ASSESSING THE HEALTH OF CRAB FISHERIES IN SOUTHERN CALIFORNIA USING PREDATION TRACES ON MODERN AND PLEISTOCENE GASTROPODS

Predation traces or scars left on prey by their predators provide a means of studying predation both in modern and fossil systems where it might be otherwise difficult to observe predation directly. By using such palaeontological techniques, it is also possible to compare pre-human and modern samples as a means of identifying potential impacts of human activity on ecosystems and potentially vulnerable species. Here we compared crab predation traces (repair scars) on assemblages of the gastropod Tegula funebralis from both living populations and Pleistocene assemblages to assess any differences in crab predation that might indicate the effects of human activity on today’s crab fisheries in southern California. We examined over 700 T. funebralis from the Late Pleistocene (LACMIP collections) and 1100 living individuals from the Palos Verdes Hills and San Diego regions of California for repair scars. Overall gastropod size and the size at attack (SAA) for each repair scar observed was recorded. There were significantly fewer repair scars on modern gastropods (p < 0.001). In addition, both gastropod size and SAA were larger for modern gastropods, suggesting a possible “arms-race” between gastropods and crabs. By comparing the relative distribution of the size of repair scars on the gastropods (a proxy for the strength of the crabs against the gastropod’s defenses), we founds that Pleistocene crabs were more successful against the gastropods available to them, as there were fewer failures on smaller gastropods in the Pleistocene. If anything, the apparent increased success of Pleistocene crabs suggests that the observed repair frequency in the Pleistocene should have been lower, with Pleistocene crabs less likely to fail against gastropods. However, there are more repair scars on Pleistocene gastropods, most likely indicating that crab attacks have decreased since the Pleistocene. We suggest that fewer crab attacks in the modern may indicate declines in crab populations in southern California and that at minimum, further investigation of crab population health is warranted. Given that there is rising concern, but limited existing data, on the health of rock crab fisheries in particular, the use of predation traces on prey can provide valuable information on the health of economically important crab fisheries.