Considering Other Consumers: Fisheries, Predators, and Atlantic Herring in the Gulf of Maine

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. INTRODUCTION Fishery managers face the dual responsibility of obtaining the optimum yield from fisheries while simultaneously protecting certain other species (Gerber et al. 1999). The bycatch of protected species such as sea turtles and marine mammals is well recognized (Crowder and Murawski 1998, National Research Council 1999), but the ecological role of commercially important fish species as prey of protected species has received less attention in the management process (Overholtz et al. 2000). Most commercial fisheries in the United States are managed on a single-species basis, but trophic interactions occur within a complex, multispecies environment. Managing the extraction of natural resources to account for larger ecosystem dynamics is a challenging task for both terrestrial and aquatic natural resource managers. Whether concerned with fish or forests, managers seek to obtain optimum benefits from resource extraction while considering biological and ecological processes such as reproduction, regeneration, predation, and competition. In some systems, however, the data required to adequately understand ecosystem processes may be difficult to obtain or unavailable. Yet, despite this difficulty, federal legislation may mandate that resources are managed to protect additional species and habitats. In the Gulf of Maine, Atlantic herring (Clupea harengus) is both a commercially valuable species and an important prey species for a large number of marine mammals, seabirds, and piscivorous fish (Caddy and Iles 1973, Overholtz et al. 1991, 2000, NEFMC 1999). The herring resource in the Gulf of Maine is currently considered to be underexploited by the New England Fishery Management Council. In 2001, the ex-vessel Efforts to model ecosystem bioenergetics are underway in many resource sectors. Approaches such as Ecopath offer a method for scientists to understand ecosystem processes such as feeding interactions or nutrient flow in the context of various harvest regimes.