An investigation of bottienose dolpinin 

 Tursiops truncatus deatiis in East 

 Matagorda Bay, Texas, January 1990 



W. George Miller 



Naval Ocean Systems Center. Code 514, San Diego, California 92152 



There are reports of massive mor- 

 talities of bottienose dolphins Tur- 

 siops truncatus over periods of 

 months in areas as large as the U.S. 

 Atlantic coast and the Persian Gulf. 

 From early June 1987 until March 

 1988, over 740 bottienose dolphins 

 (estimated at about 50% of the 

 coastal migratory stock) stranded 

 along the U.S. Atlantic coast from 

 New Jersey to Florida (Scott et al. 

 1988, Geraci 1989). Geraci con- 

 cluded the dolphins were poisoned 

 by brevetoxin, a neurotoxin pro- 

 duced by the red tide organism Pty- 

 chodiscus hrevis. 



During 23 August to 30 October 

 1986, 527 dead dolphins were found 

 on the eastern and western shores 

 of the Persian Gulf. Several dead 

 turtles, dugongs, and one 6.1 m un- 

 identified whale were also found, 

 along with many fish that washed 

 ashore. Of the 78% of mammal car- 

 casses identified to species, 64% 

 were bottienose dolphins, 34% were 

 humpback dolphins Sousa chinen- 

 sis, 1.7% were common dolphins 

 Delphinus delphis, and 0.3% were 

 finless porpoises Neophocoena pho- 

 coenoides. The dead dolphins in- 

 cluded adults, neonates, calves, and 

 juveniles. Cause(s) of the deaths 

 could not be determined, since only 

 four animals were necropsied 

 (Anonymous 1986). 



The subject of this study is an 

 unusual stranding of 26 T. trun- 

 catus that occurred in January 1990 

 around East Matagorda Bay (EMB), 

 Texas. There are no previous re- 

 ports of this number of strandings 

 in a relatively small area in a single 

 day. On 20 January 1989, a helicop- 



ter pilot reported the stranded dol- 

 phins to the U.S. Coast Guard, who 

 notified the Texas Marine Mammal 

 Stranding Network at Texas A&M 

 University and Texas Parks & Wild- 

 life Department (TPW). These or- 

 ganizations collected 26 carcasses, 

 23 from within the Bay and 3 from 

 the Gulf side of East Matagorda 

 Peninsula. I performed necropsies 

 on the dolphins on 24-25 January to 

 determine cause of death. 



Methods 



Examination of dolphins 



Each T. truncatus dolphin was as- 

 signed an identification number and 

 its stranding location noted (Fig. 1). 

 State of decomposition was noted: 

 freshly dead with no bloating (1 

 animal), detectable bloating, or 

 severe decomposition. Animals 

 were sexed and weighed, and length 

 was measured via a straight-line 

 from the notch in the tail flukes to 

 the most rostral aspect of the man- 

 dible. Measurements of blubber 

 thickness were taken at six loca- 

 tions along the animal's left side 

 using the standard protocol of the 

 Naval Ocean Systems Center (NOSC) 

 (Fig. 2). Skin condition and abnor- 

 mal marks or deteriorated areas 

 were recorded. Condition and posi- 

 tion of thoracic and abdominal 

 organs were noted before removal 

 and collection of tissue samples. 



Site inspection and 

 background information 



On 26 January 1990, 1 conducted an 



aerial survey of the stranding site, 

 comparing the actual configuration 

 of East Matagorda Bay with an ex- 

 isting map (NOAA nautical chart 

 #11319) to determine exit routes for 

 dolphins from the Bay to the deeper 

 waters of the outer coast. The main 

 exit is a narrow cut connecting the 

 Bay and the Gulf (Fig. 1). 



The Texas Parks and Wildlife 

 Department monitored water-tem- 

 perature changes in the Bay almost 

 daily during 15-29 December 1989 

 (Fig. 3). The Bay was completely 

 frozen over for 2.5 days with par- 

 tial ice remaining for 4 days. On 22 

 December, a helicopter pilot flew 

 close to the Bay to observe about 12 

 dolphins swimming and breaking 

 ice (~5cm thick) in a 4-7 km area in 

 the east-central region of the Bay. 



Rapidly-moving weather systems 

 from the north with strong norther- 

 ly winds can significantly lower 

 tidal levels in the Bay (Steve Mar- 

 witz, Texas Parks & Wildlife, Rock- 

 port, TX 78382, pers. commun.). 

 Within the period 15-22 December 

 1989, when two cold-weather sys- 

 tems moved through the area, an 

 estimated range for the mean low 

 tide level was 30-60 cm below nor- 

 mal (Mark Mazot, Tex. Parks Wildl. 

 Dep., pers. commun., Feb. 1990); 

 however, there were no official 

 measurements. Thus it is possible 

 that lowered water depths around 

 the periphery of the Bay could have 

 impeded dolphin movement be- 

 tween the Bay and the Gulf of Mex- 

 ico via the Caney Creek Gulf Cut or 

 the intercoastal canal. 



Results 



Of the 26 Tursiops truncatus ex- 

 amined, 23 dolphins were from 

 within East Matagorda Bay and 

 consisted of 6 mature males (MM), 

 5 immature males (IM), 7 mature 



Manuscript accepted 9 September 1992. 

 Fishery Bulletin, U.S. 90:791-797 (1992). 



791 



