Assessing western gull (Larus occidentalis) predation on purple sea urchins (Strongylocentrotus purpuratus) in the rocky intertidal using optimal foraging theory

Purple sea urchins are abundant grazing invertebrates that have a major impact on the rocky intertidal community. Predators can control the urchin population and indirectly reduce grazing activity. My goal was to determine the effects of western gull predation on purple sea urchins in the rocky intertidal using the framework of the optimal foraging theory. I recorded the foraging behavior of gulls, measured urchin availability, and estimated prey caloric content with bomb calorimetry. Gulls selected urchins significantly more than other prey items (snails, limpets, sea stars). Larger urchins were found to contain relatively more calories. Gulls foraged optimally with the pecking-open handling technique by frequently selecting the most profitable size class. However, gulls chose smaller urchins than expected when using the air-dropping technique. Gulls were more likely to be kleptoparasitized when foraging in groups, and when using the air-dropping technique. Both age of the foraging gull and group size significantly affected urchin size selection; juveniles, as well as those foraging in groups, chose smaller urchins for the air-dropping technique. The risk of kleptoparasitism, as well as age, may explain why gulls do not always forage optimally when air-dropping. Gull predation on urchins may benefit the local rocky intertidal community by maintaining diversity and structural complexity.