Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling?

Abstract

Populations of seals, sea lions, and sea otters have sequentially collapsed over large areas of the northern North Pacific Ocean and southern Bering Sea during the last several decades. A bottom-up nutritional limitation mechanism induced by physical oceanographic change or competition with fisheries was long thought to be largely responsible for these declines. The current weight of evidence is more consistent with top-down forcing. Increased predation by killer whales probably drove the sea otter collapse and may have been responsible for the earlier pinniped declines as well. We propose that decimation of the great whales by postWorld War II industrial whaling caused the great whales' foremost natural predators, killer whales, to begin feeding more intensively on the smaller marine mammals, thus ''fishing-down'' this element of the marine food web. The timing of these events, information on the abundance, diet, and foraging behavior of both predators and prey, and feasibility analyses based on demographic and energetic modeling are all consistent with this hypothesis.T he abrupt decline of the western stock of Steller sea lions (Eumetopias jubatus) h across most of the northern North Pacific Ocean and southern Bering Sea is one of the world's most well known yet poorly understood marine conservation problems. For years, scientists attributed this decline to nutritional limitation, the presumed consequence of a climate regime shift and͞or competition with regional fisheries (1). Although fisheries and regime shifts undoubtedly influenced both the fishes and their associated food webs (2-5), several recent reviews of the available information on sea lions and their environment, including an assessment by the National Research Council, cast doubt on the nutritional limitation hypothesis (6, 7), notwithstanding evidence from field and laboratory studies that diet quality is a factor in sea lion energetics (8). The doubt stems from three main findings. First, most measures of behavior, physiology, and morphology from surviving adult sea lions and pups in the western Gulf of Alaska and Aleutian Islands are inconsistent with nutritional limitation. These animals have better body condition, reduced foraging effort, and reduced field metabolic rates relative to similar measures from the increasing sea lion population in southeast Alaska (7). Second, sea lion prey is abundant in most areas of the decline (9). Known changes in prey availability and other features of the oceanic ecosystem are particularly incongruous with the most precipitous phase of the decline, which occurred during the mid-to late 1980s, and can be accounted for only by greatly increased adult mortality (6). Third, populations of piscivorous sea birds, many of which feed on earlier life stages of the same fish species consumed by sea lions, have remained stable or increased in the same area and over the same period that the sea lions have declined (10). Top-down forcing now appears to have been an important contributor to declines of Steller sea lion...

Causes of Mortality in Northern Fur Seals (Callorhinus Ursinus), St. Paul Island, Pribilof Islands, Alaska, 1986–2006

“…Hookworms did not appear to be a problem during this investigation and are therefore not a probable cause for the present decline (Lyons et al, 2000;Spraker et al, 2007). Similar to other pinniped populations inhabiting Alaskan waters that have suffered from unexplained declines, including Steller sea lions (Burek et al, 2005) and Pacific harbor seals (Phoca vitulina; Springer et al, 2003), there was no evidence to implicate other infectious diseases as a mechanism for northern fur seal decline over the past 20 yr.…”

Geographic variation in killer whale attacks on humpback whales in the North Pacific: implications for predation pressure

“…Evidence from the US West Coast is inconsistent with the hypothesis that the depletion of large whale populations during commercial whaling forced killer whales to shift their predation to smaller prey, thus triggering the sequential collapse of pinniped and sea otter populations (as Springer et al 2003 hypothesized for western Alaska). Along the US and Mexico west coasts, while humpback and gray whales were severely depleted through the mid-1960s (Rice 1963, Clapham et al 1997, this region has experienced substantial increases in pinniped populations (summarized by Wade et al 2006).…”

“…Springer et al (2003) suggest that the depletion of large whales during commercial whaling forced killer whales to prey more heavily on smaller marine mammals, thus triggering the sequential collapse of harbor seal Phoca vitulina, northern fur seal Callorhinus ursinus, Steller sea lion Eumetopias jubatus, and sea otter Enhydra lutris populations in the northern North Pacific and southern Bering Sea. This predation hypothesis assumes that large cetaceans were an important prey item of killer whales and that this predation occurred predominantly at their high-latitude feeding areas.…”

“…This migration hypothesis and the predation hypothesis (Springer et al 2003) described above remain controversial and are the subject of continuing debate (Clapham 2001, Connor & Corkeron 2001, Williams et al 2004, DeMaster et al 2006, Mizroch & Rice 2006, Mehta et al 2007, Reeves et al 2007). An underlying question in these hypotheses is whether the killer whale predation was significant enough to have affected the behavior of large whale populations on an evolutionary time scale.…”

Environmental controls on food web regimes: A fluvial perspective

“…At the base of marine food-webs, ranges of harmful and oxygen-depleting algal blooms are expanding (http://www.whoi.edu/redtide/HABdistribution/habexpand.html, Rabalais et al, 2002). Are these changes driven by population crashes or altered foraging behavior of apex predators due to overharvesting of these predators (Pauly et al, 1998;Schindler et al, 2001) or their preferred prey (Springer et al, 2003)? Are they due instead to changes in nutrients, primary producers, or zooplankton at the base of food webs?…”