A TRAWL SURVEY METHOD FOR ESTIMATING LOGGERHEAD TURTLE, 

 CARETTA CARETTA , ABUNDANCE IN FIVE EASTERN FLORIDA 



CHANNELS AND INLETS 



Richard W Butler. Walter A Nelson, and Tyrrell A. Henwood' 



ABSTRACT 



Five eastern Florida navigational channels were surveyed on a quarterly basis from November 1981 

 through August 1982. The purpose of the surveys was to provide estimates of loggerhead turtle 

 abundance for each channel over all seasons of the year. Standard methods for estimating loggerhead 

 turtle abundance from trawl samples were developed, and the probability of capture in a 30 m by 

 1,483 m substation (P) was estimated to be 0.28 ± 0.05 (95% confidence level). Abundance estimates 

 based on this probability of capture were then developed for each channel and survey. Of the channels 

 surveyed, only Port Canaveral harbored significant concentrations of loggerhead turtles; populations 

 ranged from 701 ± 291 turtles in February to a low of 38 ± 26 turtles in August. A few loggerhead 

 turtles were captured in the other channels, but infrequency of occurrence suggested random encoun- 

 ters rather than areas of concentration. 



In the western Atlantic Ocean, loggerhead tur- 

 tles, Caretta caretta, forage throughout the warm 

 waters of the continental shelf from Argentina 

 northward to Nova Scotia, including the Gulf of 

 Mexico and the Caribbean Sea (Carr 1952). On a 

 seasonal basis, loggerheads are common as far 

 north as the Canadian portions of the Gulf of 

 Maine (Lazell 1980), but during cooler months of 

 the year distributions shift to the south (Shoop et 

 al. 1981). Sporadic nesting occurs throughout the 

 tropical and warm temperate range of distribu- 

 tion, but the most important nesting areas are the 

 Atlantic coast of Florida, Georgia, and South Car- 

 olina (Carr and Carr 1978). The Florida nesting 

 population of Caretta has been estimated to be the 

 second largest in the world (Ross 1982). 



Although population levels of adult female log- 

 gerheads can be estimated from counts on nesting 

 beaches, the remaining animals (males, sub- 

 adults, and nonbreeding females) do not come 

 ashore and are not readily available for census. 

 To estimate the total number of loggerheads in an 

 area, all segments of the population should be 

 considered. 



In the vicinity of Cape Canaveral, FL, logger- 

 head turtles congregate in the Port Canaveral 

 ship channel (Carr et al. 1980). Because turtles 

 can be captured and studied in this unique area 



^Southeast Fisheries Center Mississippi Laboratories, Na- 

 tional Marine Fisheries Service, NOAA, P.O. Drawer 1207, 

 Pascagoula, MS 39568-1207. 



Manuscript accepted February 1987. 

 FISHERY BULLETIN; VOL.'85, NO. .3, 1987. 



throughout the year, the National Marine Fish- 

 eries Service (NMFS) has conducted surveys to 

 monitor population levels and estimate relative 

 turtle abundance. This study is a continuation 

 and expansion of research efforts which began in 

 1978. 



Presented are results of a 1-yr investigation 

 conducted in response to requests from the U.S. 

 Army Corps of Engineers (COE) and the U.S. 

 Navy, to estimate sea turtle abundance and sea- 

 sonality in five eastern Florida navigational 

 channels. Animals captured in this study were 

 subadults, adult males, and adult females. Popu- 

 lation estimates of subadult turtles may prove to 

 be particularly useful for management, as effi- 

 cacy of conservation measures should be first evi- 

 dent in the population levels of the youngest co- 

 horts. 



Results of this study provide a reliable index of 

 loggerhead turtle abundance for the study year 

 and an alternative to population estimates based 

 only on nesting females. Most importantly, for 

 the first time, a standard method has been devel- 

 oped that provides sea turtle abundance esti- 

 mates with approximate standard errors. 



STUDY AREAS 



Five eastern Florida navigational channels 

 were surveyed on a seasonal basis over the study 

 period. A description of the survey sites follows 

 (each site is diagramed in Figure 1): 



447 



