Competition between fisheries and marine mammals for prey and primary production in the Pacific Ocean

The degree of competition between fisheries and marine mammals in the Pacific Ocean was estimated for 7 statistical areas defined by the Food and Agriculture Organization of the United Nations (FAO). Catch statistics compiled from FAO sources show that the amount of fish caught in the Pacific Ocean rose from 2 million tons in the late-1940s to over 50 million tons in the early-1990s. Recent stagnation and declines occurring in some areas of the Pacific suggest that Pacific fisheries cannot continue to expand as they had previously. Based on estimates of population size, total biomass and daily consumption rates, it was estimated that the 84 species of marine mammals inhabiting the Pacific Ocean consume about three times as much food as humans harvest. A large fraction (>60%) of the food caught by marine mammals consisted of deep sea squids and very small deep sea fishes not harvestable by humans, thus limiting the extent of direct competition between fisheries and marine mammals. Moreover, the most important consumers of commercially exploited fish are other predatory fish, not marine mammals. Although direct competition between fisheries and marine mammals for prey appears rather limited, there may be considerable indirect competition for primary production. The primary production required to sustain marine mammals in each of the 7 FAO areas varies within a narrow range, suggesting that the diversity and abundance of marine mammals may have slowly evolved to fully exploit their niche and maximize their use of available primary production. This contrasts with the rapid expansion of fisheries and their relatively recent dependence on primary production, which may have led to what we call 'food web competition'.