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Marine Turtle Newsletter 47:3-5, © 1989

Shrimp Trawl-Induced Mortality of Sea Turtles
During Short Duration Trawling

Thane Wibbels
Department of Biology, Texas A&M University, College Station, Texas 77843 (Present address: Department of Zoology, University of Texas, Austin, Texas 78712)

Recent changes in turtle excluder device (TED) regulations may allow offshore shrimpers to trawl without the use of TEDs if they limit their trawling durations to 105 minutes. This change in regulations greatly increases the need for data regarding sea turtle mortality during short duration trawling. During June of 1983, I was allowed to accompany a shrimp crew while they conducted three days of normal trawling off the east coast of central Florida. The captain of the trawler was concerned about sea turtle mortality and therefore limited trawling to relatively short durations. Trawling took place approximately 1 km offshore in an approximate 14.5 km line from the Cape Canaveral ship channel to Patrick's Air Force Base. The incidental-capture data from all of the trawls during those three days are summarized in Table 1.

The high rate of incidental capture (11 captures in 18.7 hrs of trawling) most certainly reflects the relative abundance of loggerhead turtles in the area. Aerial surveys by the National Marine Fisheries Service (Schroeder and Thompson, 1987) indicate that both immature and adult loggerheads are frequently sighted along the Atlantic coast of the central Florida Cape during the spring and summer. The Atlantic coast of central Florida represents a major nesting area for loggerheads (Hopkins and Richardson, 1984; Provancha and Ehrhart, 1987; Ehrhart and Raymond, 1987) and the seasonal abundance of immature loggerheads in that area suggests that it may also represent a major feeding ground (Schroeder and Thompson, 1987). The high incidental capture rate reported above exemplifies the need to either restrict trawling activities or require the use of TEDs to protect critical habitats such as feeding grounds and the waters adjacent to nesting beaches.

Table 1: Data from 3 days of trawling off the east coast of central Florida in 1983. "Loggerhead" refers to the loggerhead sea turtle, Caretta caretta. "Dead" refers to turtles that did not regain consciousness during the approximate 1.5 hours they remained on deck.

A second aspect of the incidental capture data is the mortality during relatively short trawling durations. If all of the trawls are considered (ranging from 30-105 minutes in duration), the mortality was 45.5%. This mortality included both adult and immature turtles. It is plausible that the high mortality during the short duration trawls may relate to water temperature. Warm water temperatures should increase the metabolic rate and overall activity of sea turtles and thus decrease the length of time before a turtle needs to surface for air. Regardless, the data above indicate that short duration trawling (i.e., 105 minutes or less) can result in high sea turtle mortality.

Acknowledgements: The above data were collected during a study of sea turtle reproductive endocrinology funded by Texas A&M Sea Grant College, U.S. Department of Commerce, grant #NA85AA-D-SG128.

EHRHART, L. M. and RAYMOND, P. W. 1987. Loggerhead turtle, Caretta caretta, and green turtle, Chelonia mydas, nesting densities in South Brevard County, Florida, 1981-1984, p.21-25. In: Ecology of East Florida Sea Turtles, W.N. Witzell (ed.). NOAA Tech. Rept. NMFS 53.

HOPKINS, S. R. and RICHARDSON, J. I. 1984. A Recovery Plan for Marine Turtles. U.S. Government Printing Office, Washington D.C. 355 p.

PROVANCHA, J. A. and EHRHART, L. M. 1987. Sea turtle nesting trends at John F. Kennedy Space Center and Cape Canaveral Air Force Station, Florida, and relationships with factors influencing nest site selection, p.33-44. In: Ecology of East Florida Sea Turtles, W.N. Witzell (ed.). NOAA Tech. Rept. NMFS 53.

SCHROEDER, B. A. and THOMPSON, N. B. 1987. Distribution of the loggerhead turtle, Caretta caretta, and the leatherback turtle, Dermochelys coriacea, in the Cape Canaveral, Florida area: results of aerial surveys, p.45-53. In: Ecology of East Florida Sea Turtles, Witzell (ed.). NOAA Tech. Rept. NMFS 53.