Sea otter studies in Glacier Bay National Park and Preserve: annual report 2002

Since 1995, the number of sea otters in Glacier Bay proper has increased from around 5
to more than 1200. Sea otter distribution is mostly limited to the Lower Bay, south of
Sandy Cove, and is not continuous within that area. Concentrations occur in the vicinity
of Sita Reef and Boulder Island and between Pt. Carolus and Rush Pt. on the west side of
the Bay, although there have been occasional sightings north of Sandy Cove (Figure 1).
Large portions of the Bay remain unoccupied by sea otters, but recolonization is
occurring rapidly.

Most prey recovered by sea otters in Glacier Bay are ecologically, commercially, or
socially important species. In 2002 sea otter diet consisted of 35% clam, 26% mussel,
3% crab, 3.0% snail, 2% starfish, 11% urchins, 2% other, and 20% unidentified.
Dominant clam species include the butter clam, Saxidomus gigantea, the Greenland
cockle, Serripes groenlandicus, and the littleneck clam, Protothaca staminea. Urchins
are primarily green urchins, Strongylocentrotus droebachiensis, and the mussel is
Modiolus modiolus. Crabs observed in 2002 include the Dungeness, Cancer magister,
the kelp crab Pugettia gracilis, and the helmet crab, Telmessus cherigonus. Although we
characterize diet at broad geographic scales, we have previously 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 as well as individual
specialization.

We estimated species composition, density, biomass, and sizes of subtidal clams, urchins,
and mussels at 13 sites in Glacier Bay and 5 sites in nearby Port Althorp, where sea otters
have been present for at least 20 years. All sites were selected based on the presence of
abundant clam siphons and the absence of sea otters (Glacier Bay) or abundant shell litter
and the presence of sea otters (Port Althorp). Glacier Bay sites were selected to achieve a
broad geographic sample of dense subtidal clam beds within Glacier Bay prior to
occupation and foraging by sea otters. Port Althorp sites were chosen to achieve a
representative sample of subtidal clam beds already under prolonged foraging pressure by
sea otters. There was no direct evidence of otter foraging at any of our Glacier Bay
sampling sites.

In Glacier Bay, we sampled 15,338 bivalves (average of 1,180/site) representing 14
species of clam, 2 species of mussel, and a single scallop and we sampled 6,917 urchins
(average of 513/site). In Port Althorp, we sampled 1,034 bivalves (average of 207/site)
representing 14 species of clam. We found only 5 urchins, all S. droebachiensis. Mean
densities and biomass of all subtidal clams were significantly greater in Glacier Bay (59.2
and 99/0.25m²
compared to Port Althorp (10.3 and 5.8/0.25m²
(p<0.002 for both).

Our contrasts of subtidal clam populations between Glacier Bay and Port Althorp suggest
that clam densities will likely decline by about a factor of six and that clam biomass
estimates will decline by more than a factor of ten. Numerically dominant species of
clams, P. staminea, S. gigantea, Macoma sp. and Mya sp. were all significantly greater in
density and biomass in Glacier Bay, while C. nutalli density was low but significantly
higher in Port Althorp. Subtidal clam species diversity was significantly greater in Port
Althorp compared to Glacier Bay, although this may simply reflect habitat differences.
Sea urchin densities were high in Glacier Bay, while in Port Althorp urchins were
virtually absent.

Sea otters are now well established in limited areas of the lower portions of Glacier Bay.
It is likely that distribution and numbers of sea otters will continue to increase in Glacier
Bay in the near future. Glacier Bay supports large and diverse populations of clams that
are largely unexploited by sea otters at present. It is predictable that the density and sizes
of clam populations will decline in response to otter predation. This will result in fewer
opportunities for human harvest, but will also trigger ecosystem level changes, as prey
for other predators, such as octopus, sea stars, fishes, birds and mammals are modified.
Sea otters will also modify benthic habitats through excavation of sediments required to
extract burrowing infauna such as clams. Effects of sediment disturbance by foraging sea
otters are not understood. Glacier Bay also supports large populations of other preferred
sea otter prey, such as king (Paralithodes sp.), tanner (Chionoecetes sp.) and dungeness
(Cancer magister) crabs and green sea urchins (S. droebachiensis). As the colonization
of Park waters by sea otters continues, it is also likely that dramatic changes will occur in
the species composition, abundance, and size class distribution of many components of
the nearshore marine ecosystem. Many of the changes will occur as a direct result of
predation by sea otters. Others will result from indirect or cascading effects of sea otter
foraging, such as increased kelp production and modified prey availability for other
nearshore predators. Without recognizing and quantifying the extent of change initiated
by the colonization of Glacier Bay by sea otters, management of nearshore resources will
be severely constrained for many decades.