Marine predator surveys in Glacier Bay National Park and Preserve

Since 1999, vessel based surveys to estimate species composition, distribution and relative abundance of marine birds and mammals have been conducted along coastal and pelagic (offshore) transects in Glacier Bay, Alaska. Surveys have been conducted during winter (November-March) and summer (June). This annual report presents the results of those surveys conducted in March and June of 2001. Following completion of surveys in 2002 we will provide a final report of the results of all surveys conducted between 1999 and 2002.

Glacier Bay supports diverse and abundant assemblages of marine birds and mammals. In 2001 we identified 58 species of bird, 7 species of marine mammal, and 6 species of terrestrial mammal on transects sampled during winter and summer. Of course all species are not equally abundant. Among all taxa, in both seasons, sea ducks were the numerically dominant group. In their roles as consumers and because of their generally large size, marine mammals are also likely important in the consumption of energy produced in the Glacier Bay ecosystem. Most common and abundant marine birds and mammals can be placed in either a fish based (e.g. alcids and pinnipeds), or a benthic invertebrate (e.g. sea ducks and sea otters) based food web.

Distinct differences in the species composition and abundance of marine birds were observed between winter and summer surveys. Winter marine bird assemblages were dominated numerically (> 11,000; 65% of all birds) by a relatively few species of sea ducks (scoters, goldeneye, Bufflehead, Harlequin and Long-tailed ducks). The sea ducks were distributed almost exclusively along near shore habitats. The prevalence of sea ducks during the March surveys indicates the importance of Glacier Bay as a wintering area for this poorly understood group of animals that occupy a high trophic position in a principally benthic invertebrate (mussel and clam) food web. Marine mammal assemblages were generally consistent between seasons, although Humpback and Killer whales were not observed in winter 2001.

Summer marine bird assemblages remained numerically dominated by sea ducks, but species composition shifted between the goldeneye whose density was 44/m² in winter to < 0.2/m² in summer, to scoters, whose density was 29/m² in winter to > 60/m² in summer. Large increases in Black-legged kittiwake, murrelet (Marbled and Kittlitz’s) and Common merganser densities were detected during summer surveys. Seasonal differences in abundance of species likely reflected differences in life history attributes (e.g. reproductive biology, foraging ecology) among species.

Because of differences observed in species composition between the winter and summer, it is apparent that a single annual survey cannot accurately describe the populations of marine birds and mammals that occur in Glacier Bay. Preliminary analysis further suggests that interpretations of data resulting from this type of survey may depend to a large extent on the individual species. Because species exhibit differences in behavior, morphology, coloration, and distribution, accuracy and precision of abundance estimates likely vary among species. Confidence in survey results should be evaluated in consideration of life history and detection probabilities at the species level. However, survey results likely provide reasonable estimates of species composition and relative abundance, as well as accurate abundance estimates for those species whose detection closely approximates one.