Southern sea otter diet in a soft sediment community

Between August 2006 and February 2009, we investigated southern sea otter foraging in Elkhorn Slough, the third largest estuary in California and an important soft-sediment community for sea otters. Our study is the first peer-reviewed comprehensive account of sea otter diet in Elkhorn Slough since the 1990s and the first to look at sea otter scats in addition to visual observations. Diet and other foraging parameters were determined during 199 daytime focal observations and by collecting 115 scat samples. Foraging success was 69%, males being more successful than females. Diet included more than 21 prey items. Nineteen are found within the study area, making this the widest variety of prey items reported taken by sea otters in a soft-sediment community. Daytime prey included 78.2% Mollusca (clams and mussels), 11.2% Echiurida (inkeeper worms), 2.8% Arthropoda (crabs) and 7.8% undetermined prey. Scat samples contained a wider variety of prey than focal samples. Four species of crabs found in scat samples but seen during focal observations, suggest otters may also be feeding outside of the study area or feeding within the study area at night. Both prey pursuit and handling time increased with prey size but depended on prey type. Sea otters exhibited a high degree of prey specialization, most individuals being highly specialized on clams (91%). Most prey items obtained were <5 cm long, suggesting otters rely on greater availability or on ease of capture of this size class.