NOTE Miller: Deaths of Tursiops truncatus in East Matagorda Bay 



795 



temperatures or duration of ice on the Bay was avail- 

 able for the 1983-84 freeze. 



There may have been no possible escape route for 

 the dolphins because of the very low water level and 

 the ice formation on the surface of the Bay. Smith et 

 al. (1983) state that ice may impede the movement of 

 dolphins in an area, and Shane (1980) studied the 

 distribution of bottlenose dolphins in southern Texas 

 and found that some animals had a home range that 

 was limited to shallow bays. In our study, local fish- 

 ermen stated that they repeatedly saw the same 

 animals, which they could recognize by marks on the 

 dorsal fin and flukes, and that the approximate number 

 of dolphins in the Bay usually was "in the20's." If the 

 dolphins in East Matagorda Bay were resident, then 

 many of the older animals stranded in January 1990 

 likely had experienced and survived the severe weather 

 conditions in 1983 when the Bay froze over. 



There are no precise data available to accurately 

 determine the food biomass available to the EMB 

 dolphins during and after the December 1989 freeze, 

 but it is possible that an essential food source was not 

 available. Fish mortality is greatest during a rapid 

 decrease in water temperature 

 (Springer and Woodburn 1960). 

 Data from Dailey et al. (1991a) 

 show that the relative abundance 

 (gillnet entrapment technique, 

 n/hour) of subadult and adult 

 striped mullet Mugil cephalus along 

 the Texas coast in spring 1989 was 

 double that of previous years, while 

 the relative abundance (bag-seine 

 entrapment technique, n/ha) of 

 juveniles in 1989 was only 60% of 

 the value for the two previous 

 years. Following the December 

 1989 freeze, the relative abundance 

 of subadult and adult striped mullet 

 in the spring of 1990 was far below 

 that of spring 1989, while the 

 relative abundance of juveniles (for 

 recruitment to the population) was 

 380% higher in 1990 than it was in 

 1989. The large increase in relative 

 abundance of young for spring 

 recruitment to the population 

 following the December 1989 freeze 

 has been attributed to a lack of 

 adult predator fish (Lawrence 

 McEachron, Texas Parks Wildl., 

 Rockport, TX 78382, pers. com- 

 mun., Oct. 1991). 



Table 2 shows estimated freeze 

 kills for a variety of marine fish 



species in East Matagorda Bay for periods in 1983-84, 

 February 1989, and December 1989 (McEachron et al. 

 1991). Although freezes are common on the Texas 

 coast, fish kills of the magnitude of the December 1989 

 freeze in the Bay had never before been recorded, with 

 Mugil cephalus mortality estimated to be over 2.5 

 million fish. 



And thus, a compounding problem for the dolphins 

 in the December 1989 freeze is the unprecedented kill 

 of striped mullet. It is probable that a food source 

 essential to the Bay dolphins was severely depleted at 

 a critical time when the dolphins needed calories. Bar- 

 ros (1992), Barros and Odell (1990), and Cockcroft and 

 Ross (1990) show that bottlenose dolphins utilize a 

 variety of food resources, composed primarily of fish 

 (4 to 6 major prey species common to their respective 

 areas) and cephalopods (primarily 1 species common 

 to their respective areas), and occasionally crustaceans. 

 Pryor et al. (1990) suggest that mullet has been a staple 

 dolphin food for centuries. Gunter (1942) reported on 

 prey in freshly-killed, presumably-healthy T. truncatus: 

 1 from deeper waters and 33 from the shallows of 

 Aransas and St. Charles Bays, Aransas County, Texas. 



