Harbor seal population decline in the Aleutian Archipelago

Abstract

Populations of Steller sea lions, northern fur seals, and northern sea otters declined substantially during recent decades in the Bering Sea and Aleutian Islands region, yet the population status of harbor seals has not been assessed adequately. We determined that counts obtained during skiff-based surveys conducted in [1977][1978][1979][1980][1981][1982] represent the earliest estimate of harbor seal abundance throughout the Aleutian Islands. By comparing counts from 106 islands surveyed in 1977-1982 (8,601 seals) with counts from the same islands during a 1999 aerial survey (2,859 seals), we observed a 67% decline over the ∼20-yr period. Regionally, the largest decline of 86% was in the western Aleutians (n = 7 islands), followed by 66% in the central Aleutians (n = 64 islands), and 45% in the eastern Aleutians (n = 35 islands). Harbor seal counts decreased at the majority of islands in each region, the number of islands with >100 seals decreased ∼70%, and the number of islands with no seals counted increased ∼80%, indicating that harbor seal abundance throughout the Aleutian Islands was substantially lower in the late 1990s than in the 1970s and 1980s.Key words: population decline, harbor seal, Phoca vitulina, Aleutian Islands, Gulf of Alaska stock. 845This article is a U.S. government work, and is not subject to copyright in the United States.846 MARINE MAMMAL SCIENCE, VOL. 24, NO. 4, 2008 The population declines of Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus), and northern sea otters (Enhydra lutris kenyoni) in the Bering Sea and Aleutian Islands region during recent decades have prompted substantial efforts to identify the causes of those declines, conserve and manage the species, and interpret marine ecosystem dynamics (Estes et al. 1998, NRC 2003, Springer et al. 2003, Trites and Donnelly 2003, DeMaster et al. 2006, Wade et al. 2007. Less information has been available to determine whether harbor seals (Phoca vitulina) have also declined in the Aleutian Islands region, yet robust estimates of multi-year population trends indicate declines in several relatively small regions in the Gulf of Alaska and Southeast Alaska (Frost et al. 1999, Small et al. 2003, Mathews and Pendleton 2006.Harbor seals were considered common and relatively abundant throughout the Aleutian Archipelago during the 1960s and early 1970s (Pitcher 1985), yet a reliable estimate of abundance is not available for that period or earlier. Harbor seals in the Aleutians have been surveyed on numerous occasions during the past 70 yr: 1930s (Murie 1959), 1956-1957(Mathisen and Lopp 1963), 1960-1965(Kenyon 1960, 1962, Kenyon and Rice 1961, Kenyon and King 1965), 1975-1977(Everitt and Braham 1980(Fiscus et al. 1981), 1986(Brueggeman et al. 1988), and 1992and 2000(Doroff et al. 2003. However, harbor seals were typically not the species for which these surveys were conducted, thus survey effort and protocols varied substantially, and available counts were often incomplete for individual islands as w...

Large mammal biomass predicts the changing distribution of hunter-gatherer settlements in mid-late Holocene Alaska

“…They perhaps acted in combination: (1) marine mammal populations are lower today than they were in the past; or (2) smaller but perhaps not lower ranked prey items, (e.g., anadromous fish, marine fish, shellfish, birds) entered the diet breadth, occurring in high enough densities that they had a large influence on settlement patterns. Consistent with the first, since the 1970s marine biologists working in the Gulf of Alaska and southern Bering Sea have documented population declines for sea lions, seals, and sea otters of 50-70% (Boveng et al, 2003;Burn and Doroff, 2005;Estes et al, 2009;Jemison et al, 2006;Sease and York, 2003;Small et al, 2008;Yen et al, 2005). In support of the second explanation, however, is the faunal record at Ocean Bay sites, which shows fish, shellfish and birds were major dietary components throughout human history in the Gulf of Alaska (Casperson, 2012;Clark, 1998Clark, , 2001).…”

Variation in breeding phenology provides insights into drivers of long-term population change in harbour seals

“…Our analyses also highlight how individualbased studies can be used to develop and validate proxies that can be applied to data collected from broad-scale abundance surveys. Survey designs that are optimized to assess variation in both abundance and breeding phenology could now play a critical role in testing competing hypotheses about the drivers of unexplained declines in both harbour seals [25,46] and other pinniped populations [45,47].…”

Harbour seal Phoca vitulina movement patterns in the high-Arctic archipelago of Svalbard, Norway

“…However, the situation is quite different for northern populations, where up-to-date regional harbour seal abundance information is often lacking (despite local harvesting activities), and some populations have shown marked declines in both the North Atlantic (Lucas & Stobo 2000, Hanson et al 2013, Matthiopoulos et al 2014 and the North Pacific (e.g. Frost et al 1999, Small et al 2008, Womble et al 2010. The reasons for the observed declines are not clear in all cases, but several hypotheses have been proposed that suggest climate change-related linkages such as shifts in prey distribution, increased predation pressure, vulnerability to pathogens or competitive stress (Trites et al 2007, Matthiopoulos et al 2014.…”

“…The reasons for the observed declines are not clear in all cases, but several hypotheses have been proposed that suggest climate change-related linkages such as shifts in prey distribution, increased predation pressure, vulnerability to pathogens or competitive stress (Trites et al 2007, Matthiopoulos et al 2014. The declines in harbour seal numbers in the Aleutian Islands and the Gulf of Alaska over the last 30 yr have been linked to a large-scale oceanographic regime shift that has likely been induced by climate change (Small et al 2008, Womble et al 2010. Sharkinflicted mortality seems to be a predominant factor for seal decline in Nova Scotia, where a large proportion of pups and adult females are killed by one or more shark species (Lucas & Stobo 2000, Lucas & Natanson 2010.…”