Abalones, Shells, and Sea Otters: Dynamics of Prey Populations in Central California

Abstract

Densities, sizes, and species composition of abalones, Haliotis refescens, H. walallensis, and H. kamtschatkana, remained stable from 1972 to 1981 within a sea otter habitat in central California. Almost all censused abalones were in crevices, with densities averaging 1.8 animals/10 m2. Live abalones and empty abalone shells resulting primarily from predation by sea otters averaged 7.5 and9 cm length, respectively. In contrast, out of the range of sea otters, live abalones were over twice as large and were numerous outside crevices (when protected from human predation). At our study site within their range, sea otters selected abalones larger than were proportionately available in crevices, and they broke many large shells with stone tools. An average of 55 freshly produced empty abalone shells, equivalent to °20% of the crevice population of live abalones, were found each month in our study area over a 12—mo period. Indirect calculations of feeding rates of sea otters also indicated a high production rate of abalones, as did observations of abalones in mapped crevices. The high production rates of abalones must result from high rates of juvenile recruitment and growth, and are indicative of a highly dynamic population, with a turnover time of 1—3 yr.