Daily Activity of Sea Otter off the Monterey Peninsula, California

Abstract

During the period 24 July through 21 September 1974 sea otter (Enhydra lutris) activity was sampled along a 5.5 km stretch of coastline in Monterey County, California. Sixty-five one hour samples covering a 15 hour day were completed during the study period. Sea otter behavior was classed as resting, foraging, interacting, grooming, or swimming. There were two modes of foraging activity, at 0700 and at 1700 hours. Preliminary observations suggest a third mode of foraging activity at 2400 hours. Principal otter activities were resting and foraging. The sample data also revealed the relative importance of various sectors within the study area. Male animals appeared to move to female areas to forage. J. WILDL. MANAGE. 41(2):277-283 Sea otters are known to affect the nearshore environment profoundly (Estes and Palmisano 1974; Wild and Ames 1974). Adult sea otters weigh up to 39 kg and in captivity eat about 25 percent of their body weight per day in fish and squid (Kenyon 1969). Wild sea otters in California eat a wide variety of invertebrates including crabs, mussels, sea urchins, turbin snails, abalones, ascidians, squids, and annelids (Hall and Schaller 1964; Ebert 1968; Vandevere 1969). We wished to determine how much time free-ranging otters spent foraging diurnally and the spatial distribution of activity throughout the study area. Repeated sampling of an area for sea otter activity provided this information. The diurnal activity of California sea otters has been described quantitatively for females with dependent young (Sandegren et al. 1973). Numerous qualitative descriptions of sea otter diurnal activity exist. In California the foraging cycle has been described as uni, bi, and tri modal (Fisher 1939; Hall and Schaller 1964; Peterson, unpublished data; Odemar and Wilson unpublished data). On Amchitka Island, Alaska, the foraging cycle has been described as bi and tri modal (Kenyon 1969; Estes and Smith 1973). There are no studies of nocturnal activity. We thank Dr. J. Pearse for his encouragement and support of this project. J. Vandevere was fundamental in the completion of this study and his assistance is appreciated. Hopkins Marine Station, Stanford University, generously granted access and facilities.