Nocturnal activity by mammal‐eating killer whales at a predation hot spot in the Bering Sea

Abstract

The importance of food availability (bottom-up control) vs. predation (top-down control) in affecting population dynamics of individual species and the structure of marine communities and ecosystems is in most cases poorly known, but vigorously debated. This debate has recently focused on the dramatic declines of populations of four species of marine mammals in Alaska (Estes et al. 1998, Springer et al. 2003, 2008; Williams et al. 2004, Fritz and Hinckley 2005, Trites et al. 2007, Wade et al. 2007). Between the early 1970s and late 1990s, the abundance of harbor seals (Phoca vitulina), Steller sea lions (Eumetopias jubatus), and sea otters (Enhydra lutris) collapsed sequentially by 80%–90% throughout southwest Alaska—the Aleutian Islands, Bering Sea, and western Gulf of Alaska. The major portion of each collapse occurred over short intervals of about 10 yr. Harbor seals have since begun a slow recovery in at least a part of this range (Small et al. 2003); sea lions have variously increased somewhat, remained stable, or continued to decline (NMFS 2007); and sea otters were still declining in most areas through the early 2000s (Doroff et al. 2003, Estes et al. 2005, J. Estes1). Across that entire span of years, a fourth species, the northern fur seal (Callorhinus ursinus) on the Pribilof Islands in the Bering Sea, has experienced a less precipitous, but continuing, and now concerning, decline—for example, pup production, the index of total abundance, has fallen by 26% since 2002 and their abundance is just 24% of the peak in 1955 (Trites and Larkin 1989, Towell and Fowler 2006). Three principal hypotheses have been proposed to explain these collapses and the lack of significant recovery: direct mortality from human activities, nutritional limitation, and predation. Direct mortality of sea lions was caused primarily by commercial harvests, bycatch in commercial fisheries, and legal and illegal shooting (Atkinson et al. 2008): direct mortality has not been invoked as a cause of the declines of any of the other species in the region since the mid 1970s or earlier. The cause of purported nutritional limitation of pinnipeds is thought to be climate change or commercial fisheries, or both, which altered the availability of various species of