Predicting the Impact of Turtle Excluder Devices on Loggerhead Sea Turtle Populations

Abstract

In this paper, we modified and updated a stage—based population model for loggerhead sea turtles (Caretta caretta) and used the model to project potential population—level effects of the use of turtle excluder devices (TEDs) in trawl fisheries of the southeastern US. We reduced the seven—stage model of Crouse et al. (1987) to a five—stage model and performed sensitivity analyses on the matrix. The most sensitive matrix parameters were those dealing with survival while remaining in a stage, rather than growth from one stage to the next or reproductive output. Population growth rate was most sensitive to survival in the large juvenile stage, followed by small juvenile survival. Large juveniles are the most common size class among stranded dead turtles found on beaches; 70—80% of strandings are thought to be related to trawl fisheries. Simulations of our loggerhead model based on estimated effects of TED regulations on stage—specific survivorship suggested that southeastern US loggerhead populations should increase, but rather slowly. If TEDs were required during the shrimping season in offshore areas only (as they were from 1990 to 1992), 70 yr or more would be required for the simulated population to increase by an order of magnitude. Recent estimates of TED effects from South Carolina strandings data suggest a similar recovery rate. Good compliance with regulations requiring TEDs year—round in all waters could allow the population to increase nearly twice as fast as that expected under the "seasonal offshore" regulations. We also used a Leslie matrix version of the model to illustrate the expected transient response in the numbers of females expected on nesting beaches (due to shifting age—size structures with TED use). Rather than a monotonic increase, we expect an initial increase in the number of nesting females, followed by a leveling off or slight decline (perhaps 10—15 yr from now), followed by another increase. The magnitude of the projected population increase will depend upon the actual increases in stage—specific survivorship due to TED regulations. New, or compensatory, sources of mortality could slow or reverse this projected recovery.