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Dynamic Changes in Marine Ecosystems: Fishing, Food Webs, and Future Options
the ecosystem effects discussed in Chapter 2. This chapter focuses primarily on tradeoffs within and between fisheries; however, a larger suite of issues must be examined when considering tradeoffs in an ecosystem context. These issues are addressed in Chapter 4.
FISHERIES MANAGEMENT IMPLICATIONS OF ECOSYSTEM INTERACTIONS
The consideration of multi-species interactions requires making decisions that involve explicit allocation tradeoffs both between food-web components and between user groups. In choosing harvest strategies in a multi-species system, it is impossible to avoid making de facto distribution decisions. Making these kinds of tradeoffs goes well beyond just deciding allowable catches for target species; bycatch must also be considered in most fisheries. Tradeoffs must also be made among fisheries, other commercial uses, and nonconsumptive uses of marine resources. These decisions are not scientific but instead decide the allocation of resources, although science still has an important role to play in informing such decisions (discussed further in Chapter 4).
Harvest strategies used in the United States involve the specification of biological reference points to determine target harvest rates as well as limits of fishing mortality and biomass that ought to be avoided. In this context, a natural first step is to determine how these harvest-rate targets and limits will be determined to account for species interactions. Should lower harvest targets be used for forage fish to protect the productivity of top predators? Should rebuilding targets be set taking into account that reestablishing depleted predator populations may impact fisheries that harvest their prey? Biological reference points depend upon life-history parameters, perhaps most significantly predation mortality (Collie and Gislason 2001). The effects of interactions can only be ignored if buffering mechanisms (e.g., predators switch between alternative food sources and prey have limited vulnerability to their predators) keep natural mortality rates from varying in response to changes in trophic structure.
This chapter presents examples of different fisheries scenarios and associated management implications that come into play when accounting for the food-web effects of fishing in marine ecosystems. These examples are simplified. They are intended only to highlight considerations that managers will begin to face as ecosystem interactions are incorporated into fisheries management and tradeoffs are explicitly made among species and users.
Fishing a Single Trophic Level
The management implications of accounting for ecosystem effects of fishing vary depending upon the nature of the fisheries in question. The simplest setting (uncommon in practice) would be one in which fisheries only target the top