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Dynamic Changes in Marine Ecosystems: Fishing, Food Webs, and Future Options
managed, including the impacts of fishing on non-target resources and habitat. In response to this growing concern, the National Oceanic and Atmospheric Administration asked The National Academies’ Ocean Studies Board to form a committee of experts to review recent scientific reports and weigh the collective evidence for fisheries-induced changes to the dynamics of marine ecosystems. The committee was asked to discuss the relevance of these scientific findings for U.S. fisheries management, to identify areas for future research and analysis, and to characterize the stewardship implications for living marine resources. To help accomplish these tasks, the committee met publicly three times to hear presentations on relevant subjects ranging from fisheries biology and fisheries governance mechanisms to current modeling and analysis techniques, among others.
EVIDENCE FOR ECOSYSTEM CHANGE
Fishing can alter a wide range of biological interactions, causing changes in predator-prey relationships, cascading effects mediated through food-web interactions, and the loss or degradation of essential habitats. These impacts, along with natural fluctuations in the physical state of the ocean, can interact to intensify fishing impacts beyond targeted species. Fishing is also generally size and species selective, potentially changing the genetic structure and age composition of fished stocks, as well as decreasing the diversity of marine communities. Examples of all these effects have been documented. Although some changes are expected outcomes of management actions, in many instances the measured effects are quantitatively and qualitatively more severe than anticipated by management.
Declines in stock abundance have been measured for many species throughout the world’s oceans, but the extent and severity of these declines differ across stocks and geographical areas. Changes to food-web interactions are expected because fisheries reduce the abundance of one or more components of the food web, simultaneously altering the interactions among species and the strength of these interactions. Direct predator-prey relationships have changed—either releasing lower trophic levels from predation or reducing the availability of prey for higher-level predators—and these effects may spread to successive trophic levels up and down the food web. Such cascading effects are often unforeseen and management actions frequently have unexpected results, especially if the target species plays a critical role in the ecosystem. Some of the greatest long-term impacts of fishing have been observed in non-targeted ecosystem components. Many species, including marine mammals, seabirds, sea turtles, sharks, oysters, kelps, and sea grasses, have been negatively affected by fisheries either directly through bycatch or habitat damage, or indirectly through altered food-web interactions.
One area of active inquiry is the underlying cause for the measured reduction in mean trophic level of landings seen in many of the world’s oceans. Originally, these reductions were attributed to sequentially fishing lower trophic levels as