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...

Climate change in the last century was associated with spectacular growth of many wild Pacific salmon stocks in the North Pacific Ocean and Bering Sea, apparently through bottom-up forcing linking meteorology to ocean physics, water temperature, and plankton production. One species in particular, pink salmon, became so numerous by the 1990s that they began to dominate other species of salmon for prey resources and to exert top-down control in the open ocean ecosystem. Information from long-term monitoring of seabirds in the Aleutian Islands and Bering Sea reveals that the sphere of influence of pink salmon is much larger than previously known. Seabirds, pink salmon, other species of salmon, and by extension other higher-order predators, are tightly linked ecologically and must be included in international management and conservation policies for sustaining all species that compete for common, finite resource pools. These data further emphasize that the unique 2-y cycle in abundance of pink salmon drives interannual shifts between two alternate states of a complex marine ecosystem.ocean ecology | exploitative competition | consumer front | interaction strength | carrying capacity P redator control of community structure and ecosystem function became a tenet of intertidal and nearshore marine ecology following early studies of Paine and others (1-3), yet with few exceptions (4, 5), until more recent times the idea has been less well appreciated for open oceans. Growing attention now is being paid to the overexploitation of pelagic species, particularly those at higher trophic levels currently and in the past, and effects on ocean ecosystems of the loss, or development, of top-down forcing (6-12).The prevailing view has long held that most biological change in ocean ecosystems, apart from human exploitation, is driven from the bottom up (13-16). One striking example that has been linked to bottom-up processes driven by climate change is the burgeoning abundance of wild Pacific salmon (Oncorhynchus spp.), and in particular pink salmon (Oncorhynchus gorbuscha), in the subarctic North Pacific Ocean and Bering Sea (SNPO/BS). Underpinning the notion initially were studies that found (i) strong coherence between decadal patterns in the Aleutian Low pressure system, which exerts a large influence over climate in the North Pacific Ocean, and patterns in salmon production across a broad region of the SNPO/BS (17, 18); (ii) decadal patterns in primary production that could be explained by the effect of the Aleutian Low pressure system on basin scale wind fields (19); and (iii) decadal patterns in zooplankton, squid, and pelagic fish production that also were correlated with meteorological forcing over the North Pacific Ocean and consistent with patterns in primary production (20). Thus, the general explanation for waxing and waning abundances of salmon over the record in the 20th century was that physical forcing by shifts in the strength and position of the Aleutian Low altered winds, ocean temperatures, and primary and sec...