Crab traps do not kill terrapins at random.
In 2007, John Willson, Whit Gibbons, and I used 21 years of mark–recapture data from Kiawah Island to examine why a diamondback terrapin population was declining. The answer was visible not only in how many turtles remained, but in which turtles were missing.
Terrapins are strongly sexually dimorphic. Adult males are much smaller than adult females, and males and young females can enter most blue-crab pots. Large adult females often outgrow the openings. A terrapin entering a submerged pot may be unable to escape before drowning.
At Kiawah, crab-pot mortality was size- and sex-selective. Males and younger females remained vulnerable, while the largest females gained some protection from size alone. Over two decades, the population became older, larger, and more female-biased, with fewer young animals—exactly the pattern expected when smaller terrapins are removed disproportionately.
In 2009, Andrew Grosse and colleagues documented 133 terrapin carcasses in abandoned crab pots in one Georgia tidal marsh—more than twice the estimated surviving population there. Andrew then led a broader 2011 study showing that greater crabbing pressure was associated with lower terrapin abundance and fewer young or small individuals.
Research elsewhere found similar effects, although the details varied with local body sizes and trap openings. Even adult females can be vulnerable where they are small enough to enter.
Fishing gear can therefore remove particular sizes, ages, or sexes rather than a random sample of the population. Counting dead animals alone may underestimate the impact. Selective mortality can alter age structure, sex ratio, recruitment, and the body-size distribution of survivors long before a population disappears.
Understanding the mechanism also points toward solutions.
Rebecca McKee, Kristen Cecala, and I later examined how terrapins behaved around crab pots fitted with bycatch-reduction devices, or BRDs. Rebecca’s study showed that BRDs reduced entrapment, slowed entry, and lowered successful entries relative to attempts. Vertically oriented BRDs were more effective than horizontal ones.
Terrapins also investigated pots baited with fish more often than those baited with chicken, showing that gear design and bait choice can influence risk.
Conservation is not always about stopping human use. Sometimes small changes in fishing gear or practices can reduce predictable mortality while allowing a fishery to continue.
A marsh may still contain terrapins while losing the age and sex classes needed to sustain the population.
Presence is not the same as population health.
Selected references:
Dorcas et al. 2007. Biological Conservation 137:334–340.
Grosse et al. 2011. Journal of Wildlife Management 75:762–770.
McKee et al. 2016. Aquatic Conservation 26:1081–1089.
This Insight is part of my ongoing LinkedIn series on reptiles, amphibians, conservation biology, invasive species, and scientific writing. Follow Michael E. Dorcas on LinkedIn for new posts and discussion.