Considering Other Consumers: Fisheries, Predators, and Atlantic Herring in the Gulf of Maine
January 20, 2003
By A. J.Read and C. R. Brownstein
After decades of overexploitation and severe depletion, Atlantic herring stocks in waters of the northeastern United States have recovered. Fishery managers now consider the herring resource to be underexploited. Nevertheless, some fishery managers and sustainable fishery advocates in New England have expressed concern that the fishery management plan may not adequately consider the importance of herring as prey for marine mammals, seabirds, and piscivorous fish. Several studies suggest that consumption by these predators is significant, yet trophic interactions are not explicitly considered in stock assessment models. Instead, as in most fisheries stock assessments, predation is subsumed within the natural mortality rate, and no empirical estimates of herring consumption are used in the models. The goal of the present study was to assess the consumption of herring by marine mammals and to compare this level of consumption with estimates of natural mortality derived from herring stock assessment models. Using the most recent estimates of abundance and the best available data on diet, we estimated total annual consumption of herring by eight marine mammal species in the Gulf of Maine. Our results indicate that marine mammals consume 93,802–189,898 metric tons (mt; 1 metric ton = 1000 kg) of herring annually. In addition, piscivorous fish and seabirds are important predators of herring. We estimate that the consumption of herring by these upper trophic level predators may have exceeded the estimate of natural mortality used in stock assessment models by more than fourfold in 1991. We suggest that fisheries management must move beyond a single-species approach to one that includes formal consideration of trophic relationships.