NOAA Technical Report NMFS 147 November 1999

U.S. Department of Commerce

Investigation of Unusual Mortalities of 

Bottlenose Dolphins along the Mid-Texas 

Coastal Bay Ecosystem during 1992

A. A. Colbert

G. I. Scott

M. H. Fulton

E. F. Wirth

J. W. Daugomah

P. B. Key

E. D. Strozier

S. B. Galloway
U.S. Department of Commerce

Seattle, Washington

NOAA Technical Report NMFS 147

A Technial Report of the Fishery Bulletin

Investigation of Unusual Mortalities of

Bottlenose Dolphins along the Mid-Texas

Coastal Bay Ecosystem during 1992

A. A. Colbert

G. I. Scott

M. H. Fulton

E. F. Wirth

J. W. Daugomah

P. B. Key

E. D. Strozier

S. B. Galloway

November 1999
1

Introduction

Early in 1992, the mortality and stranding rate of bottlenose dolphins (Tursiops truncatus) increased dramatically along the mid-Texas coast1. In April 1992, the 

National Marine Fisheries Service, Southeast Fisheries 

Science Center (NMFS/SEFSC) was notified that the 

Texas Marine Mammal Stranding Network suspected 

anomalous dolphin mortalities were occurring along 

the mid-Texas coast. Causal factors of these mortalities 

were not known. Gender, species, length, and location 

data were available for most dolphins, but the overwhelming number of reported strandings prevented 

thorough examination of a majority of them. The volunteer stranding network was unable to respond effectively 

to the increased number of strandings and requested 

investigative assistance from the NMFS/SEFSC.

Determining cause and effect in toxicological risk 

assessment investigations is always tenuous. This is particularly true when dealing with taxa such as marine 

mammals, for which very little clinical and toxicological 

data exists. Most marine mammal toxicological information is chemical body burden residue data, not toxiInvestigation of Unusual Mortalities of Bottlenose Dolphins

along the Mid-Texas Coastal Bay Ecosystem during 1992

A. A. COLBERT, G. I. SCOTT, M. H. FULTON, 

E. F. WIRTH, J. W. DAUGOMAH, P. B. KEY, 

E. D. STROZIER, and S. B. GALLOWAY

Center for Coastal Environmental Health

and Biomolecular Research at Charleston

National Ocean Service, NOAA

219 Fort Johnson Road

Charleston, South Carolina 29412

ABSTRACT

An investigation was conducted into the deaths of more than 220 bottlenose dolphins (Tursiops truncatus) that occurred within the coastal bay ecosystem of mid-Texas between January 

and May 1992. The high mortality rate was unusual in that it was limited to a relatively small 

geographical area, occurred primarily within an inshore bay system separated from the Gulf 

of Mexico by barrier islands, and coincided with deaths of other taxa including birds and fish. 

Factors examined to determine the potential causes of the dolphin mortalities included microbial pathogens, natural biotoxins, industrial pollutants, other environmental contaminants, and 

direct human interactions. Emphasis was placed on nonpoint source pesticide runoff from 

agricultural areas, which had resulted from record rainfall that occurred during the period of 

increased mortality.

Analytical results from sediment, water, and biota indicated that biotoxins, trace metals, 

and industrial chemical contamination were not likely causative factors in this mortality event. 

Elevated concentrations of pesticides (atrazine and aldicarb) were detected in surface water 

samples from bays within the region, and bay salinities were reduced to <10 ppt from December 

1991 through April 1992 due to record rainfall and freshwater runoff exceeding any levels since 

1939. Prolonged exposure to low salinity could have played a significant role in the unusual mortalities because low salinity exposure may cause disruption of the permeability barrier in dolphin 

skin. The lack of established toxicity data for marine mammals, particularly dermal absorption 

and bioaccumulation, precludes accurate toxicological interpretation of results beyond a simple 

comparison to terrestrial mammalian models. Results clearly indicated that significant periods 

of agricultural runoff and accompanying low salinities co-occurred with the unusual mortality 

event in Texas, but no definitive cause of the moralities was determined.

1 Worthy, Graham. 1992. Texas Marine Mammal Stranding Network, 

4700 Avenue U, Bldg. 303, Galveston, TX 77551. Personal commun.
2 NOAA Technical Report NMFS 147

cological data per se, and scientists are forced to use 

comparative terrestrial mammalian toxicological data as 

surrogate estimates of exposure, effects, and risk. While 

human forensic scientists have discrete methods for 

ascertaining cause of death, environmental risk assessment for cause and effect are still generally based on a 

“weight of evidence” approach which lacks discrete evidence of toxicological effects. Instead it may be based 

upon exceedances of sublethal, toxicological endpoints 

which indicate exposure, physiological impairment, or 

potential reduction in health.

The Working Group on Unusual Marine Mammal 

Mortalities, a panel of experts convened in 1991 to 

advise NMFS on such situations, was consulted about 

the dolphin deaths in Texas and agreed that a field 

investigation was warranted. Data were collected during 

the investigation for use in evaluating this unusual mortality event. In this paper we present results of the 

investigation and describe potential factors which may 

have affected dolphin health, based on the circumstantial evidence identified. Our conclusions include recommendations for improving future scientific response 

to unusual marine mammal mortality events. We also 

review actions that have already been taken to improve 

future investigations of this type.

Materials and Methods

Initial Field Investigation

The study’s first priority was to define the mortality event’s 

basic characteristics in order to determine the areas of 

focus for sample collection and analysis. Nautical charts 

and maps of the Texas coastline were used to determine 

the extent of logistical support required to conduct field 

reconnaissance. These indicated that the event’s suspected magnitude was probably underestimated due to 

the region’s numerous remote habitats (Fig. 1). In these 

remote regions, aerial surveys were initially used to determine the magnitude and geographical extent of the dolphin mortalities. Overflights were continued biweekly 

from 14 April until the end of July 1992 to locate carcasses 

for retrieval and examination and to record presence and 

behavior of live dolphins in the bay ecosystem. The aerial 

surveys also provided the opportunity to locate areas 

of unusual water coloration or potential algal blooms, 

define the land-use characteristics of the area surrounding the bay ecosystem in terms of agricultural and industrial activities, and determine whether any fish kills or 

mortalities of other taxa were occurring simultaneously 

with the dolphins’ deaths. Information from numerous 

local, state, federal, and academic sources regarding 

recent algal blooms, weather patterns, hazardous chemical spills, industrial discharges, known pollution sources, 

and dredging activities was collected and evaluated immediately. The available data for the stranded dolphins, 

including species, gender, length, and stranding location, 

were examined for any identifiable trends in those parameters. The reported number of dolphin deaths was compared to historical databases maintained by the Southeast 

U.S. Marine Mammal Stranding Network. 

Local field biologists, fishermen, and other residents 

were informally interviewed concerning their past experiences and observations with the local dolphin population, in an effort to determine whether the number 

of reported strandings was different from past years. 

The local stranding network was also consulted about 

changes in member participation. 

Efforts were focused on identifying potential factors 

responsible for the increased mortality rate, using sample 

collection and analysis from the dolphin carcasses and 

a thorough environmental/ecotoxicological evaluation. 

All samples collected and analyzed were treated as case 

evidence, using strict legal documentation and chainof-custody procedures, in order to withstand scrutiny 

under potential legal action. During this mortality event, 

an official federal law enforcement case was opened, 

and enforcement agents from federal and state agencies 

occasionally assisted the on-site coordinator with field 

activities.

Examination of Dolphins

A condition code system used by the stranding network 

classifies dolphin carcasses into five levels of decomposition, with code 1 being a live animal and code 5 being 

mummified or skeletal remains. When numerous animals 

were reported in a single day, this system was used to 

prioritize their examination. Attempts were made to evaluate the parameters listed in Table 1 during thorough 

carcass examinations on all stranded dolphins. Whenever 

possible after 12 April, lung, liver, kidney, and blubber 

were collected, but generally the organs taken were barely 

identifiable due to the advanced state of decomposition. 

These samples were considered unsuitable for existing 

analyses of any type.

In order to define the age structure of the dolphins 

that died during this mortality event and to more accurately interpret toxicological results, teeth were collected from the stranded animals for age determination. 

The results were not provided for this report, but will 

be published separately. Similarly, skulls were archived 

for morphometrics, but examination for determination of 

inshore/offshore characteristics has not been completed.

Commercial fishermen and local biologists that spend 

much of their time working in the bay ecosystem were 

informally interviewed concerning their observations of 

live dolphins. They were asked if live dolphins were seen 
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 3

in the study area since December, and if the animals 

exhibited any unusual characteristics. If unusual characteristics were observed, people were asked approximately when the signs were first noticed.

In addition to carcass examination, live dolphins were 

located and followed to detect unusual behavior. A 

marine mammal behavioral specialist was contracted to 

observe live dolphins in the bay ecosystem for a period 

of approximately two weeks.

Ecotoxicological Evaluation

Mortalities of multiple taxa, including birds and fish, were 

detected by aerial survey early in the investigation, resulting in an expansion of the investigation’s scope to include 

a thorough environmental risk assessment. The fish and 

bird mortalities appeared to overlap spatially and temporally with dolphin strandings. During the initial aerial 

survey, one bay location at Sand Point, Texas was identified where dead fish, primarily hardhead catfish (Arius 

felis), black drum (Pogonias cromis), laughing gulls (Larus 

atricilla), and four bottlenose dolphins, were observed on 

a two-mile stretch of beach between Lavaca and Keller 

Bays. Ground observations indicated that the carcasses 

were scavenged by fire ants, making the laughing gulls, 

catfish, and black drum unsuitable for sample collection 

and chemical analysis. Texas Parks and Wildlife Department personnel responded to the fish kill and conducted 

their usual survey. No indication of trauma was apparent 

from the external condition, and examination by radiography showed that the birds from the Sand Point location 

had not been shot. Additional reports were received of 

dead fish and birds in other isolated locations but, due 

to logistical problems, those reports were not confirmed 

and the over-washing of the narrow beaches at high tide 

often prevented verification of reported mortality.

Meteorological information suggested that the area 

had experienced unusually heavy rainfall between December 1991 and April 1992, which drastically reduced salinity in the bay ecosystem. The excessive rainfall coincided 

with spring pesticide application periods in local agricultural operations. Because of the involvement of multiple taxa and altered environmental conditions, an 

ecotoxicological field study was designed and executed 

to monitor the bay area for the presence of chemical 

or biological stressors that could adversely affect marine 

life in the bay ecosystem. It is important to note that 

this study took place after 14 April, several weeks after 

the peak dolphin mortality period had occurred during 

March.

The ecotoxicological evaluation was focused on detecting and quantifying stressors such as biotoxins and 

anthropogenic chemical contaminants present in water, 

sediment, and biota (tissue). A total of 17 stations were 

selected for ecotoxicological evaluation, covering a geographical area from Corpus Christi, Texas, north toward 

San Luis Pass, Texas (Fig. 2; Table 2). Station locations 

were selected based upon:

1) The occurrence of multiple dolphin strandings along 

with mortalities of other species, including birds and 

fish. Initial field investigations and aerial overflights 

identified several regions where numerous bottlenose 

dolphin strandings had occurred during the period, 

including Sand Point in Lavaca Bay, the shore of the 

Intracoastal Waterway west of Port O’Connor, and an 

Table 1

List of parameters recommended for use in examination 

of stranded T. truncatus along the Texas coast. Data records 

using video and photography are recommended.

Condition code

Sample type 11 22 33,4 4 5

Clinical

 — respiration rate x5

 — heart rate x

 — urine sample x

 — blood sample x x

Life history

 — stomach and contents x x x x

 — jawbone/teeth x x x x

 — skull x x x x

 — rib/spinal section x x x x

 — morphometrics x x x

 — reproductive organs x x x

 — parasites x x x

 — blubber thickness x x

Parasitology x x x x

Microbiology x x

Toxicology x x

Histopathology x x

Necropsy x x

 — external observation

 — site observations x x

 — ecto-parasites x x

 — gross lesions x x

 — gross dissection

 — parasites x x

 — gross lesions x x

 — major organs x

1 Code 1 animals are assigned to Code 2 at death.

2 Heart blood collected shortly after death.

3 Morphometrics should be taken on late Code 3 and Code 4 

animals only for those measurements that are not altered by 

decomposition of the soft body tissues.

4 Some tissues degrade quickly and will not yield samples suitable for histopathology; judgment is required.

5 x = sample/measurement taken.
4 NOAA Technical Report NMFS 147

area in the vicinity of Rockport, Texas, near Copano 

Bay (Fig. 2); 

2) Known point sources of pollution including industrial 

areas and active dredge disposal areas; 

3) Critical areas adjacent to agricultural drainage basins; 

and

4) Known areas where domoic-acid-producing phytoplankton could be found.

The parameters measured at each station (Table 3) 

included:

1) Gross observations and photographs of any dead or 

stranded animals, including fish, birds, and marine 

mammals (all stations);

2) Measurement of general water quality parameters including water temperature (°C), pH, dissolved oxygen 

(mg/L and % saturation), and salinity (ppt) for all 

stations;

3) Surface water samples analyzed for priority pollutants 

established by the Environmental Protection Agency 

(EPA), including trace metals, polycyclic aromatic 

hydrocarbons (PAHs), and pesticides (all stations);

4) Sediment and tissue (finfish) samples analyzed for priority pollutants, including trace metals, PAHs, and pesticides (selected stations); available dolphin livers were 

also analyzed for mercury (Hg) due to the known Hg 

contamination in Lavaca Bay;

5) Water and finfish samples collected for brevetoxin 

analysis and icthyotoxicity tests (selected stations);

6) Oyster (Crassostrea virginica) samples collected and analyzed for domoic acid by J. C. Wekell (NMFS, Northwest 

Fisheries Science Center) using methods described by 

Hatfield et al. (1994) (selected stations);

Figure 1

Representative stranding locations for a portion (57/240) of the dolphins stranded during the unusual 

1992 mortality event. Note that the majority of the strandings occurred in coastal and inland bay regions 

rather than in offshore waters.
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 5

7) Plankton tows for domoic acid-producing diatoms of 

the genus Pseudo-nitzchia made by hand (50 m/site) and 

examined using light microscopy (selected stations); 

8) Laboratory toxicity tests conducted with larval grass 

shrimp, Palaemonetes pugio, exposed for 96 h to surface water samples that tested positive for pesticides 

(selected stations); and

9) Morphometrics and external observations on stranded 

dolphins (stranding sites only).

Surface water samples (3.5 L) were initially screened for 

the presence of selected pesticides (polychlorinated biphenyls 

[PCBs], cyclodiene insecticides, triazine herbicides, and the 

insecticide aldicarb) using polyclonal antibody ELISA assays 

(Envirogard® Test Kits). Lower limits of detection (LLOD) 

in these assays ranged from 0.100–5 μg/L.

Surface water samples were extracted with dichloromethane, cleaned with florisil, and analyzed for priority 

pollutants (PAHs, PCBs, and pesticides) using gas chromatography (GC)-mass spectrometry (MS), GC-electron 

capture detection (ECD), and GC-nitrogen-phosphorus 

detection (NPD) using techniques described by Scott et 

al. 2 and Bush et al. (1978). Lower limits of detection 

ranged from 3–1,000 μg/L for pesticides and PCBs and 

from 2–8 μg/L for PAHs. Similar methods of analysis 

were used for sediment and tissue samples. 

Analyses of surface waters, sediments, and tissues 

included priority pollutant trace metals (established by 

EPA) using both atomic absorption spectrometry (AAS) 

and intracoupled plasma spectrometry (ICPS). Dolphin 

2 Scott, G. I., M. H. Fulton, M. C. Crosby, P. B. Key, J. W. Daugomah, 

J. T. Waldren, E. D. Strozier, C. J. Louden, G. T. Chandler, T. F. Bidleman, K. L. Jackson, T. W. Hampton, T. Hoffman, A. Shultz, and M. 

Bradford. 1992. Agriculture insecticide runoff effects on estuarine 

organisms: correlating laboratory and field toxicity tests, ecophysiology bioassays and ecotoxicological biomonitoring. Final Report prepared for EPA, Gulf Breeze Environmental Research Laboratory, Gulf 

Breeze, FL 32561, 281 p.

Figure 2

Station locations for sampling of water, sediments, and selected biota during the ecotoxicological assessment.
6 NOAA Technical Report NMFS 147

liver tissues were freeze-dried and weighed into Teflon®

digestion vessels with nitric acid. After the vessels were 

sealed, they were microwave extracted and brought to a 

volume of 25 ml. The samples were analyzed for heavy 

metals by cold vapor AAS. The lower level of detection 

(LLOD) for the method was 2.5 μg/g. The recovery of a 

standard reference material (SRM) was 83.6±8.0%(SD). 

QA/QC criteria were based on the SRM (Dogfish Liver 

Tissue [DOLT-1]) from NRC Canada Marine Analytical 

Chemistry Standards. For every two samples, one SRM 

and one blank was run. Acceptable analytical results 

were obtained when the SRM fell within the reported 

95% CI for DOLT-1 and the blank was zero.

Results

Initial Field Investigation

Comparison of stranding data to historical databases indicated that the number and nature of the mortalities occurring in the Texas bay system were unprecedented. During 

1992, a total of 240 Tursiops truncatus stranded along the 

Texas coast (Fig. 3). More than half of these strandings 

occurred in March (99 strandings) and April (61 strandings) (Fig. 3) in remote coastal regions of Aransas and 

Calhoun counties. In the southeastern U.S., 645 cetacean 

strandings occurred the previous year (1991), including 

448 (69%) bottlenose dolphins 3. A total of 139 cetacean 

strandings occurred in Texas during 1991, including 134 

(96%) bottlenose dolphins; and in Aransas and Calhoun 

counties, a total of 5 and 8 bottlenose dolphins were 

reported stranded respectively that year. From 1982–91, 

the number of annual bottlenose dolphin strandings in 

Calhoun and Aransas counties ranged from 0–12/year; 

this indicates that the number of dolphin strandings in 

the counties during early 1992 was a dramatic increase.

Helicopter surveys of the coastline established that the 

extent of the event did not exceed the number of animals reported or extend beyond the area suspected to 

be involved. Evaluation of available data from stranded 

dolphins indicated that both genders and all size ranges 

of bottlenose dolphins (Tursiops truncatus) were affected, 

reducing the likelihood that disease factors limited to a 

specific gender or to certain age structure were involved. 

Basic data for strandings that occurred in Texas from 

January through April 1992 are listed in Table 4.

From the stranding location data and area nautical 

charts, it was apparent that a large number of dolphin 

3 Odell, Daniel K. Southeast U.S. Marine Mammal Stranding Network, 

Sea World Research Institute, 7007 Sea World Dr., Orlando, FL 32821. 

Unpubl. data.

Table 2

List of stations sampled for surface water pesticide analysis during 1992. During this sampling period locations were focused on 

regions where dolphin strandings occurred. For 1993 data see Pennigton (1994). (Station locations can be seen on Fig. 2.)

Station Station

number classification1 Location2

 1 3 Sand Point, E end

 2 3 Sand Point, W end

 3 3 Sand Point, E end where road enters bay

 4 3 Intracoastal Waterway (ICW), S of Port OConnor, W bank

 5 3 ICW, S of Port O’Connor, E bank at dredge disposal pipes

 6 3 Port O’Connor, E of Texas Parks and Wildlife Field Office

 7 3 Fulton Beach

 8 3 Corpus Christi, power plant intake

 9 3 Corpus Christi, ponds at power plant

10 3 Galveston Bay, southern pass

11 3 Upper Lavaca Bay, park and boat landing

12 3 Lavaca Bay, dredge spoil island

13 3 Lavaca Bay, Magnolia Beach

14 3 North Matagorda Bay, live capture site

15 3 Middle Matagorda Bay, Greens Bayou live capture

16 2 Caranachua Bay

17 3 Port O’Connor jetty

1 1=agricultural sites; 2=stream sites downstream from agricultural areas; 3=bay sites.

2 E=East; W=West; S=South.
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 7

deaths appeared to be concentrated along the coasts of 

the inner bay system from Caranachua Bay southwest 

to Copano Bay, but not along the outer coast of the 

barrier islands along the Gulf of Mexico (Fig. 1). The 

location of the barrier islands (Matagorda Island and 

Matagorda Peninsula) and the small size and number 

of tidal inlets greatly reduced the likelihood that such 

a large number of dolphins were washing into the bay 

system from offshore. Dolphin skulls archived from the 

die-off are being examined to determine if morphology was characteristic of inshore or offshore animals. 

Also, observations that very few dolphins had stranded 

on the Gulf coast of the barrier islands suggested that 

the dolphins probably died within the bay areas. The 

exact location of the animals at the time of death could 

not be fully established because no stranded dolphins 

were observed alive. Live dolphins were sighted throughout the bay system during the event, and locals indicated that live dolphins were often seen in the area 

year-round.

According to information collected from state, 

regional, and local authorities, as well as academic institutions, there were no reported oil/hazardous materials 

spills or other chemical discharges, no known harmful 

algal blooms other than a brown tide in Laguna Madre 

(further to the southwest), and no increase in stranding 

rates along other regions of the coast. This suggested 

that the unusual mortalities were part of an isolated 

incident of unknown cause limited to a small geographical region. Known pollution sources were identified in 

several bay areas. For example, Lavaca Bay has been 

contaminated with mercury since the 1960s, and portions of the bay have been closed to fishing due to high 

mercury levels. The Lavaca Ship Channel was dredged 

during early 1992; no information was available regarding the effects of dredging on the mercury levels in 

the water column. Isolated incidences involving chemical discharge from area industries were identified, but 

these were ruled out as potential causes because of the 

type and quantity of the chemicals.

Table 3

List of parameters measured during the field investigation of known pollution sources at sites with a heavy concentration of 

stranded T. truncatus. Matrices sampled include water (W), sediment (S), tissue (T), and phytoplankton (P).

LLOD (ppm= mg/L (W);

Parameter Matrix mg/Kg (T or S))

Physiochemical water quality

 — Temperature W

 — Salinity/conductivity W

 — Dissolved oxygen W

 — pH W

Chemical contaminants

 — Trace metals (Ag, As, Be, Cd, Cr, Cu, Hg, Pb, Ni, Sb, Th, Zn) W,S,T 0.1–1.0

 — Polycyclic aromatic hydrocarbons and other priority pollutants1 W,S,T 0.004–0.012

 — Persistent pesticides2 W,S,T 0.00001–0.0003 (W)

 0.005–0.1 (S)

 0.01–1.0 (T)

 — Polychlorinated biphenyls (aroclors 1016,1221, 1232, 1242,1248 and1260) W,S,T 0.3

Biotoxins

 — Brevetoxin and domoic acid W,T

 — Brown tide W,P

1 Compounds include: Phenol; Bis (2-chloroethyl)ether; 2-chlorophenol; 1,3 dichlorophenol; 1,4 dichlorophenol; benzyl alcohol; 1,2 dichlorobenzene; 2 methylphenol; Bis (2-chloroisopropyl)ether; 4-methylphenol; N-nitro-di-n-propylamine; hexachloroethane; nitrobenzene, isophorene; 2-nitrophenol; benzoic acid; Bis (2-chloroethoxy)methane; 2,4 Dichlorophenol; 1,2,4 trichlorobenzene; naphthalene, 4-chloroaniline; 

haxachlorobutaniene; 4-chloro-3-methylphenol; 3-methyl naphthalene; hexachloropentadiene; 2,4,6 trichlorophenol; 2,4,5 trichlorophenol; 2 chloronaphthalene; acenaphthalene; 2,6 dinitrotoluene; 3 nitroanaline; acenaphthene; 2,4 dinitrophenol; 4-nitrophenol; dibenzofuran; 2,4 dinitrotoluene; diethylphthalate; 4-chlorophenyl-phenylether; flourene; 4-nitroaniline; 4,6-dinitro-2-methylphenol; N-nitrosodiohenylamine; 4-bromophenyl-pheny ether; hexachlorobenzene; pentachlorophenol; phenanthrene; anthracene; Di-N-butylphthalate; fluoranthene; pyrene; butylbenzylphthalate; 3,3-dichlorobenzidine; benzo (a) anthracene; chrysene; bis (2-ethylhexyl) phthalate; di-N-octylphthalate; 

benzo (b) fluoranthene; benzo (k) fluoranthene; benzo (a) pyrene; dibenzo (a,h) anthracene and benzo (g,h,i) perylene.

2 Compounds include: aldrin; α-BHC; βBHC; γBHC; σBHC; chlordane; 4,4-DDE; 4,4-DDT; Diazinon; dielderin; Dursban; endosulfan (I, II 

and sulfate); endrin; permethrin; endrin aldehyde; EPN; heptachlor; heptachlor-epoxy; kelthane; lindane; malathion; methyl parathion; 

mirex; toxaphene; methoxychlor; HCB; pentachloroanisole; parathion; phosdrin; Rabon; Trichloron; atrazine; balin; Bravo; bromacil; dervinol; 

dimethoate; Dual; Eptam; Goal; hexazinone; Lasso; Ordam; oxadiazon; Paarlan; Propachlor; Propazine; Prowl; Roneet; Sencor; simazine; 

Sutan; Terbacil; Tillam; Tolban; Treflan and Vernam,.
8 NOAA Technical Report NMFS 147

Two common characteristics were apparent among 

many of the dolphin carcasses found prior to April 14. 

A gray, pasty substance (Plate 1) and severe skin blisters (Plate 2) were visible on several of the stranded dolphins. These conditions did not appear to have occurred 

post-mortem. It was necessary to try to determine when 

these dermal conditions first developed. Interviews with 

local individuals helped provide this information. Several shrimp fishermen reported observations of live dolphins with numerous blisters and sores as well as cloudy 

eyes. Local biologists who regularly work on the bay also 

reported observations of live dolphins covered with a 

gray, pasty substance and exhibiting blisters. Information obtained from these sources indicated that dolphins began exhibiting these characteristics as early as 

January 1992, immediately prior to increased mortality 

from February through early May 1992. 

Examination of Dolphins

The decomposed condition of dolphins precluded analysis of tissues for viruses, bacteria, organic contaminants, 

biotoxins, and histopathology. Several carcasses that would 

have been suitable for analyses were stranded prior to 

mid-April, but were not sampled due to the overwhelming number of strandings to which network members 

responded (Plates 1 and 2). Most carcasses were in a condition unsuitable for thorough sample collection (Plate 

3), and only length and location data were recorded. 

Several attempts were made to isolate microbes from 

the gray substance and fluid from the blisters of dead 

dolphins, but in all cases, the sample quality was compromised by decomposition; culture analysis identified 

microorganisms primarily associated with decomposition. 

All stranded dolphins examined from the bay ecosystem 

were between late code 3 and code 5 condition. Animals 

were retrieved immediately after stranding reports were 

received. Only two dolphins in code 2 (freshly dead) condition were recovered during the period of investigation; 

they were stranded in the Galveston area, outside the area 

of concern, and not considered to be directly involved in 

the local event. The two dolphins had none of the external conditions previously described. 

The behavioral specialist contracted during May to 

observe live dolphins for signs of stress identified no 

unusual behavior, and no live dolphins were observed 

with skin lesions.4 It should be noted that the behavioral observations were made after the mortality rate 

had begun to decline and after surface water salinity 

had increased.

Because the dolphins that stranded during the field 

investigation period did not provide the quality of tissues needed for accurate evaluation of potential disease 

factors and biomarkers for certain contaminant exposures, a live capture operation was planned as quickly as 

possible by the NMFS/SEFSC. The intent was to capture 

apparently healthy dolphins in the bay ecosystem and 

collect relatively noninvasive samples for selected health 

parameters without harming the animals. The results 

Figure 3

Stranding rates for each coastal county and the entire 

state of Texas for Jan.–June 1992. Note the elevated mortality rates in Aransas, Calhoun, Galveston, and Matagorda counties and the similarities in the pattern of mortality 

in each region. 4 Wursig, Berendt. Texas A&M University, College Station, TX 77843. 

Personal commun. 
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 9

Table 4

Stranding information provided by the Texas Marine Mammals Stranding Network (G. Worthy) for the Texas coast during the 

mortality event reported here. 

Region # Species Sex Length Date reported Lat Long

CC108 T. truncatus U 1/4/92 27°16'.6" 97°20'.85"

CC109 T. truncatus U 118.75 2/2/92 27°24'.3" 97°18'.3"

CC110 T. truncatus F 242 2/12/92 27°20' 97°19'.8"

CC111 T. truncatus F 271.8 3/2/92 27°15'.7" 97°21'.1"

CC112 T. truncatus M 257.8 3/12/92 27°28'.6" 97°16'.4"

CC113 T. truncatus U 218 3/26/92 26°58' 97°22'.6"

CC114 T. truncatus F 216.45 3/26/92 26°58' 97°22'.6"

GA419 Unknown M 310 1/6/92 29°3'.52" 95°8'.3"

GA420 T. truncatus M 210 1/11/92 29°32'.9" 94°23'.2"

GA421 T. truncatus M 103 1/29/62 28°56'.7" 95°17'.3"

GA422 T. truncatus F 98.5 2/22/92 29°17'.2" 94°47'.5"

GA423 T. truncatus M 215 2/24/92 29°28'.7" 94°34'.2"

GA424 T. truncatus U 2/27/92 28°52'.5" 95°22'.7"

GA425 T. truncatus M 224 2/28/92 28°55'.8" 95°18'.35"

GA426 T. truncatus F 174 3/3/92 28°58'.6" 95°15'.1"

GA427 T. truncatus M 248 3/9/92 29°3'.7" 95°8'

GA428 T. truncatus M 89 3/6/92 29°8'.7" 95°2'.8"

GA429 T. truncatus F 98 3/6/92 29°11'.72" 94°57'.9"

GA430 T. truncatus M 110 3/8/92 29°27'.2" 94°37'.6"

GA431 T. truncatus M 162 3/8/92 29°10'.8" 94°58'.5"

GA432 T. truncatus M 249 3/9/92 29°11'.1" 94°57'.9"

GA433 T. truncatus M 111 3/10/92 29°9'.8" 95°1'

GA434 T. truncatus M 227 3/18/92 28°55'.2" 95°20'.5"

GA435 T. truncatus M 107 3/21/92 29°29'.1" 94°33'.2"

GA436 T. truncatus F 255 3/21/92 29°32'.05" 94°25'.6"

GA437 T. truncatus F 102 3/21/92 29°32'.05" 94°25'.6"

GA438 T. truncatus F 237 3/21/92 29°32'.2" 94°24'.91"

GA439 T. truncatus M 259 3/22/92 29°29' 94°55'.1"

GA440 T. truncatus M 247 3/24/92 28°56'.8" 95°17'.21"

GA441 T. truncatus U 3/27/92 29°15'.35" 94°50'.75"

GA442 T. truncatus M 101 3/28/92 28°7' 94°34'.2"

GA443 T. truncatus M 118 3/28/92 29°13'.9" 94°53'.1"

GA444 T. truncatus U 106 3/29/92 29°21'.9" 94°45'.3"

GA445 T. truncatus M 249 3/29/92 29°30'.3" 94°29'.2"

GA446 T. truncatus M 255 3/30/92 29°40'.41" 94°49'.4"

GA447 T. truncatus M 109 3/30/92 29°15' 94°51'.3"

GA448 T. truncatus M 260 3/31/92 29°13' 94°54'.6"

GA449 T. truncatus M 105 3/31/92 29°9'.2" 94°9'

GA450 T. truncatus F 130 4/2/92 29°13'.7" 94°53'.6"

GA451 T. truncatus F 111 4/6/92 29°18'.33" 94°46'.1"

GA452 T. truncatus F 105 4/9/92 29°17'.28" 94°47'.5"

GA453 T. truncatus M 105 4/11/92 29°16'.6" 95°48'.5"

GA454 T. truncatus U 4/13/92 29°30'.9" 94°29'.1"

GA455 T. truncatus M 256 4/15/92 29°10'.71" 94°58'.1"

GA456 T. truncatus M 248 4/19/92 29°2' 95°3'.7"

GA457 T. truncatus F 218 4/17/92 29°16'.82" 94°48'.1"

GA458 T. truncatus F 241 4/19/92 29°28'.8" 94°34'

GA459 T. truncatus U 142 4/22/92 29°32'.45" 94°24'.45"

PA233 T. truncatus U 204 1/6/92 27°43'.3" 97°20'.5"

PA234 T. truncatus F 284 1/8/92 27°40'.3" 97°10'.2"

PA235 T. truncatus M 256 1/27/92 28°3' 97°1'.8"

PA236 T. truncatus F 230 2/5/92 27°37'.1" 97°12'.5"

PA237 T. truncatus M 170 2/5/92 28°3'.3" 97°2'

PA238 T. truncatus F 234 2/7/92 27°49'.8" 97°7'.3"

PA239 T. truncatus U 281 1/29/92 27°90'.3" 97°10'

PA240 T. truncatus F 239 2/10/92 28°2' 97°1'.5"

continued
10 NOAA Technical Report NMFS 147

Table 4 (continued)

Region # Species Sex Length Date reported Lat Long

 PA241 T. truncatus F 200 2/8/92 27°51' 97°2'.6"

PA242 T. truncatus F 88.5 2/13/92 27°45'.765" 97°6'.48"

PA243 T. truncatus M 106 2/13/92 27°36'.02" 97°12'.5"

PA244 T. truncatus F 248 2/18/92 27°52'.7" 97°1'.6"

PA245 T. truncatus M 260 2/19/92 27°55' 97°4'.7"

PA246 T. truncatus F 206 3/5/92 28°17'.4" 96°48'.5"

PA247 T. truncatus F 175 3/4/92 27°40' 97°10'.5"

PA248 T. truncatus M 270 3/5/92 28°18' 96°48'

PA249 T. truncatus F 228 3/5/92 28°18' 96°48'

PA250 T. truncatus M 258 3/9/92 28°4' 97°2'

PA251 T. truncatus M 270 3/9/92 28°6'.1" 97°1'.4"

PA252 T. truncatus U 2/28/92 28°8'.5" 96°53'

PA253 T. truncatus U 3/2/92 28°7' 96°54'

PA254 T. truncatus U 3/2/92 28°3'.5" 96°57'.5"

PA255 T. truncatus U 3/2/92 28°3'.5" 96°57'.5"

PA256 T. truncatus U 3/2/92 28°3'.5" 96°57'.5"

PA257 T. truncatus M 217 3/4/92 28°3'.8" 97°1'.9"

PA258 T. truncatus F 224 3/16/92 28°5' 97°2'

PA259 T. truncatus F 3/17/92 28°5'.7" 97°1'.9"

PA260 T. truncatus M 101 3/17/92 28°7' 96°59'

PA261 T. truncatus M 120 3/17/92 28°0' 97°3'

PA262 T. truncatus F 252 3/18/92 28°8' 97°0'

PA263 T. truncatus F 236 3/19/92 28°10'.2" 47°1'

PA264 T. truncatus M 272 3/19/92 28°8' 97°0'

PA265 T. truncatus M 199 3/19/92 28°5'.6" 97°3'.2"

PA266 T. truncatus M 250 3/20/92 28°2'.8" 97°1'.6"

PA267 T. truncatus F 269 3/20/92 28°5'.5" 97°2'

PA268 T. truncatus M 225 3/24/92 28°53'.4" 97°2.8"

PA269 T. truncatus F 245 3/23/92 28°7'.5" 96°58'.8"

PA270 T. truncatus F 212 3/24/92 28°5'.1" 97°2'

PA271 T. truncatus M 222 3/24/92 28°5'.1" 97°2'

PA272 T. truncatus M 229 3/24/92 28°6'.4" 97°1'.3"

PA273 T. truncatus F 208 3/14/92 28°5'.1" 97°4'.5"

PA274 T. truncatus F 267 3/24/92 28°5'.2" 97°13'

PA275 T. truncatus U 154 3/25/92 28°9' 97°1'.8"

PA276 T. truncatus M 285 3/30/92 28°6'.6" 97°1'.3"

PA277 T. truncatus M 261 3/31/92 28°0'.4" 97°3'.4"

PA278 T. truncatus M 261 3/31/92 28°3'.1" 97°1'.8"

PA279 T. truncatus F 264 3/30/92 28°14'.9" 96°47'.2"

PA280 T. truncatus M 210 3/19/92 28°17'.5" 96°48'.5"

PA281 T. truncatus U 269 4/1/92 28°1'.3" 97°2'.8"

PA282 T. truncatus M 123 4/2/92 28°2'.5" 97°1'.58"

PA283 T. truncatus M 96 4/4/92 27°46'.9" 97°23'.3"

PA285 T. truncatus M 190 4/6/92 27°50'.5" 97°2'.7"

PA286 T. truncatus U 4/3/92 27°59'.4" 97°4'

PA287 T. truncatus F 238 4/7/92 28°3'.9" 97°2'

PA288 T. truncatus U 4/7/92 27°54'.4" 97°3'.5"

PA289 T. truncatus F 224 4/9/92 28°3'.1" 97°1'.9"

PA290 T. truncatus F 247 4/9/92 28°1'.5" 97°2'.8"

PA291 T. truncatus M 179 4/9/92 28°4'.8" 97°5'.5"

PA292 T. truncatus M 263 4/11/92 28°39'.4" 97°10'.8"

PA293 T. truncatus U 238 4/10/92 28°18' 96°48'

PA294 T. truncatus M 98 4/11/92 28°2'.5" 97°1'.5"

PA295 T. truncatus U 4/13/92

PA296 T. truncatus U 241 4/14/92 28°3'.7" 97°7'.7"

PA297 T. truncatus U 237 4/15/92 28°9' 97°1'.5"

PA298 T. truncatus F 115 4/17/92 28°0'.8" 97°3'.2"

continued
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 11

Table 4 (continued)

Region # Species Sex Length Date reported Lat Long

PA299 T. truncatus F 239 4/20/92 28°5'.2" 97°3'.9"

PA300 T. truncatus 4/28/92

PA301 Unknown M 230.7 4/29/92 28°7'.35" 96°59'.1"

PA302 T. truncatus U 4/27/92 28°1'.9" 97°1'.5"

PA 303 T. truncatus U 3/24/92 28°8' 96°52'.5"

PA 304 T. truncatus U 3/24/92

PA 305 T. truncatus U 4/28/92 27°49'.38" 97°8'.92"

PA 306 T. truncatus M 223 4/30/92 27°52'.38" 97°5'.87"

PA 307 T. truncatus F 180.4 4/26/92 28°11'.83" 97°1'.3"

PO176 T. truncatus M 252 1/22/92 28°45.8' 95°37.2'

PO177 T. truncatus M 254 1/22/92 28°37.5' 95°54.3'

PO178 T. truncatus M 189 1/24/92 28°35.7' 95°58.8'

PO179 Unknown cetacean 2/9/92 28°27.8' 96°16'

PO180 T. truncatus F 236 2/25/92 28°40.2' 95°48.1'

PO181 T. truncatus F 210 2/25/92 28°36.05' 95°58'

PO182 T. truncatus U 211 2/20/92 28°24.4' 96°28.7'

PO183 T. truncatus U 3/10/92 28°22.6' 96°42'

PO184 T. truncatus U 250 3/10/92 28°22.2' 96°28.2'

PO185 T. truncatus U 245 3/13/92 28°26.2' 96°24.8'

PO186 T. truncatus U 295 3/13/92 28°26.2' 96°24.7'

PO187 K. breviceps U 240 3/18/92 28°7.3' 96°46.2'

PO188 T. truncatus M 212 3/20/92 28°44.2' 96°23.9'

PO189 T. truncatus U 154 3/20/92 28°33.7' 96°32.3'

PO190 T. truncatus U 205 3/22/92 28°33.7' 96°32.3'

PO191 T. truncatus U 3/19/92 28°20.7' 96°47.6'

PO192 Unknown U 3/19/92 28°20.7' 96°47.6'

PO193 T. truncatus M 215 3/24/92 28°18.8' 96°37.6'

PO194 T. truncatus U 280 3/20/92 28°20.7' 96°35.5'

PO195 T. truncatus U 255 3/20/92 28°21.3' 96°34.7'

PO196 T. truncatus M 205 3/24/92 28°23.4' 96°31.5'

PO197 T. truncatus U 245 3/24/92 28°23.8' 96°30.7'

PO198 T. truncatus M 228 3/24/92 28°24.4' 96°29.6'

PO199 T. truncatus U 240 3/24/92 28°26' 96°25.7'

PO201 T. truncatus F 3/27/92 28°36.8' 96°56.3'

PO202 T. truncatus U 3/27/92 28°36.8' 96°56.3'

PO203 Unknown dolphin 3/26/92 28°22.5' 96°23.5'

PO204 T. truncatus 3/26/92 28°26' 96°25.3'

PO205 T. truncatus U 238 3/27/92 28°41.6' 96°39.8'

PO206 T. truncatus M 214 3/15/92 28°19' 96°37.4'

PO207 T. truncatus M 270 3/20/92 28°19.7' 96°36.5'

PO208 T. truncatus M 275 3/20/92 28°20' 96°36.2'

PO209 T. truncatus U 3/20/92 28°21.1' 96°35'

PO210 T. truncatus F 113 28°19.3' 96°37'

PO211 T. truncatus U 3/15/92 28°18.9' 96°37.5'

PO212 T. truncatus M 211 3/29/92 28°26.6' 96°24.2'

PO213 T. truncatus U 3/19/92 28°35.8' 96°10.7'

PO214 T. truncatus 180 28°41.6' 96°25'

PO215 T. truncatus U 3/27/92 28°40.9' 96°17.4'

PO216 Unknown U 4/5/92 28°13.7' 96°37.4'

PO217 Unknown U 4/5/92 28°11.7' 96°40.4'

PO218 T. truncatus U 4/6/92 28°26.2' 96°25.3'

PO219 T. truncatus U 4/5/92 28°20' 96°24.7'

PO220 T. truncatus F 202 4/8/92 28°26.2' 96°24.8'

PO221 T. truncatus M 250 4/8/92 28°24.2' 96°25.2'

PO222 T. truncatus 4/8/92 28°24' 96°24.5'

PO223v T. truncatus F 111 4/12/92 28°33.7' 96°32.3'

PO224 T. truncatus F 230 4/10/92 28°39.2' 96°35.8'

continued
12 NOAA Technical Report NMFS 147

of this capture study are published separately.5 Though 

the current serological tests for dolphin morbillivirus 

were not available at the time of the live capture, tests 

for canine distemper were negative. A subsequent study 

using new technology has shown that 24% of the dolphins captured and examined during July 1992 tested 

positive for cetacean morbillivirus (Duignan et al., 1996). 

It is not known if the animals captured in July were 

present in the bay ecosystem during the peak mortality 

period.

To determine whether the live dolphins in the bay 

area were part of a resident or migratory population, 

satellite tagging and tracking of dolphins were conducted in the area where the mortality event occurred. 

Lynn (1995) reported that nearly all of the 35 dolphins 

branded during live capture appeared to be resident 

to the Matagorda-Espiritu Santo Bay area, and overall 

mark/recapture population size estimates from the 

photo-identification study demonstrated that 218±71.4 

(95% CI) dolphins used an area of 312 km2 in Matagorda and Espiritu Santo Bays.

Table 4 (continued)

Region # Species Sex Length Date reported Lat Long

PO225 T. truncatus M 177 4/14/92 28°36' 96°25.2'

PO226 T. truncatus U 253 4/14/92 28°35.1' 96°26.8'

PO229 T. truncatus M 267 4/17/92

PO230 T. truncatus M 115 4/18/92 28°27.7' 96°25'

PO231 T. truncatus M 253 4/18/92 28°35.1' 96°35.1'

PO232 T. truncatus M 234 28°24' 96°29'

PO233 T. truncatus U 28°24.1' 96°28.9'

PO234 T. truncatus M 248 4/18/92 28°26.3' 96°19.8'

PO235 T. truncatus U 4/16/92 28°35' 96°34.6'

PO236 T. truncatus M 235 4/19/92 28°19.5' 96°30.8'

Plate 1

Example of gray, pasty substance on the skin of a stranded dolphin. Photographed by Tom 

Wagner of Texas Parks and Wildlife Department.

5 Sweeney, J. C. 1992. Veterinary assessment report, Tursiops truncatus, 

Matagorda Bay, Texas, July, 1992. Contr. Rep. NOAA, NMFS, Southeast 

Fisheries Science Center, Miami Laboratory Contrib. MIA-92/93-41, 

10 p. + 5 append.
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 13

Plate 2

Example of blisters and circular lesions on the skin of a stranded dolphin. Photographed by 

Tom Wagner of Texas Parks and Wildlife Department.

Table 5

Metals data quantified from livers of stranded dolphins during 

the unusual mortality event along the Texas coast during 1992. 

Metals concentrations (μg/g dry weight)

Sample # Arsenic Cadmium Lead Mercury Selenium

PA259 0.1223 0.0086 0.0521 1.9197 1.2177

PA260 0.1385 0.0031 0.0039 0.8112 1.3533

PA294 0.0922 0.0016 0.0055 0.5980 0.5313

PA298 0.1844 0.0007 0.0129 0.6224 3.7370

PA301 0.1994 0.0424 0.1067 10.3978 5.1535

PO199 0.4644 0.1372 0.1126 33.6303 23.2716

PO210 0.0703 0.0027 0.0122 0.8686 1.9325

PO212 0.1956 0.0649 0.0539 13.3652 8.5036

PO221 0.1638 0.0538 0.1002 47.9008 23.4912

PO223 0.0059 0.0012 0.0443 1.7375 0.9801

PO224 0.4559 0.0423 0.0548 17.1141 9.3483

PO225 0.3127 0.0074 0.0496 2.7288 2.0019

PO226 1.0885 0.1980 0.3237 932.1566 363.6390

PO231 0.4128 0.1211 0.2453 172.2030 80.1658

PO232 0.1378 0.0708 0.1560 34.6706 22.1409

PO234 0.0298 0.0786 0.0623 27.2880 14.8838

PO236 0.2655 0.0896 0.1034 26.9704 16.0083

Results from the metals analysis on sampled dolphin 

livers are shown in Table 5. Ranges for the 5 metals (arsenic, cadmium, lead, mercury, and selenium) are 0.0059–

1.0885 μg/g for As, 0.0007–0.1980 for Cd, 0.0039–0.3237 

for Pb, 0.5980–932.1566 for Hg, and 0.5313–363.639 μg/g 

for Se. For each metal, the highest concentration was 

quantified from one stranded dolphin, PO226. For the 

remainder of the dolphins, trace metal concentrations 

were generally comparable among individuals. Linear 

regression analysis of individual metals concentration 

and total length of dolphins showed two clusters of R2

values. R2 values <0.2 were found for As (0.18), Hg 

(0.10), and Se (0.11). Those metals with R2 values >0.2 

were Cd (0.47) and Pb (0.45). These results indicate that 

as total length increases, generally so does metals concentration within the liver. However, it is important to 

note that length and age in marine mammals have not 

been well correlated. Additionally, some metals concentrations within the livers exceeded the Food and Drug 

Administration (FDA) action levels and other international limits for metals in fisheries products. For Hg, 

the U.S. has a limit of 1.0 ppm in fillets, while the most 

restrictive limit found is 0.1 ppm in fillets (Venezuela) 

(EPA, 1989). 
14 NOAA Technical Report NMFS 147

Plate 3

Typical condition of stranded dolphins during the 1992 unusual mortality event in Texas. Photographed by 

Ann Colbert, on-site coordinator of the investigation.

Ecotoxicological Evaluation

General Water Quality Measurements of surface water 

temperatures at each station indicated temperatures ranging from 21.8–28.8°C, averaging 25.4°C (Table 6). Dissolved oxygen ranged from 5.8–15.8 mg/L (82–200% 

saturation), averaging 7.95 mg/L (103% saturation). The 

pH ranged from 7.9–9.3, averaging 8.43. High pH (9.3) 

was observed at Station #9 (Corpus Christi power plant) 

which exceeded EPA water quality criteria for pH. Salinities throughout the area were extremely low, ranging 

from 0–24 ppt, averaging 12 ppt. Evaluation of rainfall 

(Fig. 4) and riverflow records (Fig. 5) indicated that excessive rainfall caused the extremely low salinities and the 

high river flow measured during January–April 1992.

Chemical and Toxicological Analysis of Surface Water 

Samples Screening of surface water samples for pesticides and PCBs resulted in the detection of triazine herbicides (i.e. atrazine) (0.1–1.0 μg/L) at all sites assayed. 

The insecticide aldicarb (≥2–<5 μg/L) was detected at 

four sites during April 1992 (Table 7). These results indicated that significant pesticide runoff from agricultural 

fields into shallow bays had occurred following the periods of excessive rainfall during January–April 1992. No 

PCBs or cyclodiene pesticides were detected (Table 7). 

Subsequent sampling during July 1992 also detected triazine herbicides (0.1–1.0 μg/L) at all sites assayed, and 

aldicarb (>2–<5 μg/L) at four sites (Table 8). These findings suggested that pesticide inputs continued well into 

the summer, after peak application and rainfall periods.

Subsequent GC-ECD analysis of surface water samples 

for PCBs, PAHs, and pesticides failed to detect the presence of any of these chemical contaminants. Aldicarb 

was not detected by GC-NPD analysis at levels above 

the LLOD (3 μg/L). An identifiable peak was observed 

in one sample, but it was below the LLOD (<3 μg/L). 

These data suggest that any aldicarb present was at concentration <3 μg/L. This agrees in part with the ELISA 

screening bioassay results which indicated levels >2–

<5 μg/L. Subsequent sampling and analysis during 1993 

quantified the presence of aldicarb in surface water by 

GC-NPD at concentrations of 2–3 μg/L (Pennington et 

al., 1994). During this follow up study in 1993, rainfall 
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 15

Table 6

Results of general water quality parameters at bay stations sampled during the Texas marine mammal stranding study.

Temp Salinity DO2 %

Station # (oC) (ppt) pH (mg/L) Saturation

 1A 23.7 10.0 8.4 8.1 109

 1B 27.8 11.0 8.3 6.7 91

 2 25.8 11.0 8.3 7.8 104

 3 26.8 11.0 8.3 6.9 92

 4 27.4 3.0 8.7 7.2 95

 5 25.3 8.0 8.5 7.3 NM1

 6 24.5 22.0 NM 7.4 99

 7 25.5 3.0 8.4 7.8 98

 8 28.8 23.0 8.3 5.8 95

 9A 26.7 24.0 9.3 9.1 122

 9B 26.7 24.0 9.3 15.8 >200

10 25.0 24.0 8.3 6.6 91

11 21.8 0.0 8.3 7.6 87

12 22.6 3.0 7.9 7.0 82

13 22.4 3.0 8.2 8.2 92

Average 25.40 12.0 8.4 7.9 103.4

SE± 0.54 2.33 0.10 0.60 8.00

Range 21.8–28.8 0.0–24.0 7.9–9.3 5.8–15.8 82–>200

1 NM = not measured.

Figure 4

Comparison of the average precipitation at bay and inland sites along the Texas coast, Dec. 

1991–June 1992.

levels were at or below normal for the region for the 

February to October study period.

GC-NPD analysis also indicated that the triazine 

herbicide detected by ELISA at concentrations >0.1–

1.0 μg/L was atrazine. Subsequent sampling and analysis in a follow-up study in the area during 1993 indicated atrazine concentrations ranging from 0.1–>1.0 

μg/L and that 98% of >130 samples collected were positive for atrazine (Pennington, 1996). Other GC-MS, 

GC-ECD, GC-NPD, AA, and ICP results did not detect 

the presence of any other priority pollutants in surface 

waters. Toxicological analysis of selected surface water 

samples using P. pugio larvae indicated no toxicity in any 

of the samples analyzed (e.g. those containing aldicarb 

and atrazine).

Chemical Analysis of Sediments Chemical analysis of 

sediment samples collected at selected stations indicated 

the presence of several trace metals including arsenic (As) 

(0.26–7.64 mg/kg), mercury (Hg) (<0.020–0.050 mg/kg), 

selenium (Se) (<0.033–0.063 mg/kg), barium (Ba) 

(9.0–56.0 mg/kg), chromium (Cr) (<1.0–3.00 mg/kg), 

copper (Cu) (<1.00–1.00 mg/kg), lead (Pb) (<2.00–5.00 

mg/kg), and nickel (Ni) (<1.00–1.00 mg/kg). None of 

the concentrations of trace metals detected exceeded the 

current Sediment Quality Guidelines (Effects Range Low 

[ERLs] or Effect Range Median [ERMs]) as described by 
16 NOAA Technical Report NMFS 147

Figure 5

River flow and water discharge data from dams in the Quadelupe and 

San Antonio Rivers, Dec. 1991–April 1992. Note the elevated river flow 

and water discharges during February 1992.

Long et al. (1995). Also, no detectable levels of persistent 

pesticides, PAHs, or PCBs were found in sediment samples. 

These results indicate that none of the sediments sampled 

within the study area contained contaminants at concentrations known to be toxic to living marine resources.

Chemical Analysis of Animal Tissues Chemical analysis 

of whole fish (prey species) samples collected by cast-net 

from selected stations within the bay system indicated the 

presence of DDD (<2–10 μg/Kg), DDE (<1–40 μg/Kg), 

As (0.14–0.42 mg/Kg), Se (0.19–0.49 mg/Kg), Hg 

(<0.020–0.042 mg/Kg), Ba (<1.00–11.00 mg/Kg), Cu 

(<1.00–7.00 mg/Kg), and Ni (<1.00–5.00 mg/Kg). None of 

the detectable contaminants exceeded FDA action levels/

tolerances for edible fishery products. These results indicated that fish (mullet, menhaden, catfish, and anchovy) 

within the area of concern did not contain elevated concentrations of those chemical contaminants. Rather, fish 

appeared to contain only degradation products from DDT 

(i.e. DDD and DDE) and trace metals at rather low concentrations, suggesting that dolphin mortality was probably not related to bioaccumulation through the food 

chain of any of the contaminants listed above.

Analysis of Surface Waters and Finfish for Biotoxins

Analysis of surface waters and finfish for brevetoxin were 

negative (<LLOD). Icthyotoxicity tests exposing sheepshead minnow (Cyprinodon variegatus) to extracts of surface 

water and finfish were negative, indicating that brevetoxin 

was not present in harmful concentrations in any of the 

samples analyzed. Similarly, analyses of surface water and 

adult oysters (Crassostrea virginica) for domoic acid, using 

methods described by Hatfield et al. (1994), were negative. Light microscope examination of plankton tows col-
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 17

Table 7

Results of April 1992 water samples using the Enviroguard® Test Kits as a screening tool.

Concentration

Cyclodiene PCBs Triazines Aldicarb

Site (ppb) (ppm) (ppb) (ppb)

Cherry Point, SC (reference) —1 — >0.1, <1.0 < llod 2

Sand Point, East Bay < llod < llod > 1.0 >2, <5 

Sand Point, West Bay < llod < llod > 1.0 >2, <5 

Port O’Connor, ICW — — > 1.0 < llod

Port O’Connor, ICW dredge spoil — — > 1.0 < llod

Fulton Beach, Rockport — — > 1.0 < llod

Corpus Christi, aquaculture ponds < llod < llod > 1.0 < llod

Corpus Christi, power plant intakes — — >0.1, <1.0 ≥2

San Luis Pass — — >0.1, <1.0 ≥2

1 — = Not screened.

2 <llod = Lower limits of detection are 0.5 ppb for cyclodienes; 0.1 ppm for PCBs and 2.0 ppm for aldicarb.

Table 8

Results of July 1992 water samples using the Envirogard® Test Kits as a screening tool.

Concentration (ppb)

Site Station Triazines Aldicarb

Upper Lavaca Bay; park and boat landing 11 > 0.1, < 1.0 < LLOD 1

Corpus Christi; ponds at power plant 9 > 0.1, < 1.0 < LLOD

Port O’Connor Jetty 17 < LLOD < LLOD

Lavaca Bay; Magnolia Beach 13 > 0.1, < 1.0 < LLOD

Lavaca Bay; Dredge Spoil Island 12 > 0.1, < 1.0 < LLOD

Sand Point, west end of spit 2 > 0.1, < 1.0 < LLOD

Port O’Connor ICW at dredge disposal pipe 5 > 0.1, < 1.0 > 2.0, < 5.0

Port O’Connor ICW at Dewberry Island 4 > 0.1, < 1.0 < LLOD

Fulton Beach 7 > 0.1, < 1.0 < LLOD

North Matagorda Bay; live capture site 14 > 0.1, < 1.0 > 2.0, < 5.0

Middle Matagorda Bay; Green’s Bayou 15 > 0.1, < 1.0 > 2.0, < 5.0

ICW; S Port O’Connor; Channel Marker #19 * < LLOD > 2.0, < 5.0

1 LLOD = lower limits of detection: for triazines = 0.1 ppb and aldicarb = 2.0 ppb.

* This sample was taken only once at the end of the study, it is located within the bay, south of stations 4 and 5. 

lected in the area during the investigation (after 12 April) 

revealed no representatives of the domoic acid-producing 

diatom genus Pseudo-nitzschia nor other harmful phytoplankton species. These results suggest that known biotoxin-producing organisms were not present in the water 

column at effective concentrations during the investigation period after peak mortality had passed.

Discussion

The deaths of more than 220 dolphins during a 4-month 

period within the Texas inner bay estuarine ecosystems 

were unprecedented and clearly unusual. Evaluation of 

information collected during the investigation and environmental contaminant data indicated no specific causes 

of this unusual mortality event. The stranded animals’ 

advanced stage of decomposition prevented the thorough 
18 NOAA Technical Report NMFS 147

histopathology and virology examinations that might have 

been crucial to the investigation, as well as forensic contaminant analyses for non-persistent pesticides. Reijnders 

(1988) noted the potential problem of establishing cause 

and effect relationships during periods of marine mammal 

population declines or strandings. He concluded that 

future research should focus on baseline chemical contaminant surveys which establish uptake kinetics, and identification of marine mammals’ physiological responses 

to chemical contaminants. Without information such as 

condition of the animals’ lymph nodes, blubber, lungs, 

livers, and other organs, it is impossible to draw definitive conclusions about their health or the potential causes 

of their deaths. Circumstances of this unusual mortality 

investigation further emphasize the importance of assessing dermal toxicity of chemical contaminants in marine 

mammals and understanding the combination of dermal, 

oral, and food chain exposures in formulating environmental risk assessments. 

Results of environmental studies indicated that significant rainfall and freshwater runoff from agriculture 

sources entered the bay ecosystem from December 1991 

through March 1992. The effects of prolonged low 

salinity exposure on dolphins have not been rigorously 

studied. Early research (Simpson and Gardner, 1972; 

Greenwood et al., 1974; Harrison and Thurley, 1974) 

suggested that low salinity can cause increased sloughing of the skin; epidermal-dermal separation resulting 

in blistering; and formation of a gray, pasty substance 

on the skin, as was observed on many of the stranded 

dolphins and live animals during the 1992 Texas event. 

These altered skin conditions were observed on live 

dolphins from mid-January through March 1992, but 

the skin conditions apparently began improving during 

early to mid-April as salinities in the area returned to 

normal. Animals with the pasty, grayish skin coloration 

were observed after mid-April, but none with blisters 

and open wounds. No animals in this condition were 

observed during the live-capture operation in July. This 

trend coincides with a gradual increase in salinity due 

to the reduction of the record rainfall and the accompanying freshwater runoff that was observed in this 

area from December 1991 through March 1992. The 

possibility exists that members of a resident dolphin 

population might not leave their area of residency if 

unfavorable environmental conditions such as low salinity develop (Wells).6 In addition, Shane (1977) noted 

that the offshore population of bottlenose dolphins off 

the Texas coast rarely interacted with the bay population. These findings may be significant if long-term low 

salinity exposure and/or dermal absorption of environmental contaminants played roles in the observed dolphin mortalities.

Field observations confirmed that dolphins were present in the Texas bay ecosystem during the period of 

investigation when extremely low salinity conditions had 

developed from the unusually high rainfall observed 

during early 1992 and when pesticides were detected in 

the inner bay areas. For some species such as bottlenose 

dolphins, resident inshore populations spend extensive 

periods in close proximity to both point and nonpoint 

sources of contamination, where surface water, sediment, and prey species contaminant concentrations are 

likely to be much higher than in offshore areas.

Comparisons of total Hg levels in liver tissue from 

stranded dolphins examined during this unusual mortality event with previous unusual mortality events (Texas, 

19907; east coast of U.S., 1987–888; South Carolina 

coasts9) indicated that Hg levels were lower or comparable in dolphins from Texas during 1992. Even though 

different laboratories and analytical techniques were 

used to obtain and compare the mercury values between 

these studies, these findings generally suggest that liver 

Hg concentrations in dolphins from the 1992 unusual 

mortality event in Texas were not unusually high levels 

compared to previous dolphin strandings along the 

mid-Atlantic and Gulf coasts. This finding is significant 

because the Lavaca Bay area had been historically contaminated with mercury, and there was concern that 

mercury poisoning might have played a role in the mortality event. We recognize that age data and mercury 

speciation would have been important in evaluating the 

mercury data, and it is unfortunate that these data were 

not available for interpretation of results.

Exposure to pollution point sources would generally 

be confined to the immediate proximity of the discharge and buffer zone, since contaminant concentrations would be diluted within the immediate proximity 

of the input (Marcus and Scott, 1990). The primary 

concerns for point source pollution inputs are environmentally persistent pollutants, which may pose a potential chronic toxicity to estuarine living marine resources, 

including marine mammals. Environmental monitoring 

studies of marine mammals have clearly indicated the 

presence of environmentally persistent pesticides and 

PCBs in cetacean tissues8 (Borrell and Aguilar, 1990; 

O’Shea and Brownell, 1994). 

7 Hansen, L. J. (ed.). 1992. Report on investigation of 1990 Gulf of 

Mexico bottlenose dolphin strandings. NMFS, Southeast Fisheries Science Center, Contrib. MIA-92/93-21, 219 p.

8 Geraci, J. A. 1989. Clinical investigation of the 1987-88 mass mortality 

of bottlenose dolphins along the central and south Atlantic coast. Final 

Report to the National Marine Fisheries Service, U.S. Navy Office of 

Naval Research, and Marine Mammal Commission, 63 p.

9 Beck, Kevin. SEFSC Charleston Laboratory, NMFS, 219 Fort Johnson 

Rd., Charleston, SC 29412. Personal commun.

6 Wells, Randall. Mote Marine Laboratory, 1600 Thompson Parkway, 

Sarasota, FL 34236. Personal commun.
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 19

Exposure to nonpoint source pollution poses a somewhat different problem. Nonpoint source pollution 

generally describes a large volume of pollution input 

relative to the watershed size, emanating from a wide 

geographical area, generally containing low concentrations of environmental pollutants such as nutrients, bacteria, and environmental contaminants including trace 

metals, PAHs, and pesticides10, 11 (Hoffman et al., 1984). 

Nonpoint source pollution exposures frequently occur 

concomitant with low salinity10, 11. In a study of nonpoint source agricultural pesticide runoff, insecticide 

(azinphosmethyl, endosulfan, and fenvalerate) concentrations were measured in estuarine tidal creeks that 

exceeded acute toxicity levels found in laboratory toxicity tests on other marine species10, 11. This runoff 

resulted in local fish kills. Acute in situ toxicity tests 

measured significant inhibition of the enzyme acetylcholinesterase (AChE) in finfish, altered bioenergetic 

metabolism (respiration, nitrogen excretion, condition 

index, and gonadal index) in finfish and mollusks, and 

inhibited reproduction in the fish, crustacean, molluscan, and benthic copepod fauna examined. These studies were confined to the headwaters of tidal creeks. 

Bottlenose dolphins had been observed feeding within 

a 0.5 km range of the study sites used by Scott and 

coworkers and would have potentially been exposed to 

contaminants in surface waters and finfish. However, 

no significant dolphin strandings were observed in this 

region of South Carolina. Assessment of environmental 

monitoring data collected in this study did not indicate 

the presence of priority pollutants at toxicologically significant concentrations (> water quality criteria) in the 

surface water and sediment samples of the area.

Surface water analysis indicated the presence of pesticides (e.g. atrazine and aldicarb), but analysis of 

selected prey fish tissues collected after the peak of dolphin mortality did not indicate the presence of these 

pesticides or other toxicologically significant contaminants. This is not surprising given the short persistence and low bioaccumulation potential for many of 

these nonpersistent compounds, as well as the fact that 

sampling occurred weeks after peak dolphin mortality 

had occurred. Another notable factor is that detectable 

levels of pesticides were observed using randomly col10 Scott, G. I., M. H. Fulton, D. W. Moore, G. T. Chandler, P. B. Key, 

T. W. Hampton, J. M. Marcus, K. L. Jackson, D. S. Baughman, A. H. 

Trim, L. Williams, C. J. Lauden, and E. R. Patterson. 1988. Agricultural insecticide runoff effects on estuarine organisms: correlating 

laboratory and field toxicity testing with ecotoxicological biomonitoring. Report to U.S. EPA, Gulf Breeze Environmental Research Laboratory, Gulf Breeze, FL 32561, 688 p. 11 Scott, G. I., M. H. Fulton, D. W. Moore, G. T. Chandler, P. B. Key, J. 

M. Marcus, K.L . Jackson, D. S. Baughman, A. H. Trim, L. Williams, C. 

J. Louden, and E. R. Patterson. 1990. Agricultural insecticide runoff 

effects on estuarine organisms: correlating laboratory and field toxicity testing with ecotoxicological biomonitoring. Report to U.S. EPA, 

Gulf Breeze Environmental Research Laboratory, Gulf Breeze, FL 

32561, 232 p.

lected temporal samples rather than biased rain event 

samples. Higher levels of pesticides would be detected 

immediately following runoff events10, 11. The detection of these pesticides in random water samples suggests that pesticide runoff had occurred and that peak 

pesticide concentrations in runoff were probably not 

sampled during 1992. Pennington et al. (1994), in a subsequent study of pesticide runoff in this area of Texas 

in 1993, only found aldicarb in ditches adjacent to agricultural fields at similar concentrations measured in 

Matagorda Bay in 1992 (2–3 μg/L). It is very important 

that normal rainfall was observed during 1993, whereas 

record rainfall levels were measured during 1992. These 

findings clearly indicate the extent of the record rainfall 

during 1992, because pesticide levels in bays were similar to pesticide levels in ditches adjacent to agricultural 

fields during normal periods of rainfall. Because dolphins were observed in this area during the period, and 

tagging studies suggested a resident population, this 

information demonstrates the likely increased exposure 

of dolphins to pesticides and concomitant low salinity 

conditions during early 1992.

In addition to aldicarb, atrazine was also measured 

in surface water samples. Atrazine usage accounts for 

16.7% of the applied herbicides along the Gulf of 

Mexico. Adjacent to Matagorda Bay, Texas, atrazine 

usage ranks this watershed with the third highest application rate in the region 12. As with aldicarb, there have 

been no studies of toxicity for atrazine in bottlenose 

dolphins, although atrazine generally has much lower 

mammalian toxicity.

The fresh-water outflow into the Texas bays was higher 

for the period from December 1991 through May 1992 

than had been recorded in that area since 1939 13. The 

most critical implication of significant low salinity exposure was supported by water flow records and the pesticide measurements in mid and outer bay sampling 

stations. In much of the area, agricultural fields extend 

up to the bay beaches. Because the event coincided with 

agricultural planting and pesticide application periods, 

it was likely that the amount of contaminants in agricultural runoff would be higher than normal during this 

period. Pait et al.12 similarly reported peak pesticide 

usage in Texas during this same time period. Additionally, Pait et al. reported that the top four estuarine drainage areas in the U.S. were located in Texas.14 These 

12 Pait, A. S., A. E. DeSouza, and D.R. Farrow. 1992. Agricultural pesticide use in coastal areas: a national summary. NOS, NOAA, Rockville, 

MD, 112 p. 13 Texas Water Commission. 1992. Texas Water Commission District 12, 

4410 Dillon Ave., Suite 47, Corpus Christie, TX 78415-5326. Unpubl. 

data. 14 Pait, A. S., D. R. Farrow, J. A. Lowe, and P. A. Pacheco. 1989. Agricultural pesticide use in estuarine drainage areas: a preliminary summary 

for selected pesticides. Ocean Assessments Division, NOS, NOAA, 

Rockville, MD, 134 p.
20 NOAA Technical Report NMFS 147

estuarine areas contained 47%–75% agricultural drainage. Subsequent monitoring from February–October 

1993 indicated that peak pesticide levels occurred in 

the February–March sampling period (Pennington et 

al., 1994). These findings suggest that the peak pesticide concentrations were probably not sampled during 

1992 as sampling did not begin until April 1992.

The water samples collected at Sand Point, Texas, 

where the dead fish, birds, and dolphins were observed, 

showed presence of the carbamate insecticide aldicarb, 

as well as triazine herbicides. These pesticides were also 

detected at other sites in the bay ecosystem. Aldicarb 

and triazine herbicides were detected during mid-April 

1992 and again during July 1992, as far out into Matagorda Bay as the inland coast of Matagorda Peninsula 

(Tables 7 and 8). Both of these sampling periods took 

place long after (1–4 months) the major rain events had 

caused depressed salinity, and after the dolphin mortality rate had begun to decrease.

While aldicarb usage on agriculture in Texas (7.8% 

of total U.S. usage)15 is relatively low compared to 

other pesticides, given its high water solubility and high 

mammalian toxicity potential, measurement of levels 

approaching 2–5 μg/L in seawater may be toxicologically important. The oral LD50 for aldicarb in rats is 

0.9–1.0 mg/kg and the acute dermal LD50 in rats is 

2.5–3.0 mg/kg. Wildlife toxicity values ranging from 

1.0–2.0 mg/kg have been reported in bobwhite quail 

and mallard ducks. The mode of action for aldicarb 

toxicity is inhibition of the enzyme acetylcholinesterase 

(AChE). Clinical effects of AChE poisoning have been 

reported at aldicarb doses as low as 0.002 mg/kg/day in 

humans. The major metabolite, aldicarb sulfoxide, has 

76 times greater anti-AChE activity than the parent compound.16 Increasing salinity was found to increase aldicarb toxicity to a freshwater fish (Japanese Medaka) by 

enhancing the formation of aldicarb sulfoxide through 

cytochrome P450 and flavin monooxygenase enzyme 

systems (El-Alfy and Schlenk, 1997). While enhanced 

toxicity was observed with increasing salinities in this 

freshwater species, these findings clearly indicate alterations in aldicarb toxicity are possible. The extreme 

conditions found during this study do not rule out the 

potential for metabolically altered aldicarb toxicity in 

exposed organisms. Unfortunately, the dose-response 

relationship and toxicity of aldicarb in marine mammals is unknown. Because oral and dermal LD50 values 

15 Gianessi, L. P., and C. A. Puffer. 1990. The use of selected pesticide 

active ingredients included in the national pesticide survey, p. 128. 

Report to the Environmental Protection Agency. Quality of the Environment Division, Resources for the Future, Inc., 1616 P Street, NW, 

Washington, DC 20036.

16 DHEW/NCI. 1979. Toxicology and carcinogenesis studies of aldicarb. National Institute of Health. Report #136, NIH Pub. #79-1391, 

p. 2.

have not been established for bottlenose dolphins and 

other marine mammals undergoing continuous aqueous exposure when pesticides are present in their environment, the toxicological significance of the aldicarb 

and atrazine concentrations measured in Texas bay 

waters is not clear, even though the concentrations were 

well below oral and dermal LD50 values for terrestrial 

mammals. 

Another factor may be that marine mammals are 

more sensitive to neurotoxic chemicals, such as pesticides, than other mammalian species. Anderson (1994) 

reported that whales appeared to be much more sensitive to neurotoxic biotoxins, such as saxitoxin, than 

humans. Anderson (1994) suggests that increased neurotoxic sensitivity may have resulted from three possible 

factors. First, whales may receive a continuous dose of 

a toxin, whereas human estimates are based on single 

exposures (oral or dermal). Second, during swimming, 

the mammalian diving reflex channels more blood and 

oxygen to the heart and brain, bypassing the kidney and 

liver which are major organs of detoxification. This may 

result in greater toxicity potential in marine mammals. 

Third, whales may die indirectly from other causes following sublethal exposure to neurotoxins. Due to the 

lack of information from animal tissues and water before 

and during the peak mortality period, it is unclear 

whether or not similar phenomena may have caused 

enhanced susceptibility to potentially neurotoxic pesticides in bottlenose dolphins swimming and feeding 

in the shallow Texas bays. In future unusual mortality 

events, increased neurotoxic sensitivity of marine mammals should be considered when evaluating mammalian 

toxicity data, particularly if Class I mammalian neurotoxic agents are present in potential exposure pathways 

(water, sediment, or biota).

Aldicarb and atrazine are potential endocrine disrupting chemicals that may affect endocrine function 

and immune response in mammals, fish, and other species (Porter et al., 1985; Olson et al., 1987; Boyd et al., 

1990; Gill et al., 1991; Colborn and Clements, 1992). 

Soto et al. (1994) have further shown that the effect 

of endocrine disrupting chemicals can be cumulative, 

as subclinical pesticide doses were as effective in altering endocrine function as a single large dose of an individual pesticide in a human breast cell line. Olson et 

al. (1987) reported that exposure of mice to aldicarb in 

drinking water significantly affected immune function. 

There were significant reductions in splenic plaqueforming cell response in low dose (1 ppb) aldicarb 

exposure. An inverse dose-response effect on immune 

function was observed, which was measured only in low 

concentrations and not at higher aldicarb doses. Also, 

exposure of rats to aldicarb in combination with other 

insecticides (methomyl) and herbicides (metribuzin) 

significantly affected thyroid function (e.g. increased thy-
Investigation of Unusual Mortalities of Bottlenose Dolphins along the Mid-Texas Coastal Bay Ecosystem during 1992 21

roxin levels) and caused neurological changes, learning 

impairment, and altered immune and endocrine function when combined with triazine herbicides (Porter et 

al., 1993). 

Significant (p≤0.05) correlations have been reported 

between increased persistent pesticide concentrations 

(DDE, total PCB) and declining health status (e.g. suppressed immunity) among dolphins studied during the 

live capture in Texas bays during July 1992. Additionally, 

persistent pesticide concentrations in blubber samples 

were elevated among dolphins infected with the cetacean morbillivirus.17 Exposure to low salinity conditions 

and nonpersistent pesticides could have provided additional stress to resident populations.

Two critical facets of the investigation of organic contaminants as causative agents in a mortality event are 

seriously limited by the lack of marine mammal scientific information. One of those aspects is the mode of 

toxicity. For example, if water analysis creates cause for 

concern regarding a particular contaminant, and that 

contaminant is known not to bioaccumulate in tissues 

but instead is toxic by inhibiting enzyme activity, then 

distinct problems arise in establishing a cause-effect relationship. First, the routinely conducted contaminant 

analysis of blubber tissue would not be an effective investigative tool for detection of that specific contaminant. 

Secondly, if fresh blood and brain tissue are not available to test for enzyme activity levels, and a database 

large enough to have established normal and abnormal 

values for the parameters is also not available, then there 

is not currently an effective forensic tool to evaluate 

exposure to the particular contaminant. The other critical aspect is the fact that simple detection of an environmental contaminant is not usually enough to establish a 

causal relationship in terms of adverse effects on many 

marine species. Evidence needs to be obtained from 

animal tissues or from controlled studies demonstrating 

effects in models for the species involved, in order for 

any causal hypotheses to be verified. Scientific data needs 

to be available that suggests the particular type of animal 

involved is in some way adversely affected by exposure to 

a particular contaminant’s detected concentration.

The possibility exists that exposure to some additional acute environmental factor, which can compromise immune systems and allow for various declining 

health conditions to develop in the animals, could be 

additive and cumulative. Thus, the primary cause of 

death for each individual animal may not necessarily be 

consistent in the population as the primary cause of the 

entire event. This is particularly true when dealing with 

potential endocrine disrupting chemicals which may 

affect young and old, as well as males and females, differently. Likewise, a disease condition in the animals could 

weaken the defense mechanisms against environmental 

contaminants, facilitating various conditions leading to 

death. Because no serum from dolphins was available for 

morbillivirus tests during the unusual mortality event, 

and the current PCR-based tests were not available, we 

cannot rule out the possibility that this virus may have 

played a role in the event. Results from analyses conducted following the live capture study clearly indicated 

24% of the dolphins captured had been exposed to 

morbillivirus (Duignan et al., 1996) and that these dolphins had a greater environmental contaminant load16. 

A number of questions remain unanswered due to the 

decomposition stage of stranded animals. Were the dead 

stranded dolphins ever exposed to morbillivirus and did 

they have even higher levels of environmental contaminants? Additionally, what does a 24% prevalence rate for 

morbillivirus represent among dolphins in this area?

The scope of future investigations can be better 

planned as more data becomes available to support 

conclusions drawn on analytical results. Subsequent 

research on dolphin skin morphological characteristics 

likely to affect dermal absorption has suggested that 

use of terrestrial animal models for dermal toxicology 

in dolphins is inappropriate (Colbert, 1996). In addition, research is underway to establish baseline levels 

for AChE (the target enzyme for carbamate insecticides 

such as aldicarb) activity in bottlenose dolphins. All of 

the information collected during past investigations is 

important in developing parameters for better assessment of future mortality events. Research is currently 

underway regarding the effects of certain contaminants 

on marine mammal species, and health assessment studies of wild dolphin populations are being conducted 

when possible.

Since this investigation, several actions have been 

taken to alleviate some of these methodological problems. The Working Group on Unusual Marine Mammal 

Mortalities was formally established under the Marine 

Mammal Health and Stranding Response Act of 1992. 

This group annually reviews relative information on 

marine mammal mortalities and makes recommendations to the federal agencies involved. A Small Cetacean 

Necropsy Guide18 has been drafted to help guide and 

standardize the collection of tissues for specific analyses. 

Training is being provided to Marine Mammal Stranding Network personnel in marine mammal evidence 

collection and associated investigation protocols.19 Also, 

a contingency fund for response has been established 

under the Act, but appropriation of funds will vary over 

17 Reif, J. S., L. Hansen, S. Galloway, and G. Mitchum. 1996. The relationship between chlorinated hydrocarbon contaminants and selected 

health parameters in bottlenose dolphins (Tursiops truncatus) from 

Matagorda Bay, Texas, 1992. Manuscr. in prep.

18 Galloway, S. B., and A. Colbert (eds.). 1997. NMFS Charleston Laboratory, 219 Fort Johnson Rd., Charleston, SC 29412. Manuscr. in prep. 19 Rowles, T. 1998 Ongoing training workshop plans. Office of Protected Resources, NMFS, Silver Spring, MD 20910.
22 NOAA Technical Report NMFS 147

time and with each event.

In conclusion, results of field studies and investigations conducted in the unusual mortalities of bottlenose 

dolphins along the mid-Texas coast during 1992 indicated no specific cause of the observed deaths. Rather, 

four factors were identified which may have contributed to an overall cumulative effect on environmental 

conditions that had the potential to play a role in the 

1992 unusual mortality event. These included evidence 

that dolphins were exposed to prolonged low salinity 

conditions which could have altered the permeability 

barrier of the skin; the co-occurrence of nonpersistent 

but toxic pesticides which are potential endocrine disrupting chemicals that may chronically affect immune 

function and health; the presence of elevated levels 

of persistent pesticides in surviving dolphins of poor 

health status; and the correlation of elevated persistent 

pesticide concentrations in surviving dolphins testing 

positive for morbillivirus. While no statistical correlation was found with the specific timing of runoff events 

and stranding rates, the mortality event coincided with 

a period of extensive record rainfall, low salinity conditions within the bay, and increased agricultural pesticide runoff only in inland bays where the strandings 

occurred and not in offshore waters. Aldicarb and atrazine concentrations detected in July after dolphin mortality had declined did not co-occur with extensive low 

salinity conditions and accompanying acute, pulsed pesticide exposure at higher concentrations, suggesting 

pesticide exposure per se was not the cause of this event. 

Rather, observed dolphin mortalities may have resulted 

from a combination of prolonged rainfall, low salinity 

conditions, and the increased discharge of pesticides 

and other chemical contaminants into Texas bays. 

Acknowledgments

The 1992 investigation was conducted with the support 

of many agencies and institutions, researchers, and other 

individuals. We hesitate to list any individuals for fear of 

leaving someone out, but we sincerely thank all of the participants from the agencies and institutions listed below 

for their information, intense efforts, and dedication. An 

investigation such as the one described here requires time 

and physical efforts far beyond the call of duty from many 

of the people involved. Many researchers donated their 

time, expertise, and other resources to assist in this investigation. The people of Texas were gracious to our field 

team during our extended visit along their coast, and we 

thank them for making the experience as pleasant as possible under the circumstances.

Thanks to Texas A&M University, Texas Department 

of Health, Texas Marine Mammal Stranding Network, 

Texas Parks and Wildlife Department, Texas Veterinary 

Medical Diagnostic Laboratories, Texas Water Commission, University of Texas, U.S. Coast Guard, U.S. Fish and 

Wildlife Service, The Working Group on Unusual Marine 

Mammal Mortalities; and to National Marine Fisheries 

Service Southeast Fisheries Science Center Laboratories 

in Galveston, Texas, Miami, Florida, Beaufort, North Carolina, and Charleston, South Carolina; National Marine 

Fisheries Service Office of Protected Resources, Silver 

Spring, Maryland; and National Marine Fisheries Service 

Office of Protected Species, St. Petersburg, Florida.

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