Notes Observations of Sea Otters Digging for Clams at Monterey Harbor, California

Although the feeding behavior of the sea· otter, Enhydra lutris, is frequently observed from the surface, few underwater observations of foraging sea otters have been published. Faro (1969) and Houk and Geibel (1974) described the underwater behavior and tool use of sea otters when they removed abalone from rock substrates. Shimek (1977) observed a sea otter foraging for snails and presumably other invertebrates by patting the surface of rocks and feeling into cracks. Shimek also described a sea otter digging up the echiuroid worm, Urechis caupo, from a silt and cobble substrate. Further deductions about underwater foraging behavior have been made from collections of abalone shells with the characteristic "otter break" hole in the middle (Wild and Ames l ) and from observations ofaluminum beverage cans bitten by otters to remove octopus (McCleneghan and Ames 1976). Some sea otters can be enticed underwater to take food offered them (pers. obs). However, these latter observations of underwater food manipulation are oflimited value because the otters also take items unpalatable to them (e.g., the holothuroid Stichopus californicus), and because the otters were clearly interacting with the diver observer. These accounts of underwater foraging indicate that sea otters use primarily tactile sensitivity of the forelimbs to locate and capture prey, whereas all other marine mammals (pinnipeds and cetaceans) use their jaws to capture prey. Radinsky (1968) hypothesized that the sea otter evolved forelimb tactile sensitivity separately from the aonychoid otters. The large impact of sea otters on Pismo clam, Tivela stultorum, populations in California has been documented (Stephenson 1977; Miller et al,2), and in Prince William Sound, Alaska, 81% of the food items taken by sea otters were bivalves, especially Saxidomus gigantea (Calkins 1978).