Co-occurrence of Pacific sleeper sharks Somniosus pacificus and harbor seals Phoca vitulina in Glacier Bay

Abstract

We present evidence that Pacific sleeper sharks Somniosus pacificus co-occur with harbor seals Phoca vitulina in Glacier Bay, Alaska, and that these sharks scavenge or prey on marine mammals. In 2002, 415 stations were fished throughout Glacier Bay on a systematic sampling grid. Pacific sleeper sharks were caught at 3 of the 415 stations, and at one station a Pacific halibut Hippoglossus stenolepis was caught with a fresh bite, identified as the bite of a sleeper shark. All 3 sharks and the shark-bitten halibut were caught at stations near the mouth of Johns Hopkins Inlet, a glacial fjord with the highest concentration of seals in Glacier Bay. Using a bootstrap technique, we estimated the probability of sampling the sharks (and the shark-bitten halibut) in the vicinity of Johns Hopkins Inlet. If sharks were randomly distributed in Glacier Bay, the probability of sampling all 4 pots at the mouth of Johns Hopkins Inlet was very low (P = 0.00002). The highly non-random distribution of the sleeper sharks located near the largest harbor seal pupping and breeding colony in Glacier Bay suggests that these 2 species co-occur and may interact ecologically in or near Johns Hopkins Inlet. be active predators on marine mammals (Crovetto et al. 1992), including seals (Bright 1959; Ridoux et al. 1998). Although there is no direct information on abundance or trend for Pacific sleeper sharks in Glacier Bay, significant increases were detected between 1984 and 1996 in the central Gulf of Alaska (Mueter and Norcross 2002). If the number of sleeper sharks in Glacier Bay has also increased, then it may help explain the seal declines observed. We present evidence that Pacific sleeper sharks co-occur with breeding harbor seals in Glacier Bay. We hypothesize that the co-occurrence may be caused by an overlap in prey species or that these sharks scavenge or prey on marine mammals. METHODS In July and August, 2002, top-loading conical Tanner crab pots were fished at 415 stations on a systematic grid throughout Glacier Bay to assess the distribution and relative abundance of Tanner crab Chionoecetes bairdi and red king crab Paralithodes camtschaticus (Figure 1). All locations and depths were sampled Authors: S. JAMES TAGGART and ALEXANDER G. ANDREWS are with the U.S. Geological Survey, Glacier Bay Field Station, 3100 National Park Road, Juneau, Alaska 99801. E-mail: jim_taggart@usgs.gov. JENNIFER MONDRAGON was with the U.S. Geological Survey, Glacier Bay Field Station, 3100 National Park Road, Juneau, Alaska 99801 and is now with NOAA Fisheries, 709 West 9th Street, Juneau, Alaska 99801. ELIZABETH A. MATHEWS is with the University of Alaska Southeast, Natural Sciences Department, 11120 Glacier Hwy., Juneau, Alaska 99801. Acknowledgments: We especially thank J. de La Bruere for his experience and skill in operating the R/V Alaskan Gyre and J. Nielsen for her enthusiastic effort toward this work. We thank S. Tamone, G. Eckert, J. Douglas, Z. Hoyt, C. Hoover, J. Mitchell and L. Eisner for helping with the field sampling. Stomach samples were sorted and identified with the assistance of L. Hulbert formerly with NOAA Fisheries, now with the Alaska Department of Fish and Game. We thank Jeep Rice and 2 anonymous reviewers for their constructive comments on the manuscript. Sponsorship: Funding for this work was provided by the U.S. Geological Survey and the National Park Service. INTRODUCTION Glacier Bay National Park has had one of the largest breeding colonies of harbor seals Phoca vitulina in Alaska (Calambokidis 1987; Mathews 1995). However, the number of seals has declined steeply. In 1992, 6,200 seals were counted at haulouts during August surveys, but by August 2001 only 2,600 seals were counted at these same haulouts. Harbor seals in Glacier Bay have experienced one of the largest documented population declines in Alaska—a decline of 63% over 11 years within Johns Hopkins Inlet, a large glacial fjord, and a decline of 75% over 10 years at terrestrial resting sites (Mathews and Pendleton 2006). The cause of this population decline is not known, but increased mortality, possibly due to predation by Pacific sleeper sharks Somniosus pacificus, Steller sea lions Eumetopias jubatus, or killer whales Orcinus orca, is one of several proposed hypotheses (Mathews and Pendleton 2006). Sleeper sharks are known to feed on marine mammal carrion in the North Pacific (S. pacificus; Smith et al. 2002) and North Atlantic (S. microcephalus; Bigelow and Schroeder 1948; Compagno 1984); they may also