Davis, Ian Patrick (2018-08). THE TROPHIC DYNAMICS OF SEA OTTERS IN SIMPSON BAY, ALASKA. Doctoral Dissertation. Thesis uri icon


  • Sea otters are generalist predators that feed on benthic megainvertebrates in littoral waters of the North Pacific. Because of their elevated metabolism, they consume up to 24% of their body weight daily and have a strong, top-down influence on invertebrate populations. Simpson Bay is a shallow, turbid, outwash fjord within Prince William Sound located in southcentral Alaska. Sea otters reoccupied Simpson Bay in the late 1970s, and the annual number of adults (~70 with a summer peak of 98) using the bay has been stable since 2002. The goal of this study was to measure the abundance and distribution of large bivalves relative to habitat type and then compare their abundance to annual consumption by sea otters. In addition, carbon flow in the bay was modeled for four trophic levels from primary production to sea otters. The seafloor of the bay is 33.1% mud, 45.1% mud-gravel, and 21.8% rocky substrate. The total fresh mass of bivalves for all species in Simpson Bay was 9.27 x 10^5 kg. Butter clams (Saxidomus gigantea) were the most abundant hard-shelled bivalve which occurred primarily in mud-gravel (76%) but also in mud. Stained macomas (Macoma inquinata) were the second most abundant bivalve which occurred almost exclusively in mud (91%). Together, these two species represented 71% of the bivalve biomass in the bay. The overall average numerical and mass densities of clams for the entire bay were 9.1 clams m^-2 and 47.5 g m^-2. Sea otters foraged on bivalves in proportion to their presence in the two benthic sediment types. The estimated annual primary productivity in Simpson Bay was of 132 g C m^-2 yr^-1 with a peak during the spring bloom of 720 g C m^-2 yr^-1. Carbon from primary productivity moved through particulate organic carbon and suspension feeding bivalves to sea otters. On an annual basis, about 3% of the carbon flowing to bivalves goes to sea otters which consume about 12% of the bivalve standing stock annually. This abundance of large bivalves has been sufficient to sustain a sea otter density of ~3.3 adult otters km-2 that has been stable for 17 years and probably longer.

publication date

  • August 2018