Effects Of Sea Otter Colonization On Soft-Sediment Intertidal Prey Assemblages In Glacier Bay, Alaska

The influence of sea otters (Enhydra lutris) on nearshore marine communities has been widely studied in kelp-forest ecosystems, however less is known about their impacts in other systems. In Southeast Alaska, the recolonization by sea otters into areas where they have been long absent (~100 years), provides a natural experiment with which to examine the response of soft-substrate intertidal prey communities to the return of a top predator. Sea otter abundance in Glacier Bay, Alaska increased from zero in 1993 to >8,500 individuals in 2012. Their aggregated diet from intertidal foraging bouts consisted predominantly of clam (56%), horse mussels (15%) and urchins (18%) and was found to diversify over space and time as sea otters colonized and persisted. To assess intertidal community response to sea otters 45 intertidal sites were sampled prior to, or concurrent with sea otter colonization, and again up to 12 years later. Using GIS and aerial surveys I estimated the cumulative density of the sea otters at each site. At a sea otter density of roughly 6-otters/km2 clam populations were found to have 50% probability of population decline, and preferred prey species such as Saxidomus giganteus were found to likely decline at sea otter densities <2-otters/km2. At sites where sea otters were at low densities or absent, clam biomass generally remained unchanged or increased. Energy recovery rates decreased significantly with cumulative otter density from roughly 20 kcal/min at 0.36 otters/km2 to 9 kcal/min at 4.54 otters/km2, requiring individual otters to increase the amount of energy they expend foraging. The effect of sea otters on intertidal clam populations in soft-sediment was consistent with findings in other habitats, where reduced densities and sizes of prey were documented. This study established the occurrence of top-down structuring in soft-sediment systems over a multi-decadal time scale