Crab trapping causes population decline and demographic changes in diamondback terrapins over two decades
Michael E. Dorcas1, John D. Willson2, and J. Whitfield
Gibbons2
1Department of Biology, Davidson College, Davidson, NC 28035-7118,
USA
2Savannah River Ecology Laboratory, The University of Georgia,
Aiken, SC 29802, USA
Abstract
Diamondback
terrapins (Malaclemys terrapin) are thought to be declining throughout
their range. Although many factors have been proposed to contribute to
terrapin declines, including increased predation of nests and adults,
habitat loss and degradation, road mortality, commercial harvest for food,
and mortality as bycatch in crab traps, few studies have provided evidence
linking these agents to population declines. Because male and small female
terrapins are most susceptible to mortality in crab traps, population
declines should coincide with shifts in the age and size distributions
of the population and a shift to a more female-biased sex ratio. We used
twenty-one years of mark-recapture data (>2800 captures of 1399 individuals)
from a declining diamondback terrapin population in South Carolina to
test the prediction that the decline is the result of mortality in crab
traps. Since the 1980s, the modal size of both male and female terrapins
has increased substantially and the proportion that are females is higher
than in earlier samples. Additionally, the population now contains more
old and fewer young individuals than before. The changes in demography
and sex ratio we observed suggest that this terrapin population has declined
as a result of selective mortality of smaller individuals in crab traps.
The use of bycatch-reduction devices on crab traps may help prevent terrapins
from entering the traps, but current models are too large to prevent mortality
of males and many females in this population. Future research should focus
on design and testing of effective bycatch-reduction devices for specific
regions and other methods to prevent terrapin mortality in crab traps.
Keywords
by-catch reduction device, crab trap, Malaclemys terrapin, mortality,
salt marsh, turtle
SREL Reprint #3035
Dorcas, M.
E., J. D. Willson, and J. W. Gibbons. 2007. Crab
trapping causes population decline and demographic changes in diamondback
terrapins over two decades. Biological Conservation 137(2007):334-340.
