Glacier Bay National Park and Preserve - Glacier Bay Harbor Seals (U.S. National Park Service)

Harbor Seal Declines in Glacier Bay

Until recently, Glacier Bay was the site of one of Alaska’s largest documented breeding colonies of harbor seals (Phoca vitulina) at Johns Hopkins Inlet, where they rest and nurse on drifting ice bergs (Mathews 1995, Mathews and Kelly 1996). Ice haulouts (resting areas) generated in tidewater glacial fjords are an extremely important habitat for pupping and nursing harbor seals in Alaska. Since this scenic fjord is also an important attraction for visitors to Glacier Bay National Park, special protection measures are in effect during the sensitive pupping months of May and June.

In 1992, 6,370 seals were counted in a parkwide survey of Glacier Bay, with about two thirds found in the ice-filled sections of Johns Hopkins Inlet. Approximately half of the remainder were counted at the Spider Island reef complex in the Beardslee Island Wilderness, where seals haul out on tidally influenced reefs. However, analysis of subsequent counts has shown numbers of seals at both types of haulouts to be steadily declining since then (Mathews and Pendleton 2000). From 1992 to 2001 the minimal estimate of seals on iceberg and terrestrial haulouts in Glacier Bay declined from 6,370 to 2,650, with about the same percentage still distributed between ice and land. Trend analysis, which incorporates environmental (e.g., tide height) and other variables that influence the number of seals ashore, indicates that 60% fewer seals were in Johns Hopkins Inlet in August 2001 compared to August 1992 and 76% fewer seals were on haulouts throughout the rest of Glacier Bay over the same 10-year period. Annual decline rates are –9.6% (95% CI = -10.5 to -8.7%) and –14.46% (95% CI = -17.1 to -11.9) for Johns Hopkins Inlet and all remaining sites, respectively. Rates of declines in harbor seals in Glacier Bay are comparable to those observed in the Gulf of Alaska, where an area-wide decline in harbor seals, Steller sea lions and fish-eating seabirds has been documented.

Harbor seal numbers to the west in the Gulf of Alaska have declined by 50-90%, yet up until recently the population in southeastern Alaska has appeared to be relatively stable or increasing slightly. Because of severe declines elsewhere, and the fact that harbor seals are currently the most numerous top-level mammal in Glacier Bay’s marine ecosystem, researchers have been closely monitoring seal trends. Beginning in 1992, systematic counts of seals during the late-summer molt (shedding) have been conducted using a combination of aerial photographic surveys and counts from land in Johns Hopkins Inlet. Population monitoring of harbor seals in Glacier Bay is conducted through a cooperative program between the National Park Service and the University of Alaska Southeast with help from the National Science Foundation's Research Experiences for Undergraduates. (For more information on that program, you may click on the following link):

http://www.uas.alaska.edu/uas/biology/nsf.html

Harbor Seal Breeding and Pupping

Harbor seals in southeastern Alaska, including Glacier Bay, breed in May and June, and they mate during July and August. Pups grow rapidly on fat-enriched milk, doubling their birth weight of about 11.5 kg (25 lbs), before they are weaned after 3-6 weeks. In Glacier Bay, during spring and summer months, harbor seals use approximately 20 different land haulouts (i.e., tidally-influenced reefs, beaches, and sand bars) as well as iceberg haulouts in Johns Hopkins and McBride Inlets. Despite the importance of glacial fjords as breeding habitat for harbor seals, Johns Hopkins Inlet is the only tidewater glacial fjord in Alaska where long-term monitoring of harbor seals has occurred. Land-based counts of seals in Johns Hopkins Inlet in the early 1970s (Streveler 1979) and in 1984 (Calambokidis et al. 1984, Calambokidis et al. 1987) have provided valuable baseline data for this important breeding area. Other glacial fjords in Alaska have not been monitored because of difficulties in counting seals distributed widely on drifting icebergs using conventional aerial photography or due to access limitations. In summer 2001, the National Marine Fisheries Service initiated the use of high-resolution aerial photography as an alternate method for counting harbor seals on glacial ice, and the seal biologists who work in Glacier Bay will compare their shore-based counts to counts made from extensive patchworks of these photographs

Possible Causes of the Seal Population Decline

Causes of declining numbers of seals on haulouts could result from increased mortality, reduced birth rates, or emigration from Glacier Bay to other areas. Further, increased time in the water, possibly as a result of increased human disturbance or shifts in prey distribution, abundance, or quality, could cause apparent declining trends in seals on haulouts. However, boat-based surveys for marine mammals in Glacier Bay indicate that there are a fewer seals in the water now compared to a decade ago. Increased predation by sleeper sharks (Somniosus pacificus) and Steller sea lions (Eumetopias jubatus) and shifts in diet in the last decade are hypothesized causes of the declines that researchers are testing or plan to test in the near future.

In addition to monitoring the distribution and abundance of seals in the Bay, researchers are also conducting studies on seal diet, genetics, and behavior, including responses to vessel traffic. Determining if the observed declines in harbor seals in Glacier Bay are the result of natural fluctuations in the marine environment or due to human activities will likely be the focus of future research.

Reports on Effects of Vessels on Harbor Seals in Glacier Bay

Lewis, T. M. and E. A. Mathews. 2000. Effects of human visitors on the behavior of harbor seals ( Phoca vitulina richardsi ) at McBride Glacier fjord, Glacier Bay National Park . Glacier Bay National Park and Preserve. Unpublished report. 22 pp. 119 KB

Mathews, E. A. 1994. The effects of seismic reflection surveys and vessel traffic on harbor seals in Johns Hopkins Inlet, Glacier Bay National Park : A preliminary assessment. U.S. National Park Service, Glacier Bay National Park and Preserve. Unpublished Report. 25 pp. 195 KB

Mathews, E. A. 1997. Preliminary assessment of haulout behavior of harbor seals (Phoca vitulina) and sources of disturbance at the Spider Island reefs, Glacier Bay National Park . U.S. National Park Service, Glacier Bay National Park and Preserve. Final Report. 22 pp. 661 KB

Mathews, E. A. 2000. Measuring the effects of vessels on harbor seals (Phoca vitulina richardsi) at North Marble Island , a terrestrial haulout in Glacier Bay National Park . University of Alaska Southeast. Final report to National Park Service. 17 pp. 495 KB

Mathews, E. A. and J. Driscoll. 2001. Disturbance of harbor seals (Phoca vitulina) and potential effects on counts from aerial surveys, Glacier Bay National Park, 1991-1999 . U.S. National Park Service, Glacier Bay National Park and Preserve. Final Report. CA 9910-7-0026. 26 pp. 579 KB

Report on Harbor Seal Diet in Glacier Bay

Mathews, E. A. 2002. Diet of harbor seals at a glacial fjord and a terrestrial haulout in Glacier Bay: 1996-2001. Glacier Bay National Park and Preserve. 17 pp. Report to National Park Service. Cooperative Agreement 9910-97-0026. 17 pp. 232 KB

Literature Cited
Calambokidis, J., G. H. Steiger, and L. E. Healey. 1984. Harbor seal (Phoca vitulina) population, behavior, and reaction to vessels in Glacier Bay, Alaska. Pages 47 in M. Gladziszewski, I. A. Worley, and G. Vequist, eds. First Glacier Bay Science Symposium. U.S.National Park Service, Glacier Bay National Park & Preserve, AK.

Calambokidis, J., B. L. Taylor, S. D. Carter, G. H. Steiger, P. K. Dawson, and L. D. Antrim. 1987. Distribution and haul-out behavior of harbor seals in Glacier Bay, Alaska. Canadian Journal of Zoology 65: 1391-1396.

Mathews, E. A. 1995. Longterm trends in abundance of harbor seals (Phoca vitulina richardsi) and development of monitoring methods in Glacier Bay National Park, Southeast Alaska. Pages 254-263 in D. R. Engstrom, ed. Third Glacier Bay Science Symposium. U.S. National Park Service, Glacier Bay National Park & Preserve, AK.

Mathews, E. A., and B. P. Kelly. 1996. Extreme temporal variation in harbor seal (Phoca vitulina richardsi) numbers in Glacier Bay, a glacial fjord in Southeast Alaska. Marine Mammal Science 12: 483-488.

Mathews, E. A. and G. W. Pendleton. 2000. Declining trends in harbor seal (Phoca vitulina richardsi) numbers at glacial ice and terrestrial haulouts in Glacier Bay National Park, 1992-1998. U.S. National Park Service, Glacier Bay National Park and Preserve. Final Report. 9910-97-0026. 25 pp. [487K .PDF document]

Streveler, G. P. 1979. Distribution, population ecology, and impact susceptibility of the harbor seal in Glacier Bay, Alaska. Pages 49. U.S. National Park Service, Glacier Bay National Park and Preserve.