD/YYYY; e.g., 09/21/1998 

B Type Type of Sampling 

T-POP=Trout Population Estimate, LARTAG=Sucker tagging, LONG=Longitud

C Sample

Contains 2 digit year, 3 letters for river and 2 digits for sample number. e.g., 01ANI03 = 2001, Animas River, S

D RM Approximate river mile of sampled fish Measured upstream from San Juan Rive

E RO RM Round-off river mile Rounded off for purposes of data blocki

F Species 3 letter species code See Table 5 for species codes 

G TL_mm Total length measured in millimeters 1 to 999 H Wt_g Weight of fish measured in grams 1 to 9,999 

I SL_mm Standard length measured in millimeters 1 to 999 

J Count Fish not measured or weighted - count in number 1 to 999 

K Reach Specific to trout 2-pass mark recapture. 

1 = Purple Cliffs to Basin Creek, 2 = BaWeaselskin Bridge, 3 = Weaselskin BridBondad to Colorado Stateline 

L Pass Specific to trout 2-pass mark recapture. Pass 1 = mark pass, Pass 2 = recapture p

M Status Applies to both trout and native suckers. 

M=marked trout, C=captured trout-not mtrout, new= new FLOY Tag, recap=reca

N Lat. Latitude e.g., 36.78143 O Long. Longitude e.g., -108.09953 

P Northing UTM Northing in zone 13 e.g., 4075118 

Q Easting UTM Easting in zone 13 e.g., 223390 

R Elec_min Electro-fishing minutes 1 to 999 

S Tag_num FLOY tag number 1 to 9,999 

T Tag_Color FLOY tag color B = blue, G = green, Y = yellow 

U Tag_Type Type of tag F = Floy, P = PIT, C = coded wire tag V Tag_Info FLOY tag information 

Responsible party; USBOR = Bureau of Southern Ute Indian Tribe 

W Comments Comments related to specific fish sampled Text comments 

10
Animas River Fisheries Database July 25, 2008 

11

Column Field Names Description Codes or Measureme

X Sex Sex of specific fish sampled 

M = male, F = female, m/milt = male wfemale with eggs, TVB male = tubercled tubercled female 

Y fsample Field sample number 1 to 99 

Z Boat Electro-fishing rafts. SUIT = Southern Ute Raft, BOR = Recl

AA GPS GPS waypoint number 1 to 99 

AB Mile Range of sample measured in river miles 0 to 99 AC Report_Sample Specific only to 2001 data 1.1 to 2.4 

AD Location Description of specific location of sampled fish Text description 
Animas River Fisheries Database July 25, 2008 

3.3 Fish Species Composition 

Large-bodied fish were collected with raft electrofishing in the Animas River 

from Purple Cliffs to the confluence of the San Juan River, and species composition was 

determined from the pool of all years of sampling (1997-2005, 2007). Bluehead sucker 

(37.12%) and flannelmouth sucker (20.99%) dominated fish composition (Figure 2, 

Table 6) followed by brown trout (14.32%), rainbow trout (13.72%), and Snake River 

cutthroat trout (6.59%). 

Raft electrofishing captured primarily large-bodied fish and species composition 

does not include young of any species nor does it appropriately represent small-bodied 

forms such as mosquitofish, mottled sculpin, or speckled dace. These small-bodied 

forms are undoubtedly important to the Animas River ecosystem and their importance 

should not be discounted. Other sampling gears or methods should be employed to better 

account for these small-bodied fishes. 

Figure 2. Fish composition of the Animas River between Purple Cliffs and the confluence of the 

San Juan River, 1997-2007 (see Table 6 for species codes; MISC = miscellaneous). 

12
Animas River Fisheries Database July 25, 2008

13

Table 6. Number and proportions of fish species captured in the Animas River between Purple 

Cliffs and the confluence of the San Juan River, 1997-2007.

Common Name Scientific Name Code Number Percent 

Black bullhead Ictalurus melas BBH 185 0.43 

Bluehead sucker Catostomus discobolus BHS 15,976 37.12 

Bluehead x white hybrid BW 168 0.39 

Bluehead x flannelmouth BF 15 0.03 

Brown trout Salmo trutta BNT 6,162 14.32 

Channel catfish Ictalurus punctatus CHC 2 0.00 

Colorado pikeminnow Ptychocheilus lucius CPM 3 0.01 

Common carp Cyprinus carpio CCP 276 0.64 

Cutthroat x rainbow hybrid CB 175 0.41 

Fathead minnow Pimephales promelas FHM 105 0.24 

Flannelmouth sucker Catostomus latipinnis FMS 9,033 20.99 

Flannelmouth x white hybrid FW 159 0.37 

Green sunfish Lepomis cyanellus GSF 15 0.03 

Largemouth bass Micropterus salmoides LMB 86 0.20 

Mosquitofish Gambusia affinis GAM 28 0.07 

Mottled sculpin Cottus bairdi MSC 1,194 2.77 

Rainbow trout Oncorhynchus mykiss RBT 5,904 13.72 

Roundtail chub Gila robusta RTC 4 0.01 

Snake River cutthroat trout Oncorhynchus clarki SRC 2,837 6.59 

Speckled dace Rhinichthys osculus SPD 220 0.51 

Sucker-unidentified Catostomus sp. SU 3 0.01 

White sucker Catostomus commersonii WHS 488 1.13 

Grand Total 43,038 100.00 

 Of the 17 species captured in the Animas River between Purple Cliffs and the 

confluence of the San Juan River (about 91 km), only six were native, including bluehead 

sucker, Colorado pikeminnow, flannelmouth sucker, mottled sculpin, roundtail chub, and 

speckled dace. Only four roundtail chub were caught in 10 years of sampling, and the 

population in the Animas River is alarmingly low. The Colorado pikeminnow is an 

endangered species and three were captured on July 15, 2004 at RM 1.9 and 2.3. The 

fish were 226, 239, and 246 mm TL and had been previously marked as part of the U.S. 

Fish and Wildlife Service reintroduction program of Colorado pikeminnow in the San 

Juan River near Farmington. These fish had apparently moved upstream into the Animas 

River following their release in the San Juan River. 

 The remaining 11 fish species were nonnative forms that have been introduced 

into the Animas River by various means. Brown trout, rainbow trout, and Snake River 

cutthroat trout are introduced regularly as game fish. Channel catfish, largemouth bass, 

and green sunfish have been distributed variously for many years as game fish and are 

found primarily in the San Juan River and below the Farmington Lake Diversion. 

Fathead minnow and mosquitofish are common throughout the San Juan River Basin and 

the white sucker has established populations in various tributaries and occurs as small 

concentrations in the Animas River, where they hybridize with native suckers and where 

hybrid swarms are becoming evident.
Animas River Fisheries Database July 25, 2008 

14

Table 7. Number (No.) and percentage (Per.) of fish species captured in the Animas River, 1997-2007 (see Table 6 for spec

1997 1998 1999 2001 2002 2003 2004 2

Species No. Per. No. Per. No. Per. No. Per. No. Per. No. Per. No. Per. NBBH 0 0.00 0 0.00 0 0.00 2 0.06 1 0.07 1 0.01 181 2.50 BF 0 0.00 0 0.00 6 0.10 1 0.03 2 0.13 4 0.04 0 0.00 

BHS 228 7.59 702 28.32 2132 35.14 893 26.19 616 40.37 2747 27.25 6537 90.20 1

BNT 1048 34.88 381 15.37 849 13.99 716 21.00 8 0.52 1168 11.59 34 0.47 

BW 0 0.00 2 0.08 63 1.04 10 0.29 8 0.52 58 0.58 6 0.08 

CB 0 0.00 1 0.04 87 1.43 85 2.49 0 0.00 0.00 0 0.00 

CCP 4 0.13 9 0.36 36 0.59 26 0.76 68 4.46 63 0.63 6 0.08 

CPM 0 0.00 0 0.00 0 0.00 0 0.00 0 0.00 0 0.00 3 0.04 

CHC 0 0.00 0 0.00 0 0.00 0 0.00 1 0.07 1 0.01 0 0.00 

FHM 0 0.00 0 0.00 0 0.00 0 0.00 102 6.68 3 0.03 0 0.00 

FMS 44 1.46 282 11.38 1235 20.35 483 14.16 448 29.36 4068 40.36 449 6.20 1

FW 0 0.00 3 0.12 43 0.71 17 0.50 0 0.00 60 0.60 2 0.03 

GAM 0 0.00 0 0.00 0 0.00 0 0.00 28 1.83 0.00 0 0.00 

GSF 0 0.00 0 0.00 0 0.00 4 0.12 11 0.72 0.00 0 0.00 

LMB 0 0.00 0 0.00 0 0.00 17 0.50 69 4.52 0.00 0 0.00 

MSC 73 2.43 0 0.00 453 7.47 75 2.20 4 0.26 173 1.72 6 0.08 

RBT 509 16.94 726 29.29 532 8.77 222 6.51 37 2.42 1452 14.41 10 0.14 1

RTC 0 0.00 0 0.00 2 0.03 0 0.00 2 0.13 0.00 0 0.00 

SPD 1 0.03 0 0.00 6 0.10 50 1.47 71 4.65 34 0.34 6 0.08 

SRC 1087 36.17 320 12.91 500 8.24 777 22.79 6 0.39 123 1.22 0 0.00 SU 0 0.00 0 0.00 0 0.00 0 0.00 3 0.20 0.00 0 0.00 

WHS 11 0.37 53 2.14 124 2.04 32 0.94 41 2.69 124 1.23 7 0.10 

Grand Total 3005 100.00 2479 100.00 6068 100.00 3410 100.00 1526 100.00 10079 100.00 7247 100.00 6
Animas River Fisheries Database July 25, 2008 

3.4 Trout Population Size 

3.4.1 Population Estimates—SUIT Waters 

Population sizes of brown trout and rainbow trout were estimated in Reach 1 

(Purple Cliffs to Basin Creek), Reach 2 (Basin Creek to Weasel Skin Bridge), and Reach 

3 (Weasel Skin Bridge to Bondad) for 1997, 2001, 2003, 2005, and 2007. Numbers of 

Snake River cutthroat trout could not be estimated for Reach 3 in 2005 and for all reaches 

in 2007 because of small numbers of fish captured and recaptured. The last stocking of 

Snake River cutthroat trout in the Animas River was in 2001. Population estimates for 

brown trout were the most precise of the three trout species (Table 8, Figure 4). 

Coefficient of variation ranged from 0.05 to 0.47 where the numbers of marks were 

sufficient for reliable estimates. Precision of population estimates for rainbow trout 

ranged from 0.13 to 0.43 where numbers of marks were sufficient, and precision for 

Snake River cutthroat trout ranged from 0.05 to 0.88. Additional analyses of fish density 

and biomass will be computed for trout in Phase II. 

Average numbers of brown trout per mile for all years in SUIT Reaches 1, 2, and 

3 were 204, 186, and 139, respectively (Figure 3). Average numbers of rainbow trout per 

mile for all years in Reaches 1, 2, and 3 were 345, 454, and 118, respectively. Average 

numbers of Snake River cutthroat trout per mile for all years in Reaches 1, 2, and 3 were 

38, 317, and 49, respectively. 

Figure 3. Average number of brown trout (BNT), rainbow trout (RBT), and Snake River cutthroat 

trout (SRC) per mile for all years in three reaches of the Animas River within SUIT lands. 

15
Animas River Fisheries Database July 25, 2008 

Table 8. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from SUIT mark-recapture data. n1 = number of fish marked and released 

in pass 1, m2 = number of marked fish recaptured in pass 2, n2 = total number of fish captured in 

pass 2, N-hat = population estimate with Chapman modification of Petersen-Lincoln estimator, 

Var N-hat = variance of population estimate, 95% CI = 95% confidence interval, p-hat = capture 

probability, SE = standard error, CV = coefficient of variation. 

Year 

Reach 1 - Purple Cliffs to 

Basin Creek (RM 56.4-

52.1 = 4.3 mi) 

Reach 2 - Basin Creek to 

Weasel Skin Bridge (RM 

52.1-49.5 = 2.6 mi) 

Reach 3 - Weasel Skin Bridge 

to Bondad (RM 49.5-41.9 = 

7.6 mi) 

1997 BNT RBT SRC BNT RBT SRC BNT RBT SRC 

n1 149 128 47 45 21 115 226 70 305

m2 26 23 9 4 2 7 52 12 73

n2 205 178 79 55 27 140 368 84 401

N-hat 1143 961 383 514 204 2044 1579 463 1661

Var N-hat 33329 26097 9285 35908 8127 407877 30362 10651 22791

95% CI 358 317 189 371 177 1252 342 202 296

p-hat 0.13 0.13 0.11 0.07 0.07 0.05 0.14 0.14 0.18

SE 225.19 205.49 201.84 816.10 406.34 3577.87 134.94 154.36 74.97

CV 0.20 0.21 0.53 1.59 1.99 1.75 0.09 0.33 0.05

2001 BNT RBT SRC BNT RBT SRC BNT RBT SRC 

n1 108 54 29 93 15 327 145 23 27

m2 17 3 5 17 2 39 23 7 14

n2 140 55 48 67 18 244 150 39 93

N-hat 853 769 244 354 100 2008 918 119 174

Var N-hat 27943 102101 6019 3945 1755 72323 23720 852 750

95% CI 328 626 152 123 82 527 302 57 54

p-hat 0.12 0.05 0.10 0.25 0.11 0.16 0.15 0.18 0.15

SE 261.15 1926.43 214.96 42.88 125.39 221.85 164.72 38.74 28.84

CV 0.31 2.51 0.88 0.12 1.25 0.11 0.18 0.33 0.17

2003 BNT RBT SRC BNT RBT SRC BNT RBT SRC 

n1 188 291 36 64 205 17 132 228 12

m2 47 36 7 15 31 4 22 24 1

n2 198 333 35 70 187 16 96 115 5

N-hat 783 2635 166 287 1209 60 560 1062 38

Var N-hat 7092 141983 1877 2857 31108 317 8270 30346 285

95% CI 165 739 85 105 346 35 178 341 33

p-hat 0.24 0.11 0.20 0.21 0.17 0.25 0.23 0.21 0.20

SE 37.93 489.60 53.62 45.35 152.49 19.83 63.13 133.68 25.91

CV 0.05 0.19 0.32 0.16 0.13 0.33 0.11 0.13 0.68

2005 BNT RBT SRC BNT RBT SRC BNT RBT SRC 

n1 157 234 7 36 201 3 117 194 1

m2 34 41 1 3 13 0 12 34 1

n2 207 358 5 43 182 2 106 273 4

N-hat 938 2008 23 406 2639 11 970 1526 4

Var N-hat 15856 68046 95 26861 399483 35 52670 46329 -1

95% CI 247 511 19 321 1239 12 450 422 --

p-hat 0.16 0.11 0.20 0.07 0.07 0.00 0.11 0.12 0.25

SE 101.64 292.04 15.83 767.46 1997.42 17.50 454.05 240.05 --

CV 0.11 0.15 0.69 1.89 0.76 1.59 0.47 0.16 --

16
Animas River Fisheries Database July 25, 2008 

Table 8. Continued 

Year 

Reach 1 - Purple Cliffs to 

Basin Creek (RM 56.4-

52.1 = 4.3 mi) 

Reach 2 - Basin Creek to 

Weasel Skin Bridge (RM 

52.1-49.5 = 2.6 mi) 

Reach 3 - Weasel Skin Bridge 

to Bondad (RM 49.5-41.9 = 

7.6 mi) 

2007 BNT RBT SRC BNT RBT SRC BNT RBT SRC

n1 103 110 0 70 109 0 229 139 0

m2 22 15 0 5 9 0 61 20 0

n2 147 149 1 71 158 0 341 199 1

N-hat 668 1040 1 851 1748 0 1268 1332 1

Var N-hat 12274 48702 -1 87025 236909 -1 15278 61474 -1

95% CI 217 433 -- 578 954 -- 242 486 --

p-hat 0.15 0.10 0.00 0.07 0.06 -- 0.18 0.10 0.00

SE 120.33 446.81 1.00 1261.23 2193.60 1.00 67.01 445.46 1.00

CV 0.18 0.43 1.00 1.48 1.25 -- 0.05 0.33 1.00

The sums of estimates for all three reaches, averaged for the 5 years were: brown 

trout (2,419), rainbow trout (3,563), and Snake River cutthroat trout (1,364). An estimate 

by Nehring (1992) for brown trout (2,421) from Purple Cliffs to the New Mexico state 

line was strikingly similar to the average annual sum estimate from this analysis, but 

Nehring’s estimate of 512 rainbow trout was considerably lower than this estimate of 

3,563. In 1994, Miller et al. (1995) provided Lincoln-Peterson mark-recapture estimates 

of brown trout, rainbow trout, and Snake River cutthroat trout for approximately the same 

reaches of river. The sums of those estimates were brown trout (329), rainbow trout 

(277), and Snake River cutthroat trout (62). There is a wide disparity between the 

estimates of Nehring (1992) and the present estimates when compared with those of 

Miller et al. (1995) that cannot be explained. Further analysis is needed of the data 

collected by Nehring (1992) and Miller et al. (1995) together with the present database. 

This will require incorporating all data into electronic format to facilitate analysis. 

17
Animas River Fisheries Database July 25, 2008 

Figure 4. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from SUIT mark-recapture data in Reaches 1 (top), 2 (center), and 3 

(bottom) within SUIT lands. Vertical bars are 95% confidence intervals around the mean. 

18
Animas River Fisheries Database July 25, 2008 

3.4.2 Population Estimates—CDOW Waters 

Population sizes were also estimated for brown trout, rainbow trout, and Snake 

River cutthroat trout from mark-recapture data collected by CDOW from two reaches 

near the City of Durango and upstream from SUIT lands (Table 9, Figure 5). Animas #1 

represents the ‘gold medal’ section of the Animas River, and Animas #2 is a section with 

standard fishing regulations. Estimates of brown trout and rainbow trout were possible in 

both sections for 2002, 2004, and 2006, but estimates for Snake River cutthroat trout 

were not possible for Animas #1 in 2004 and 2006 because of small numbers of fish 

captured. The CV of these estimates for all three trout species ranged from 0.11 to 0.82. 

Table 9. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from CDOW mark-recapture data. n1 = number of fish marked and released 

in pass 1, m2 = number of marked fish recaptured in pass 2, n2 = total number of fish captured in 

pass 2, N-hat = population estimate with Chapman modification of Petersen-Lincoln estimator, 

Var N-hat = variance of population estimate, 95% CI = 95% confidence interval, p-hat = capture 

probability, SE = standard error, CV = coefficient of variation. 

Year Animas #1 - BMX park to high bridge Animas #2 - DHS footbridge to 9th St. bridge 

2002 BNT RBT SRN BNT RBT SRC 

n1 148 176 22 105 205 26

m2 19 24 5 13 12 2

n2 92 103 15 95 130 2

N-hat 692 735 60 726 2075 26

Var N-hat 15534 13602 247 26110 259754 -1

95% CI 244 229 31 317 999 --

p-hat 0.21 0.23 0.33 0.14 0.09 1.00

SE 105.67 77.72 11.77 251.06 1273.30 -0.04

CV 0.15 0.11 0.20 0.35 0.61 0.00

2004 BNT RBT SRC LOC RBT SRC 

n1 88 118 11 105 160 0

m2 11 16 0 11 16 0

n2 69 110 10 72 142 0

N-hat 518 776 -- 644 1353 --

Var N-hat 14862 24345 -- 23701 80301 --

95% CI 239 306 -- 302 555 --

p-hat 0.16 0.15 -- 0.15 0.11 --

SE 170.83 208.08 -- 227.89 505.04 --

CV 0.33 0.27 -- 0.35 0.37 --

2006 BNT RBT SRC BNT RBT SRC 

n1 62 83 0 111 206 0

m2 15 10 0 9 24 0

n2 85 124 0 106 180 0

N-hat 338 954 -- 1197 1498 --

Var N-hat 4095 60179 -- 107790 65461 --

95% CI 125 481 -- 643 501 --

p-hat 0.18 0.08 -- 0.08 0.13 --

SE 67.13 733.89 -- 979.91 319.32 --

CV 0.20 0.77 -- 0.82 0.21 --

19
Animas River Fisheries Database July 25, 2008 

Figure 5. Population estimates of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) from CDOW mark-recapture data in Animas 1 (top) and 2 (bottom). 

Vertical bars are 95% confidence intervals around the mean. 

20
Animas River Fisheries Database July 25, 2008 

3.4.3 Proportional Stock Density 

Proportional stock density (PSD) was computed for the three trout species on the 

basis of numbers of fish captured with raft electrofishing. The PSD for brown trout in 

SUIT waters was consistently above 50 in all years evaluated (Figure 6). This is a 

relatively high PSD and indicates that a large proportion of brown trout in the Animas 

River are of trophy quality (≥ 406 mm, 16”). The annual PSD for rainbow trout varied 

more than for brown trout and ranged from 14 to 66. This could be affected by stocking 

of fish where increased numbers of smaller stocked fish affect the PSD. For Snake River 

cutthroat trout, PSD ranged from 4 to 45. The highest PSD of 45 occurred in 2005 long 

after stocking of this species and when only large fish remained. 

Figure 6. Proportional stock density of brown trout (BNT), rainbow trout (RBT), and Snake River 

cutthroat trout (SRC) in the Animas River within SUIT lands. 

 Nehring (1988) reported a PSD for brown trout in the Animas River upstream of 

Purple Cliffs (CDOW waters) that ranged from 24 to 61, which is similar to that reported 

in this analysis within the SUIT waters. 

Proportional stock densities for the Animas River compare favorably with other 

top fisheries. The PSD for rainbow trout was 55 in the Kootenai River, Idaho (Walters 

2005) and 16-68 in the South Fork Boise River, Idaho (Hebdon 2007), compared to 14-66 

for the Animas River. Comparable PSDs for brown trout and Snake River cutthroat trout 

could not be found in literature. Nevertheless, the high PSDs for brown trout in the 

Animas River indicate a high proportion of trophy fish in the system. 

21
Animas River Fisheries Database July 25, 2008 

3.5 Catch Rates of Trout and Suckers 

Although fish sampling may have not been designed to specifically obtain and use 

catch rate data to characterize fish population size, these are the only comprehensive data 

for assessing numbers of large-bodied fish that may reflect long-term population size and 

trends. Catch rate indices were computed as numbers of large-bodied fish per hour of raft 

electrofishing (Figures 7 and 8). Small-bodied fish were not consistently and efficiently 

captured with raft electrofishing and are not included in this analysis. Nevertheless, it 

should be noted that small-bodied fish were abundant in the Animas River, but their 

numbers are not reflected in these catch rate data. 

These catch rate indices were highly variable with low precision; CV ranged from 

0.34 to 4.90. About 25% of all data distributions were not normally distributed with 

negative binomial distributions. Kruskal-Wallis ANOV and comparison of mean ranks 

were used to determine differences among means. Compatible groups that are not 

significantly different (α > 0.05) are shown in Figures 7 and 8 as the same letters. 

Detailed descriptive statistics for these analyses are provided in the file “ANIMAS 

ANALYSIS.” 

These catch rate data have limited utility as a metric because they lack the 

sensitivity to detect changes in fish densities. Most means differed by about 75% or more 

before they were significantly different; hence fish density (and presumably population 

size) would have to change by at least 75% before it could be detected with the present 

data at a significance level of α ≤ 0.05. If catch rate data for the Animas River could be 

more precise, this index could be one valuable metric for tracking abundances of largebodied fish populations as part of fish monitoring. Power analysis is recommended in 

Phase II to determine sample sizes necessary to reduce precision to CVs of less than 0.15. 

These catch rate indices also lacked the accuracy to reflect population size when 

validated against simultaneous population estimates (Figure 9). Coefficient of 

determination (R2

) for brown trout and rainbow trout were 0.05 and 0.52. The high R2 of 

0.99 for Snake River cutthroat trout is probably not valid because it is attributed to three 

small catch rates and population estimates regressed with a single large catch rate and 

population estimate. Additional data collection is also need to validate catch rate indices 

against population estimates. 

Numbers of white sucker and white sucker crosses with bluehead sucker and 

flannelmouth sucker appear to have increased from 1997 to 2007. The majority of these 

fish were between RM 30 and RM 57 (Table 10); sampling for these fishes did not occur 

above RM 57 and white suckers and their crosses may be present here as well. The white 

sucker is not native to the Colorado River Basin and readily hybridizes with the native 

suckers, sometimes forming hybrid swarms that threatened to dilute the genetic viability 

of the native suckers (Rees et al. 2005). Hybridization of native suckers by white suckers 

could be one of the most significant threats to the native sucker populations of the 

Animas River. Concentrations and sources of these nonnative white suckers should be 

identified and actions taken to reduce their numbers in the Animas River. 

22
Animas River Fisheries Database July 25, 2008 

Figure 7. Annual electrofishing catch rate estimates for brown trout (top), rainbow trout (center), 

and Snake River cutthroat trout (bottom) from the Animas River within SUIT lands (RM 37-56). 

Vertical bars are 95% confidence intervals around the mean, and same letters beneath years 

indicate compatible groups with means that are not significantly different (α > 0.05). 

23
Animas River Fisheries Database July 25, 2008 

Figure 8. Annual electrofishing catch rate estimates for bluehead sucker (top), flannelmouth 

sucker (center), and white sucker (bottom) from the Animas River within SUIT lands (RM 37-

56). Vertical bars are 95% confidence intervals around the mean, and same letters beneath years 

indicate compatible groups with means that are not significantly different (α > 0.05). 

24
Animas River Fisheries Database July 25, 2008 

igure 9. Catch rate indices regressed against simultaneous population estimates for brown trout 

op), rainbow trout (center), and Snake River cutthroat trout (bottom) from the Animas River 

within SUIT lands (RM 37-56). 

F

(t

25
Animas River Fisheries Database July 25, 2008 

Figure 10. Total numbers of white sucker (top), bluehead x white crosses (center), flannelmouth x 

white crosses (bottom) captured by river mile in the Animas River within SUIT lands (RM 37-

56), 1997-2007. 

26
Animas River Fisheries Database July 25, 2008 

3.6 Length-Frequency Analyses

3.6.1 Length Distributions of Trout and Suckers 

Nearly every fish captured was measured for total length and the data were good 

for generating length-frequency histograms for trout based on 10-mm bins. This lengthfrequency analysis helps to determine modes, medians, and means for lengths of most 

cohorts. Bhattacharya’s modal separation and the routine NORMSEP found in FiSAT II 

were used to help distinguish cohorts of fish for each species. Cohort distinction was fair 

to good for the youngest two or three cohorts of brown trout, rainbow trout, and Snake 

River cutthroat trout (Figure 11). However, cohort separation was difficult for bluehead 

sucker, flannelmouth sucker, and white sucker because: (1) the 1- and 2-year old fish 

were rarely caught, and (2) growth of these species slows dramatically after maturity in 

about the 4th year of life leading to overlap in lengths of different age fish (Figure 12). 

ter), and two for Snake River cutthroat trout (bottom) using Bhattachayra’s 

modal separation and the routine NORMSEP found in FiSAT II.

Figure 11. Modal separation of lengths showing five cohorts for brown trout (top), six for 

rainbow trout (cen

Snake River Cutthroat Trout 

Rainbow Trout 

Brown Trout 

27
Animas River Fisheries Database July 25, 2008 

Figure 12. Modal separation of lengths showing two apparent cohorts for bluehead sucker (top), 

six for flannelmouth sucker (center), and seven for white sucker (bottom). Modal separation with 

Bhattachayra’s modal separation and the routine NORMSEP found in FiSAT 

Length-frequency histograms for the major fish species in each year are provided 

in the files “ANIMAS DATA 1 of 2” and “ANIMAS DATA 2 of 2.” Cohort distinction 

using modal separation was adequate for estimating growth and survival of brown trout, 

rainbow tr ut using additional analyses described below 

under section 3.6 (Length-Weight and Age-Growth of Trout) and section 3.8 (Survival of 

Trout). However, a length-at-age relationship is needed to validate cohort separation for 

these species using scales, otoliths, or other structures that exhibit annual growth rings. 

The difficulty in distinguishing cohorts with length data of bluehead sucker, 

annelmouth sucker, and white sucker was anticipated, and opercles were collected an 

Bluehead Sucker 

Flannelmouth Sucker 

White Sucker 

out, and Snake River cutthroat tro

fl d

analyzed to develop length-at-age relationships. This process and the results are 

described below under section 3.7 (Length-Weight and Age-Growth of Suckers). 

28
Animas River Fisheries Database July 25, 2008 

3.6.2 Length Discrepancies in Native Suckers

Len th sucker and bluehead sucker were compared above and 

below the Farmer’s Ditch Diversion at RM 21.9 (Figures 13 and 14). The Farmer’s 

Diversion (RM 21.9) and the Farmington Lake Diversion (RM 11.9) are partial barriers 

to fish movement, and for the purpose of this discussion, data were partitioned with 

respect to the upper-most Farmer’s Diversion. Comparable samples were taken above 

and below this diversion in 2001, 2002, 2003, 2004, and 2005. A prior analysis of fish 

passage at these diversions concluded that the diversion dams across the Animas River 

are partial barriers to upstream fish movement, but effects to the populations of native 

suckers were inconclusive (SWCA 2007). 

The Kolmogorov-Smironov test was used to compare length-frequency 

distributions by species above and below the Farmer’s Ditch Diversion for 2001-2005 

(Table 10). Most samples were taken in July, August, and September, and samples in 

2003 and 2004 were also taken in October and December. Time of year affected size 

distribution in that the young fish were larger later in the year. All paired lengthfrequency distributions within a year were significantly different (two-tailed test, p<0.05) 

indicating that lengths of fish differed above and below the diversion. Also, in all cases, 

minimum differences were significant (one-tailed test, p<0.05) meaning that abovediversion distributions contained fewer small fish than below-diversion distributions. 

Conversely, maximum differences were not significant (one-tailed test, p>0.05) such that 

numbers of large fish below and above the diversion did not differ. 

 Evidently, the two diversions in the lower Animas River impede movement of 

especially small fish and the length-frequency distributions show a larger proportion of 

small suckers below the diversions in most years. The numbers of small fish below the 

diversions may depend on time of year and flow conditions. Nevertheless, the effects to 

e sucker populations cannot be fully understood without a comprehensive analysis of 

the Farmer’s Ditch 

Year 

gths of flannelmou

th

long-term population trends through a recruitment model. 

Table 10. Kolmogorov-Smirnov test comparing distributions above and below 

Diversion at RM 21.9 on the Animas River. 

Sample Size 

Above/Below 

Two-Tailed Test 

Above <> Below 

One-Tailed 

Above < Below 

One-Tailed 

Above > Below 

Bluehead Sucker 

2001 492/401 0.65 (p<0.0000) -0.65 (p<0.0000) 0.00 (p=1.0000) 

2002 300/312 0.15 (p<0.0017) -0.15 (p<0.0008) 0.00 (p=1.0000) 

2003 2,037/57 0.85 (p<0.0000) -0.85 (p<0.0000) 0.00 (p=1.0000) 

2004 1,367/5,169 0.82 (p<0.0000) -0.82 (p<0.0000) 0.00 (p=1.0000) 

2005 284/1,441 0.40 (p<0.0000) -0.40 (p<0.0000) 0.00 (p=1.0000) 

Flannelmouth Sucker 

2001 143/339 0.49 (p<0.0000) -0.49 (p<0.0000) 0.00 (p=1.0000) 

2002 209/235 0.34 (p<0.0000) -0.34 (p<0.0000) 0.03 (p=0.8050) 

2003 2,059/388 0.23 (p<0.0000) -0.23 (p<0.0000) 0.00 (p=1

2004 269/179 0.64 (p<0.0000) -0.64 (p<0.0000) 0.00 (p=1.0

2005 792/932 0.38 (p<0.0000) -0.38 (p<0.0000) 0.00 (p=1.00

.0000) 

000) 

00) 

29
Animas River Fisheries Database July 25, 2008 

Figure 13. Length-frequency distr lmouth sucker in the Anim

(left column) and below (right col atio

ibutions for flanne as River above 

umn) the two irrig n diversions. 

30
Animas River Fisheries Database July 25, 2008 

Figure 14. Length-frequency distributions for bluehead sucker in the Animas River above (left 

column) and below (right column) the two irrigation diversions.

31
Animas River Fisheries Database July 25, 2008 

3.7 Length-Weight and Age-Growth of Trout 

3.7.1 Length-Weight and Condition 

Length and weights were measured from between 25% and 30% of all fish 

handled. These data were sufficient to generate length-weight relationships for the 

principal species in some years (see “ANIMAS DATA 1 of 2” and “ANIMAS DATA 2 

of 2”). A power function was used to describe the relationship between length and 

weight of the principal fish species (Figure 15) and condition factor was also computed 

for each species as an index of condition (Figure 16, Table 11). 

Figure 15. Length-weight relationships for brown trout (top), rainbow trout (center), and Snake 

River cutthroat trout (bottom) from the Animas River in 1999 within SUIT lands (RM 37-56). 

32
Animas River Fisheries Database July 25, 2008 

Figure 16. ke River 

utthroat trout (bottom) of the Animas River within SUIT lands (RM 37-56). Vertical bars are 

 Relative condition of brown trout (top), rainbow trout (center), and Sna

c

95% confidence intervals around the mean, and same letters beneath years indicate compatible 

groups with means that are not significantly different (α > 0.05).

33
Animas River Fisheries Database July 25, 2008 

Table 11. Len and 

Species (s

size) 

Condition 

Factor 

(KTL) 

gth-weight relationships and condition factors for brown trout, rainbow trout,

Snake River cutthroat trout of the Animas River within SUIT lands (RM 37-56). 

ample Year Length-Weight Relationship 

Coefficient of 

Determination 

(R2) 

Brown trout (103) 1998 log W = -5.0969 + 3.0463 log L 0.91 1.21 

(506) 1.01 

(34) 0.98 

Rainbow trou

(242) 1.02 

1999 log W = -5.1549 + 3.0666 log L 0.96 

2001 log W = -4.4949 +2.8163 log L 0.97 

t 1998 log W = -4.9586 + 2.976 log L 0.86 

(306) 0.95 

(92) 1.02 

Snake River 

cutthroat trout 

(132) 

0.97 

1999 log W = -4.3979 + 2.7813 log L 0.94 

2001 log W = -4.4089 + 2.7807 log L 0.97 

1998 log W = -6.2218 + 3.4819 log L 0.80 

(229) 0.97 

(378) 1.00 

1999 log W = -5.5528 + 3.2286 log L 0.90 

2001 log W = -5.1549 + 3.0689 log L 0.91 

trout in the Anim years. 

parame

667 mm

predicted w

respectively. m sizes of 

record

Table 12. The ∞), 

and rate of gr

 K 

3.7.2 Age and Growth 

Based on length-frequency analyses, the majority of brown trout and rainbow 

as River live for 6-7 years with high mortality in the 5th and 6th

Snake River cutthroat trout survive for 1-2 years after stocking. The von Bertalanffy 

growth functions (VBGF) were derived for each of the three trout species (Table 12, 

Figure 17). The VBGF is a logarithmic growth function that describes the rate of growth 

of a group of fish and predicts the maximum length. The VBGF yields two valuable 

ters of growth, L∞ and K. 

The parameter L∞ predicted maximum sizes for brown trout, rainbow trout, and 

Snake River cutthroat trout in the Animas River as 720 mm (28 in.), 678 mm (27 in.), and 

 (26 in.), respectively. Based on length-weight relationships (see Table 9), 

eights for these lengths are 4,049 g (9 lb.), 2,996 g (7 lb.), and 3,674 g (8 lb.), 

 These estimated sizes are reasonable, given that actual maximu

brown trout, rainbow trout, and Snake River cutthroat trout from the Animas River 

fisheries database were 680, 646, and 631 mm TL, respectively. Weights were not 

ed for these fish and predicted weights cannot be confirmed. 

 von Bertalanffy growth function (VBGF), maximum predicted size of fish (L

owth (K) for the six principle fish species of the Animas River. 

Species VBGF L∞

Brown trout L(t) = 719.78 [1 – exp (-0.160 (t – (-0.16)))] 720 mm TL (28 in.) 0.16 

4,049 g (9 lb.) 

Rainbow trout L(t) = 677.78 [1 – exp (-0.110 (t – (-0.4)))] 678 mm TL (27 in.) 

2,996 g (7 lb.) 

0.11 

Snake River 

cutthroat trout 

L(t) = 667.28 [1 – exp (-0.130 (t – (-0.2)))] 667 mm TL (26 in.) 

3,674 g (8 lb.) 

0.13 

34
Animas River Fisheries Database July 25, 2008 

owth te t 

sing the routine ELEFAN in program FiSAT II to rtalanffy 

Figure 17. Gr

trout (bottom) u

growth functions. 

rates of brown trout (top), rainbow trout (cen r), and Snake River cu throat

generate von Be

Brown Trout

Rainbow Trout

Snake River Cutthroat Trout

35
Animas River Fisheries Database July 25, 2008 

assessed when the mark-recapture data are recon

requencies 1 

ciled. Use of ELEFAN provided an 

estimate of growth based on length f to 2 years apart. Growth rate estimates 

of Animas River trout need to be refined for improved accuracy through age-growth 

analysis of scales, from close-order sampling, or from mark-recapture analyses. 

Table 13. Total length at age and annual percent increase in length for brown trout (BNT), 

rainbow trout (RBT), and Snake River cutthroat trout (SRC) in the Animas River. 

Age-Æ 1 2 3 4 5 6 7 8 Average 

Total length (mm) at age 

BNT 122 210 286 350 405 451 491 525 --

RBT 97 157 211 260 304 343 377 --

SRC 96 166 227 281 328 369 --

Annual percent increase in total length 

BNT 72% 36% 22% 16% 11% 09% 7% 16%

RBT 62% 34% 23% 17% 13% 10% 11%

SRC 73% 37% 24% 17% 13% 13%

contaminants. 

 Average annual growth rate of rainbow trout for 7 years was 11%. Like brown 

trout, growth rate decreased with age and size of fish. Growth rate of rainbow trout from 

the Animas River was lower than that of three other riverine populations and one lake 

population. No explanation is offered for the lower overall growth rate of rainbow trout 

in the Animas River, but heavy metals contaminants may be a factor. 

Average annual growth rate of Snake River cutthroat trout for 6 years was 13%. 

Most fish survived only 1 or 2 years after stocking and average annual growth rate for the 

first 2 years was 35%. Growth rate of Snake River cutthroat trout from the Animas River 

was similar to that of fish from the Salmon River, Idaho. 

 Because only one sample was taken per year in every other year, estimates of fish 

growth in the Animas River may not be accurate. Sensitivity of these estimates can be 

Average annual growth rates of brown trout, rainbow trout, and Snake River 

cutthroat trout were 0.16, 0.11, and 0.13, respectively (Table 13). These rates reflect 

average growth of each species for all ages; growth rates for each age can be computed 

from the VBGFs provided in Table 9. Annual percent increase in total length was similar 

for all three species with 72%, 62%, and 73% from year 1 to 2, and 36%, 34%, and 37% 

from year 2 to 3, respectively. 

Average annual growth of brown trout for 8 years was 16%. Growth rate 

decreased with age and ranged from 41% in the first year to 6% in the seventh year. 

Growth of brown trout for the first 4 to 5 years was similar to growth of three other 

riverine populations; only the growth rate of fish from Pathfinder Lake was greater 

(Figure 18). However, growth of brown trout from the Animas River was lowest after 6 

years of all populations compared. Lower growth rate of older fish in the Animas River 

is not readily explained, but may be a function of food supply or heavy metals 

36
Animas River Fisheries Database July 25, 2008 

Figure 18. Growth rates of brown trout (top), rainbow trout (center), and Snake River cutthroat 

trout (bottom) from the Animas River based on von Bertalanffy’s growth function derived with 

ELEFAN in the program FiSAT II and compared with growth rates from other populations. 

37
Animas River Fisheries Database July 25, 2008 

3.8 

 sucker in several years are presented in Table 14 and 

xamples of these relationships are shown in Figure 19. Precision of length-weight 

relation ion 

n 

at weights were taken in the field by various personnel and under various 

eather conditions, an undetermined error rate is suspected. The only way to assess error 

er, flannelmouth 

cker, and white sucker of the Animas River within SUIT lands (RM 37-56). 

(R ) 

n 

(KTL) 

Length-Weight and Age-Growth of Suckers 

3.8.1 Length-Weight and Condition 

The length-weight relationships and condition factors for bluehead sucker, 

flannelmouth sucker, and white

e

ships was variable (R2 = 0.80-0.97 for trout; 0.44-0.94 for suckers). Precis

was higher for trout because they are short-lived and generally adhere to predictable 

growth rates; however, suckers are long-lived and growth is more variable. Precisio

was also affected by several weight measures that were apparently in error. Weight 

measures that were apparent aberrancies were excluded from the database, but those 

measures that could have been legitimate were included in the analysis. 

Given th

w

would be to double weigh a sample of fish; i.e., two people weigh the same fish on 

separate scales. No weight data were available in 2005 and 2007, and insufficient 

numbers of weights were taken within SUIT lands in 2002, 2003, and 2004. 

Table 14. Length-weight relationships and condition factors for bluehead suck

su

Species (sample size) Year Length-Weight Relationship 

Coefficient of 

Determination 

2

Conditio

Factor 

Bluehead sucker (272) 1998 log W = -2.7447 + 2.1565 log L 0.64 1.02 

(962) 

(38

Fla 8 

1999 log W = -4.8239 + 2.9688 log L 0.83 0.98 

1) 2001 log W = -5.1079 + 3.0642 log L 0.94 1.01 

nnelmouth sucker (123) 1998 log W = -4.0000 + 2.6429 log L 0.77 0.9

(55

(11

W

6) 1999 log W = -4.0458 + 2.6459 log L 0.62 1.03 

4) 2001 log W = -4.1675 + 2.6995 log L 0.76 1.00 

hite sucker (13) 1998 log W = -2.9281 + 2.2149 log L 0.44 1.03 

(32 1.05 

(19) 2001 log W = -4.6990 + 2.9045 log L 0.80 1.05 

) 1999 log W = -5.5376 + 3.2363 log L 0.88 

Relative condition may be a useful metric to assess the health of trout or suckers 

in the Anim a ement enario Figur 20 and sho

r ce intervals for the six p inciple fish species. 

Although sample size for trout subs al in e cas n = 9 6), C ried 

f .11 to 0 Th uld b proved with increased sample size, but probably 

m cu in weight measurements afield. Similarly, 

s o uckers varied substant (n = 962) 

CV varied from 0.11 to 0.16; condition factor pr n w tter ese t ucke

s s than for out

as River under different m

elative condition with 95

nag sc s. es 21 w 

% confiden r

was tanti som es ( 2-50 V va

rom 0 .23. is co e im

ore profoundly through increased ac

 size for b eh d f lm

racy

ample lu ead an lanne uth s ially 114-

wo

and 

ecisio as be for th s r 

pecie tr . 

38
Animas River Fisheries Database July 25, 2008 

Figure 19. Length-weight relationships for bluehead sucker (top), flannelmouth sucker (center), 

and white sucker (bottom) from the Animas River in 1999 within SUIT lands (RM 37-56). 

39
Animas River Fisheries Database July 25, 2008 

nd the mean, and same letters beneath years indicate compatible groups 

with means that are not significantly different (α > 0.05). 

Figure 20. Relative condition of bluehead sucker (top), flannelmouth sucker (center), and white 

sucker (bottom) of the Animas River within SUIT lands (RM 37-56). Vertical bars are 95% 

confidence intervals arou

40
Animas River Fisheries Database July 25, 2008 

Figure 21. Relative condition of brown trout (top), rainbow trout (center), and Snake River 

) of the Animas River within SUIT lands (RM 37-56). Vertical bars are 

5% confidence intervals around the mean, and same letters beneath years indicate compatible 

groups with means that are not significantly different (α > 0.05).

cutthroat trout (bottom

9

41
Animas River Fisheries Database July 25, 2008 

3.8.2 Age and Growth 

The most direct way to assess age and growth of fish is through analysis of bony 

structures that show annual growth rings, such as scales, otoliths, or opercles. Age and 

growth of fish is specific to a population and body of water, and age and growth 

information from other populations is generally not applicable. This direct measure of 

age for fishes in the Animas River has been done only from interpretation of opercles 

from bluehead suckers and flannelmouth suckers (Zimmerman 2003). 

Alternative methods for quantifying growth of fish are: (1) differences in 

measured lengths of marked and recaptured fish, and (2) progression of length modes. 

The mark-recapture data in this database have not been reconciled and hence, the modal 

progression technique was applied only to trout. This technique was not applicable to 

suckers because of the great overlap in lengths of same-age fish (see 3.7 LengthFrequency). 

Opercles were examined for annual growth rings from 71 bluehead sucker and 25 

flannelmouth sucker from the Animas River. Carlander’s third degree polynomial best 

described the relationship of opercular radius to total length and length-at-age was 

computed for each year using standard back-calculation techniques (Figures 22 and 23). 

Although the paired data for opercular radius and total length are a good fit to the third 

degree polynomial model (R2

 = 0.92-0.97), additional opercles are needed to better define 

the relationship, especially for small to intermediate size fish. As is evident from Figure 

19, there were five or fewer opercular pairs examined from fish less than 300 mm TL. 

 A more defined set of length-at-age relationships for bluehead sucker and 

flannelmouth sucker will help to better understand age composition of these populations, 

growth rates, and longevity. The oldest fish examined were 15 and 21 years of age for 

bluehead sucker and flannelmouth sucker, respectively. Information on age composition, 

growth, and longevity is also important for constructing population recruitment models, 

which are proposed for the next phase of work for the Animas River fisheries database 

analysis. These models are helpful for evaluating impacts of management actions where 

data are lacking or difficult to collect. 

Cohort separation was difficult for bluehead sucker, flannelmouth sucker, and 

white sucker (Figure 12) because: (1) the 1- and 2-year old fish were rarely caught, and 

(2) growth of these species slows dramatically after they reach maturity in about the 4th

year of life leading to overlap in lengths of different age fish. When compared with the 

length-frequency mode separation for trout using NORMSEP (Figure 11), using sucker 

length information for cohort determination, growth, and survival is clearly problematic. 

42
Animas River Fisheries Database July 25, 2008 

Figure 22. Relationship of opercular radius to total length (top) and length-at-age relationship 

(bottom) for bluehead sucker in the Animas River. 

43
Animas River Fisheries Database July 25, 2008 

Figure 23. Relationship of opercular radius to total length (top) and length-at-age relationship 

(bottom) for flannelmouth sucker in the Animas River. 

44
Animas River Fisheries Database July 25, 2008 

3 Survival of Trout .9 

rt occurs 2 years 

ter an

p 

s using 

nd Holt model (Table 15, Figure 24). 

Survival of fish is generally estimated by tracking population size over time by 

marking samples of fish and recapturing them in a single event, two recapture events, or

from multiple mark-recapture events called the Jolly-Seber model (Ricker 1975; Seber 

1982; summarized by Miranda and Bettoli 2007). The trout tagging portion of the 

Animas River Database is a mark-recapture design with mark and recapture efforts 

spaced generally less than 2 weeks apart. The next mark-recapture effo

la d because the fish are fin-clipped, marks do not remain visible or unique on fish. 

These data are inadequate for survival estimates of trout. 

 An alternative but less precise way to estimate survival is with Bhattachayra’s 

model progression routine in Program FiSAT II which tracks mean lengths of age grou

modes over time. This routine generated Ricker’s ‘Z’ function from mean length

the Beverton a

Table 15. Survival of brown trout (BNT), rainbow trout (RBT), and Snake River cutthroat trout 

(SRC) from the Animas River. 

Percent Survivors After Specified Years Species Size N Z value 1 2 3 4 5 6 7 

BNT-all All 1000 0.32 72.61 52.73 38.29 27.80 20.19 14.66 10.65

RBT-all All 1000 0.478 62.00 38.44 23.84 14.78 9.16 5.68 3.52

RBT-200+ 200+ 1000 0.416 43.52 28.71 18.94 12.49 8.24 5.4

RBT-404+ 406+ 1000 0.921 39.81 15.85 6.31 2.51 1.00

4

SRC-all All 1000 1.462 23.18 5.37 1.25 0.29 0.07 0.02 0.004

 Because all three trout species are stocked in the Animas River, survival is not 

indicative of age, but rather years in the river. For rainbow trout, there is little apparent 

natural reproduction and survival reflects hatchery fish that are in the river for 1 to 7 

years. This analysis indicates that 62% of stocked fish survive after 1 year in the Animas 

River, 38% after 2 years, 24% after 3 years, 15% after 4 years, 9% after 5 years, 6% after 

6 years, and less than 4% after 7 years in the river, respectively. 

There is natural reproduction of brown trout in the Animas River and the specie

was stocked annually from 2000 to 2007 by the CDOW, and in 2003 and 2006 by the 

SUIT. Hence, survival of this mixed stock is difficult to estimate without mark-recaptur

data. Modal progression showed that survival of brown trout was 73%, 53%, 38%, 28%,

20%, 15%, and 11% after 1 to 7 years in the river. Because of the mixed stock, it is 

difficult to determine if this greater survival rate by brown trout is for the same age fish 

s for rainbow trout. 

s 

e 

 There is no natural reproduction of Snake River cutthroat trout in the Animas 

River and most fish do not survive after their second year in the river. Only 23%, 5%, 

and 1% of stocked fish remained after 1 to 3 years in the river, respectively. 

a

45
Animas River Fisheries Database July 25, 2008 

Figure 24. Survival of brown trout (BNT), rainbow trout (RBT), and Snake River cutthroat trout 

(SRC) from the Animas River. 

 These survival estimates are approximations for fish in the river after they have 

been stocked. These estimates do not represent survival of stocked fish. In order to 

estimate survival of stocked fish, it will be necessary to batch-mark the fish with coded 

wire tags. This will enable estimates of survival to be made for the entire time that the 

fish are in the river. 

A more reliable, precise, and accurate method is needed to estimate survival of 

fish in the Animas River. The most accurate is a multiple mark-recapture estimator such 

as the Jolly-Seber model used on an annual basis. Increased precision is needed in this 

parameter to increase sensitivity of detecting a change in survival from management 

actions. 

3.10 Trout Stocking Protocols and Patterns

Trout stocking records were provided by the SUIT for the Animas River during 

 at 

 SUIT stocked primarily Snake River 

cutthroat trout through 2001, and then stocked primarily rainbow trout with smaller, 

infrequent releases of brown trout. The CDOW has released seven strains of fish (see 

Table 17), which were consolidated into the three principal species (brown trout, rainbow 

trout, Snake River cutthroat trout) for this analysis, since field distinctions are not made 

among strains (Table 18, Figure 25). The net effect of stocking on the riverine trout 

population is difficult to assess because: (1) stocked fish were not marked, (2) survival 

and movement at stocking are not known, (3) different size and age fish were stocked. 

1996-2007 (Table 16) and by the CDOW du ng 1998-2007 (Table 17). Fish stocked by 

the SUIT are released within the SUIT lands between river miles 56.2 and 37.1, usually

key access locations, such as Basin Creek. The

ri

46
Animas River Fisheries Database July 25, 2008 

Table 16. Dates, sizes, strains, and sources of fish stocked in the Animas River by the SUIT 

within SUIT lands (RM 37.1-56.2) during 1996-2007. 

Year Date Number Stocked Size, Strain, and Source of Fish Stocked 

15-May 1,000 8-12" Snake R. cutthroat trout from William's Creek NFH 

11-Jun 905 8-12" Snake R. cutthroat trout from William's Creek NFH 

9-Jul 1,714 8-12" Snake R. cutthroat trout from William's Creek NFH 

21-Aug 1,000 8-12" Snake R. cutthroat trout from William's Creek NFH 

1996 

Total 4,619 

13-May 2,000 8-12" Snake R. cutthroat trout from William's Creek NFH 

10-Jun 1,500 8-12" Snake R. cutthroat trout from William's Creek NFH 

14-Jul 1,512 8-12" Snake R. cutthroat trout from William's Creek NFH 

28-Aug 1,128 8-12" Snake R. cutthroat trout from William's Creek NFH 

1997 

Total 6,140 

22-Apr 430 12" Snake R. cutthroat trout Lake Capote transplant fish 

2-Jun 3,500 8-12" Snake R. cutthroat trout from William's Creek NFH 

11-Aug 5,700 8-12" Rainbow trout from Mescalero NFH 

27-Oct 4,500 4" Snake R. cutthroat trout 

1998 

Total 14,130 

3-May 3,500 8-12" Snake R. cutthroat trout from William's Creek NFH 

15-Jun 2,800 8-12" Snake R. cutthroat trout from William's Creek NFH 

1999 

14-Jul 4,900 8-12" Snake R. cutthroat trout from William's Creek NFH 

Total 11,200 

10-May 7,500 8-12" Snake R. cutthroat trout from William's Creek NFH 

29-Jun 3,500 8-12" Snake R. cutthroat trout from William's Creek NFH 

19-Jul 6,000 8-12" Snake R. cutthroat trout from William's Creek NFH 

17-Oct 9,000 4" Rainbow trout from Mescalero NFH 

2000 

Total 26,000 

19-Jun 4,100 8-12" Snake R. cutthroat trout from William's Creek NFH 

1-Oct 4,500 4" Snake R. cutthroat trout 

2001 

Total 8,600 

5-Jun 6,000 10" Rainbow trout from Alchesay NFH 

2002 

Total 6,000 

14-May 8,700 Catchable Rainbow trout from Silver Springs, Kamloop Strain 

3-Dec 6,000 6" Brown trout from Alchesay NFH 

2003 

Total 14,700 

8-Apr 6,000 5" Rainbow trout from Hotchkiss NFH, Irving Strain 

2004 

Total 6,000 

6-Jun 3,556 10-11" Rainbow trout from Silver Springs, Kamloop Strain 

5-Aug 3,500 10-11" Rainbow trout from Silver Springs, Kamloop Strain

2005 

Total 7,056 

8-Jun 10,304 3" Brown trout Jones Holes 

30-Jun 4,135 10.8" Rainbow trout Mescalero TFH Erwin Strain 

21-Sep 4,200 3" Rainbow trout Jones Hole 

30-Aug 1,959 10.3-11.6" Rainbow trout Mescalero TFH Erwin Strain 

2006

Total 18,639 

28-Jun 5,300 8" Rainbow trout Hotchkiss 

26-Jul 2,128 10" Rainbow trout Crowther's Freshwater Trout 

31-Jul 1,065 10" Rainbow trout Crowther's Freshwater Trout 

9-Aug 2,130 10" Rainbow trout Crowther's Freshwater Trout 

2007 

Total 10,623 

47
Animas River Fisheries Database July 25, 2008 

Table 17. Numbers of trout stocked

tanda regulations waters near D

 in the Animas River by the CDOW in the gold medal and 

rd urango, Colorado (RM 57.4-62.2). CR1=Colorado River 

inbow

rand 

otal 

s

ra , CRR=Colorado River rainbow, LOC=brown trout, RBT=rainbow trout, RXN=rainbow

x cutthroat hybrid, SRN=Snake River cutthroat trout, WEM=Weminuche Colorado River 

cutthroat trout. 

Year CR1 

(RBT) 

CRR 

(RBT) 

LOC 

(BNT) RBT RXN (RBT) 

SRN 

(SRC) 

WEM 

(RBT) 

G

T

1998 37766 37,766

1999 31992 31,992

2000 15,002 20,038 20,004 55,044

2001 20,000 20,154 32010 27,361 99,525

2002 24,309 28,361 10,497 20,026 83,193

2003 19,998 18,284 1667 833 18,285 59,067

2004 20,004 46,902 900 2,126 25,070 95,002

2005 19,996 20,004 17,882 2500 60,382

2006 20,004 22,509 2500 21,069 66,082

2007 32,810 20,144 2880 55,834

Grand 

Total 19,996 172,131 194,274 112,215 13,456 110,746 21,069 643,887

Table 18. Summary of tr nuall y t uth te SU d

Colorado Divis f W (CD i n i 6 , 

R ow t SR ke cu t 

SUIT CDOW 

out stocked an

ildlife 

y b he o

imas R

 S e U

ver (RM

rn Indian Tribe (

 37.1-5

I an

T=brow

T) t

n trout

he 

ion o OW) n the A .2). BN

BT=rainb rout, C=Sna River tthroa trout. 

Year BNT RBT SRC Total B s NT RBT RC S Totals

1996 4,619 4,619 

1997 6,140 6,140 

1998 5,700 8,430 14,130 37,766 37,766

1999 11,200 11,200 31,992 31,992

2000 9,000 17,000 26,000 20,038 15,002 20,004 55,044

2001 8,600 8,600 20,154 52,010 27,361 99,525

2002 6,000 6,000 28,361 34,806 20,026 83,193

2003 6,000 8,700 14,700 18,284 22,498 18,285 59,067

2004 6,000 6,000 46,902 23,030 25,070 95,002

2005 7,056 7,056 17,882 42,500 60,382

2006 10,304 6,094 16,398 22,509 43,573 66,082

2007 10,623 10,623 20,144 35,690 55,834

Grand 

Totals 16,304 59,173 55,989 131,466 194,274 338,867 110,746 643,887

48
Animas River Fisheries Database July 25, 2008 

igure 25. Numbers of brown trout (BNT), rainbow trout (RBT), and Snake River cutthroat trout 

(SRC) s

F

tocked by the SUIT during 1996-2007 (top) and CDOW during 1998-2007 (bottom). 

49
Animas River Fisheries Database July 25, 2008 

 Overall, the SUIT stocked an average of 10,956 trout per year over the 12-yea

period (1996-2007), and the CDOW stocked an av

r 

erage of 64,389 trout per year over the 

0-year period (1998-2007). None of these stocked fish were marked so their survival, 

m ent, and ultim t b

other stre s expecte large nu

so any mi any fis

m dow into SU ters, an

distribute downstream, although high water te

Animas R bably re ownstre

Ho these sto ish do i

certainty of the la arked 

numbers of fish that surviv initial s kings into the river are unknown. However, 

because there appears to be reprodu

Sn Riv roat trou sures of

sp s in r are larg r stocke . 

A ison of fi ked by

was made three tro ies to e

populations. Trout stocked by the CDO

which are the gold medal and standard fi ing regulations sections near Durango, 

Colorado, and extend from mile 62

an lly b es were t for this

es ma apture d

for Reaches 1, 2, and 3 and population e e reaches as 

well, which extend from r le 56.4

The brown trout population in the CDOW section (RM 62.2-57.4) remained 

st at 1 1,535 fis to 320 s stocked in 

th ctio re 26). T bers of brown trout in the SUIT section (RM 56.4-37.1) 

also rema tively st 2,141 t

tw ocki 6-inch fis 003 and ve population 

sta ity fo n trout m he resu

It is unclear if the brown tr pulatio . 

The rainbow trout tion in 

2, fish o 585 fish despite

(Figure 27). The numbers bow tr ncreased from 807 fish 

in 01 to in 2005 ( 98 fish

st d an . In the c rainbow s of fish stocked in the 

CDOW section may affect t numb

moving downstream. Clearly, stocking helps to sustain the riverine population of 

rainbow tr he Snake cutthro tocking 

st ed in IT sectio 001, des tion 

th h 20 gure 28). pecies i sh live 

for only 1-2 years. 

1

ovem ate destiny canno e traced. Based on behavior of stocked trout in 

ams, it i d that mbers of these stocked fish moved downstream, 

me perhaps for m

ove

les. M h stocked by the CDOW near Durango likely 

nstream IT wa d fish stocked by SUIT in their lands also 

mperatures and low flows in the lower 

iver pro strict d am movement or result in low survival. 

w well cked f n the Animas River cannot be determined with 

because ck of m fish and post-stocking monitoring. Hence, the 

e the toc

 little ction by rainbow trout and no reproduction by 

ake

ecie

er cutth

 e

t, mea the growth, survival, and abundance

 initial release

 of these 

the riv ely fo d fish that survived the

compar sh stoc the CDOW and SUIT to population estimates 

 for the ut spec valuate the effect of stocking on riverine 

W are released in “Animas #1” and “Animas #2” 

sh

 river .2 to 57.4. The total numbers of fish stocked 

nua

timates derived from

y speci otaled evaluation as well as the total population 

rk-rec ata. Trout stocked by the SUIT were also totaled

stimates were summed for the thre

i i ver m to 37.1. 

able

at se

,162 to h (242 fish/mile) despite variable number

n (Figu he num

ined rela able at o 3,189 fish (111 to 165 fish/mile) despite only 

o st ngs of h in 2 3-inch fish in 2006. This relati

bil r brow ay be t lt of natural reproduction and habitat limitation. 

out po n is sustained—or even affected—by stocking

popula the CDOW section remained stable at 2,129 to 

810 (444 t /mile) variable stockings of about 15,000 to 52,000 fish 

 of rain out in the SUIT section i

 20

ocke

 5,761 42 to 2 /mile) despite a fairly even number of fish 

nually ase of trout, the higher number

 the ne ers in the river in the SUIT section from fish 

out. T River at trout population declined to zero after s

opp

roug

 the SU n in 2 pite continued stocking in the CDOW sec

04 (Fi This s s sustained entirely by stocking and the fi

50
Animas River Fisheries Database July 25, 2008 

igure 26. Numbers of brown trout stocked in the Animas River compared to population 

timates for CDOW waters (top) and SUIT waters (bottom). 

F

es

51
Animas River Fisheries Database July 25, 2008 

Figure 27. Numbers of rainbow trout stocked in the Animas River compared to population 

estimates for CDOW waters (top) and SUIT waters (bottom). 

52
Animas River Fisheries Database July 25, 2008 

Figure 28. Numbers of Snake River cutthroat trout stocked in the Animas River compared to 

population estimates for CDOW waters (top) and SUIT waters (bottom). 

53
Animas River Fisheries Database July 25, 2008 

4.0 DISCUSSION 

4.1 Data Suitability and Utility for Fish Demographic 

Parameters 

The Animas River fisheries database is stored in an Excel platform with columnspecific data consisting of unique numeric and alphanumeric codes. This format 

facilitates data entry and analyses. Some data may be available from data sheets that 

were not previously entered, such as fish weights for 2005 and 2007. Data sheets should 

be reviewed to determine if additional data are available that could enhance analyses. 

The Animas River fisheries database is suitable for evaluation of lengths and 

weight and is limited for evaluation of age and growth, abundance, and survival. The 

length-weight data lack some precision, which can be rectified through cross checks with 

data sheets and deletion of apparent aberrant measures. Future collection of fish lengths 

and weights should be done in a more precise and accurate manner. 

The length data for trout were mostly suitable for determination of age and 

growth, but this information should be supplemented with aging of fish from scales 

and/or otoliths. Age determinations of fish will greatly enhance population age-structure 

evaluation and is necessary for development of a trout age-specific recruitment model. 

The length data for native suckers was, as expected, not suitable for determination 

of age and growth because of the large overlap in size of different age fish. In 

anticipation of this limitation, opercles were collected and aged from bluehead sucker, 

flannelmouth sucker, and white sucker. Additional opercles are needed from small and 

medium-size fish for a more robust assessment of age and growth of these species. This 

information is also vital for development of a sucker age-specific recruitment model. 

Demographic parameters such as length-weight, condition, age-growth, survival, 

and proportional stock density may be valuable measures of fish health when monitoring 

effects of the Animas-La Plata Project. 

Historic fish collection data for 1934 to 1993 were presented by Miller et al. 

(1995). These data should be incorporated into the present database to provide the 

opportunity to expand analyses and provide more insight into historic fish populations 

and trends. 

Although the data were not specifically collected to use for catch rate estimates as 

a metric of fish abundance, existing data for brown trout, rainbow trout, bluehead sucker, 

and flannelmouth sucker provide the longest and most consistent record of fish capture 

information available for the Animas River. Although the precision of current data is 

low, these data help provide insight into fish abundances and distributions. These data 

4.2 Suitability and Utility of Fish Abundance Estimates 

54
Animas River Fisheries Database July 25, 2008 

will also help to determine the most appropriate sampling design for precise and accurate 

assessment of fish populations. This can be accomplished through either increased 

sample size or a stratified random sampling design that reduces sample variability. 

A second drawback of the current catch rate data is that these estimates of 

numbers of fish per hour of electrofishing apparently have little relationship to markrecapture abundances estimates. These catch rate estimates should reflect true population 

abundance if they are to be used as indices of population status and trends. As discussed 

above, an appropriate sampling design will be necessary to achieve better precision and 

accuracy, and to improve detection of changes in fish populations. 

Population estimates for trout are based on close-order mark and recapture events 

that are spaced less than two weeks apart. These are fairly robust estimates with 

reasonable precision. However, these data cannot be used to estimate survival because 

the mark and recapture events are closely spaced, the marks are temporary fin clips, and 

the estimates are spaced two years apart. If population estimates of trout or native 

suckers are desirable or necessary, a better sampling strategy will be annual estimates for 

three consecutive years with two intervening years of no sampling. Marks on fish will 

need to be permanent (e.g., PIT tags, Floy tags, etc.) in order to procure across-year 

estimates and survival. 

More precise fish population estimates may be possible through marking and 

releasing fish in short river subreaches rather than on a river-wide scale. Sampling can be 

restricted to pools and runs with distinct geomorphic breaks, such as long shallow riffles 

that minimize escapement of marked fish. Estimates from these short subreaches would 

need to be expanded for an estimate of the entire reach. 

4.3 Effectiveness of Trout Stocking Program 

The effect of stocking trout on the population of fish in the Animas River could 

not be fully evaluated because of the lack of marked stocked fish and annual population 

estimates. The fish of at least one year of stocking should be marked with coded wire 

tags in order to follow their survival in the river as well as their movement. Based on the 

available data, there was no apparent negative effect of stocking on the population of 

trout in the river. All three trout species were robust with good condition and populations 

appeared to be maintained with brown trout and rainbow trout living up to 6-7 years of 

age. 

Snake River cutthroat trout did not appear to survive longer than 1 or 2 years after 

iver may be affected by 

elevated levels of heavy metals from historic mining activity in the upper watershed. The 

Snake River cutthroat trout is considered a “novelty fish” by anglers seeking an unusual 

catch. Periodic stocking of this species, when available could help to attract anglers to 

the Animas River on the chance of catching one of these attractive fish. 

stocking. Their short longevity in the river following stocking cannot be explained, 

although the longevity of all trout species in the Animas R

55
Animas River Fisheries Database July 25, 2008 

It is unclear if stocking of small brown trout helps to sustain the wild population 

or if that population is entirely sustained by natural reproduction. This uncertainty should 

be further evaluated in light of the large numbers of brown trout stocked in the Animas 

River. Stocking of rainbow trout appears to be vital to maintenance of the riverine 

population, although the amount of natural reproduction is not known. Snake River 

cutthroat trout survive for only 1 to 2 years after stocking, although a few individuals 

survive longer and become truly trophy fish. 

The numbers of fish stocked and the frequency of stocking do not appear to be 

detrimental to the population and survival of brown trout and rainbow trout for the first 5 

years in the river is high. Few Snake River cutthroat trout survive 2 years after stocking. 

Length-weight relationships and condition of all three trout species are comparable to 

other populations of similar species (Carlander 1969). Proportional stock density with 

fish greater than 16 inches long is exceptional for all three trout species, especially for 

brown trout. Also, the numbers of fish per mile is not excessive and could be several 

times higher in other comparable rivers. 

It appears that rainbow trout stocked by the CDOW move downstream in 

sufficient numbers to supplement and maintain the population in SUIT waters. This is 

most apparent for rainbow trout, but cannot be determined for brown trout. It is noted 

that the population of Snake River cutthroat trout virtually disappeared in SUIT waters 

after stocking ceased in those waters in 2001, despite over 20,000 fish per year being 

stocked in upstream sections by CDOW. This evidence shows little or no downstream 

movement by Snake River cutthroat trout after stocking. 

4.4 Population Projections for Native Suckers 

 The numbers of flannelmouth sucker and bluehead sucker in the Animas River 

appear to be large in some reaches when compared to populations in other rivers of the 

Upper Colorado River Basin. However, many of the fish are old adults and it is unknown 

if the numbers of young fish are sufficient to sustain the population long-term. These 

species are long-lived with successful reproduction and recruitment occurring 

sporadically. Capture information of these populations in the Animas River shows a 

disproportionate number of large, old adults and fewer numbers of small juveniles. This 

apparent age distribution may be a function of sampling gear; i.e., raft electrofishing 

captures primarily large-bodied fish. However, it is unlikely that juveniles of both 

ecies are present but not captured with raft electrofishing because crews do not rep t 

ing 

ents. 

Furthermore, young native suckers show up in raft electrofishing samples downstream of 

the Farmer’s Ditch diversion and the Farmington Lake diversion. 

 The viability of the flannelmouth sucker and bluehead sucker populations is not 

well understood. The fish appear to be reproducing but levels of survival and recruitment 

are unknown. Furthermore, two irrigation diversion dams impede movement of fish and 

sp or

sighting many small fish during electrofishing. Alternatively, the small fish may be us

habitats not sampled by raft electrofishing. This is also unlikely since the Animas River 

is primarily a single channel system with few side channels or embaym

56
Animas River Fisheries Database July 25, 2008 

may be affecting viability of th e diversion dams have 

een in place for 40 to 50 year cker populations upstream 

ment had 

been significantly affected, the populations would be expected to be severely depleted or 

xtirpated. Nevertheless, even a moderate reduction in recruitment could, over time, 

cause th

 is 

 of 

ncubate for 5-7 days 

and the larv

 the 

a 

short distance from

sufficient recruitm

er. A monitoring program should be developed under Phase II and in 

bo

o

s. 

ese upstream populations. Thes

b s and the persistence of native su

is strong evidence that successful reproduction and recruitment is occurring.

Flannelmouth sucker and bluehead sucker live for 15-20 years and if recruit

e

e population to slowly decline. Hence, the effect of these dams—as well as 

future management of river flows—on the long-term viability of these populations

inconclusive. An age-specific recruitment model is the best way to assess the viability

these populations. This will provide an opportunity to simulate population trajectories 

under different survival, recruitment, and movement scenarios. 

4.5 Drift of Young Native Suckers 

 Flannelmouth suckers and bluehead suckers are broadcast spawners that scattered 

semi-adhesive eggs on cobble and gravel substrates in the main river channel. Spawning 

occurs in spring on the descending limb of spring runoff. The eggs i

ae emerge and are swept downstream by river currents. The larvae are 14-16 

mm long, without fully developed fins, and rely on a yolk sac for nutrition for about

first 2-3 weeks of their life. These larvae become entrained in eddies, shoreline 

embayments, and floodplains where they develop functional fins and a mouth and 

become active feeders of tiny zooplanktors and insects. Many larvae become entrained 

 their natal areas, but some may drift for many miles downstream, 

depending on channel geomorphology and complexity. 

 Reproduction and the presence of large numbers of larval suckers have been 

documented and the data are part of this database (see ANIMAS DATA 1 OF 2—2000 

Larva). It is unknown how far these larvae drift in the Animas River and it is also 

unknown if enough larvae are retained upstream of the two diversion dams to provide 

ent to the populations for long-term sustainability. The relationship of 

young-of-year nursery habitat for native suckers to river stage should be quantified and 

modeled in order to predict how flow variation in the Animas River can affect this 

habitat. 

4.6 Insights into Developing a Fish Monitoring Program 

This Phase I was not intended to develop a monitoring program for fish in the 

Animas Riv

colla ration with managers and biostatisticians. The following provides insight into a 

fish m nitoring program. 

The first step in developing a fish monitoring program for the Animas River will 

be to identify the needs of the program. These needs should be identified by the 

managers in collaboration with fish biologists and biostatisticians as a series of question

These needs drive the sampling design and help to determine the types of data to be 

collected, the timing of those collections, as well as the frequency. 

57
Animas River Fisheries Database July 25, 2008 

The second step of developing a monitoring program is to develop acceptable 

levels of precision, accuracy, sensitivity, and risk. Precision is the variability around

sample statistics that describe the goodness of the estimates. Precision can generally

 the 

 be 

proved through an appropriate sampling design (to control resource variability), 

approp ber of 

ree 

ed under Phase II. 

little 

h 

 at least 75 % before it could be detected with the present data at 

a significance level of α ≤ 0.05. Acceptable sensitivity levels need to be established by 

manage

f risk is greatly reduced when a 

atabase exists, such as the Animas River Fisheries Database, from which one can draw 

in types of data. 

im

riate sample methods (to control sampling variability), and a sufficient num

samples. Precision determines one’s ability to detect changes with some specified deg

of confidence (usually 95%). Managers need to recognize that high precision may not be 

achievable with certain resources, and alternative means of impact evaluation may be 

necessary. The expected precision from monitoring can be approximated by using the 

existing data and performing simulations; these analyses are propos

Accuracy is a reflection of how well the sample data portray the real population 

size and dynamics. As was demonstrated above, the existing catch rate data bear 

relationship to population size. Any metric used to portray the status and trends of fish 

populations should have some known relationship to true abundance. 

Sensitivity refers to the ability to detect a change in a metric, such as population 

size or a catch rate estimator. The sensitivity for detecting a change in brown trout catc

rate was 75% or greater; in other words, catch rate (and presumably population size) 

would have to change by

rs and the sample design and cost need to be determined in advance. 

Finally, it is important to understand the risk associated with monitoring of certain

resources and what contingencies, if any, need to be implemented if a particular sampling

design does not yield desired information. The degree o

d

inferences about the expected characteristics and behavior of certa

58
Animas River Fisheries Database July 25, 2008 

5.0 RECOMMENDATIONS 

1. Maintain the Animas River Fisheries Database and record and enter all future

fisheries data in compatible format. 

2. Cross check data inconsistencies, discrepancies, and missing data in electro

database against field data sheets. 

nic 

ng 

of 

tion estimates (e.g., PIT tags, Floy tags). 

species in the Animas 

River in order to refine age-growth relationships. 

, 

e 

ide a 

14. Quantify species composition and abundances of small-bodied fishes of the 

3. Reconcile tag numbers and colors for native suckers so that data can be used 

for population estimates. 

4. Conduct population estimates for three consecutive years with two interveni

years of no sampling in order to procure more precise and reliable estimates 

abundance and survival. 

5. Use long-term tags for popula

6. Improve accuracy and precision of fish weight measurements in the field. 

7. Improve accuracy and precision of native sucker tagging and documentation

procedures. 

8. Collect scales and/or otoliths from all trout species in the Animas River in 

order to develop age-growth relationships 

9. Continue to collect opercles and otoliths from all sucker 

10. Develop age-specific recruitment models for native suckers and for trout; i.e.

one model for flannelmouth sucker and bluehead sucker, and one model for 

brown trout, rainbow trout, and Snake River cutthroat trout. 

11. Procure prior fisheries data from the Animas River to incorporate into the 

existing database; e.g., Nehring, Miller. 

12. Conduct a more thorough evaluation of the trout stocking program for th

Animas River with coded wire tagged fish and the use of age-specific 

recruitment models; e.g., determine if stocking of brown trout is necessary. 

13. Continue to stock Snake River cutthroat trout, as available, to prov

novelty fish for anglers. 

Animas River through different gears and sampling methods. 

59
Animas River Fisheries Database July 25, 2008 

15. Convene a workshop of ALP administrators, managers, biologists, and 

statisticians to define information needs and design a monitoring program 

accordingly. 

16. Develop a formal monitoring plan that defines information needs, sampling 

design, sample sizes, and desired precision, accuracy, and acceptable level

detectable change. 

17. Translocate flannelmouth sucker and bluehead sucker upstream of the 

Farmer’s Ditch and Farmington Lake Diversion during low river flows. 

s of 

18. Remove and euthanize nonnative white sucker and their hybrids from the 

Animas River. 

19. Locate sources of white sucker to the Animas River, and if possible, 

implement measures to reduce escapement. 

20. Evaluate habitat of young-of-year flannelmouth sucker and bluehead sucker at 

the full flow range of the Animas River. 

60
Animas River Fisheries Database July 25, 2008 

LITERATURE CITED 

Bagenal, T.B. and F.W. Tesch. 1978. Age and growth. Pages 101-136 in T.B.Bagenal 

(ed.). Methods of assessment of fish production in freshwater, 3rd edition. 

Blackwell Scientific Publications, Oxford, England. 

Beverton, R.J.H., and S.J. Holt. 1957. On the dynamics of exploited fish populations. 

Fisheries Investigations, Ministry of Agriculture: Fisheries and Food, Great 

Britain, 19: 533 pp. 

Carlander, K.D. 1969. Handbook of freshwater

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Animas River Fisheries Database July 25, 2008 

APPENDIX A: Photographic Record 

Photo S nd navigating past the 

armer’s Ditch diversion (bottom). 

et 1. Electrofishing raft in operation on the Animas River (top) a

F

A-1
Animas River Fisheries Database July 25, 2008 

Photo Set 2. Bluehead sucker (top), flannelmouth sucker (center), and white sucker (bottom) from 

the Animas River. 

A-2
Animas River Fisheries Database July 25, 2008 

Photo Set 3. Brown trout (top), rainbow trout (center), and Snake River cutthroat trout (bottom) 

from the Animas River. 

A-3
Animas River Fisheries Database July 25, 2008 

Photo Set 4. Farmer’s Ditch diversion during low flow (top), during medium flow (center), and 

Farmington Lake diversion (bottom). 

A-4