WHAT'S CRACKING: INVESTIGATING GULL PREDATION ON PISMO CLAM POPULATIONS AT PISMO BEACH

Ryan N Bloom; Cal Poly, San Luis Obispo; rbloom127@gmail.com; Marissa Bills, Ben I. Ruttenberg

Pismo Clams (Tivela stultorum) were once abundant on Pismo Beach until the 1980s when populations declined. However, recent years have seen a notable increase in their numbers. With the potential return of these clam populations to their legal size, we sought to investigate the factors that contribute to their predation, such as humans, otters, and shorebirds. This study focused on quantifying the extent of shorebird predation. We observed a specific clam-dropping predation behavior, where shorebirds dropped clams onto compact sand to crack them open, allowing them to feed on the clam. Our research aimed to identify the species of shorebirds involved, the size of clams they targeted, and the spatial distribution of predation on the beach. Through surveys on Pismo Beach, we found that Western Gulls (Larus occidentalis) were the sole shorebirds engaging in this behavior. The median clam size targeted was 71 millimeters, and predation almost entirely occurred in areas of the beach in which cars weren’t allowed. Based on this spatial trend, we hypothesize that anthropogenic disturbance from vehicles is a major factor influencing predation frequency. Future work will further explore the impact of Off-Highway Vehicles and other environmental factors on gull predation.

Poster Session   Student Paper