Sea otter studies in Glacier Bay National Park and Preserve: Aerial surveys, foraging observations, and intertidal clam sampling

Abstract

Following translocations to the outer coast of Southeast Alaska in 1965, sea otters have been expanding their range and increasing in abundance. We began conducting surveys for sea otters in Cross Sound, Icy Strait and Glacier Bay, Alaska in 1994, following initial reports of their presence in Glacier Bay in 1993. Since 1995, the number of sea otters in Glacier Bay proper has increased from about 5 to more than 500. Between 1993 and 1997 sea otters were apparently only occasional visitors to Glacier Bay, but in 1998 long-term residence was established as indicated by the presence of adult females and their dependent pups. Sea otter distribution is limited to the Lower Bay, south of Sandy Cove, and is not continuous within that area. Concentration occur in the vicinity of Sita Reef and Boulder Island and between Pt. Carolus and Rush Pt. on the west side of the Bay (Figure 1). We describe the diet of sea otters in Glacier Bay and south Icy Strait through visual observations of prey during >4,000 successful forage dives. In 2,399 successful foraging dives observed in Glacier Bay proper, diet consisted of 40% clam, 21% urchins, 18% mussel, 4% crab, 5% other and 12% unidentified. Most prey recovered by sea otters are commercially, socially, or ecological important species. Species of clam are primarily Saxidomus gigantea , Protothaca staminea , and Serripes groenlandicus . Urchins are primarily Strongylocentrotus droebachiensis while both mussles, Modiolus modiolus and Mytilus trossulus , are taken. Crabs include species of Cancer, Chinoecetes, Paralithodes, and Telmessus. Although we characterize diet at broad geographic scales, we found diet to vary between sites separated by as little as several hundred meters. Dietary variation among and within sites can reflect differences in prey availability and individual choice. We estimated species composition, density, biomass, and sizes of intertidal clams at 59 sites in Glacier Bay, 14 sites in Idaho Inlet, 12 sites in Port Althorp and 2 sites in Dundas Bay. There is no direct evidence of otter foraging at any of our clam sampling sites except at Port Althorp where sea otters have been present for >20 years and regularly forage intertidally. There is some indication of intertidal foraging in Idaho Inlet, based on reduced mean size of preferred clam species. Sea otters have been present in Idaho Inlet for at least 12 years. We sampled 48 systematically selected sites to allow inference throughout Glacier Bay intertidal areas and 12 preferred habitat intertidal sites to estimate maximum clam densities in the Bay. We also sampled 14 and 12 random sites in Idaho Inlet and Port Althorp, respectively, to provide contrast between sites with and without sea otters. Densities and biomass of intertidal clams were greater in the Lower Bay than either the East or West Arms. Mean densities (#/0.25m 2 ) of all species of clams > 10.0 mm total length were 96.5 at preferred sites, 32.8 in the Lower Bay, 12.2 in the East Arm, 6.6 in the West Arm, 11.32 at Port Althorp and 27.1 at Idaho Inlet. Clam densities were lower in the Upper Arms of Glacier Bay, compared to the Lower Bay and were similar to densities at Port Althorp. In the Lower Bay, clam densities were nearly twice as high at preferred clam sites compared to those systematically sampled. Species of Macoma were the numerically dominant intertidal clam at most sites in Glacier Bay, while Protothaca staminea was dominant at Idaho Inlet and Port Althorp. Biomas (g/0.25m 2 ) was higher in the Lower Bay (23.5) than either Arm (2.1 and .91) and higher at preferred sites (73.4) than systematically selected sites in Glacier Bay. Biomass estimates at Port Althorp were 5.2 and 9.7 at Idaho Inlet. Biomass estimates were dominated by species of Saxidomus , Protothaca and Mya in Glacier Bay and by Protothaca and Saxidomus at Idaho Inlet and Port Althrop. We suspect differences in density and biomass relate to habitat differences between areas within Glacier Bay