FEATURE ARTICLE
Ecologically Sustainable Yield
Marine conservation requires a new ecosystem-based concept for fisheries management that looks beyond sustainable yield for individual fish species
Stephen L. Katz, Richard Zabel, Chris Harvey, Thomas Good, Phillip Levin
Ecologically Sustainable Yield
We have seen that the traditional approach to fisheries management relies on single-species models of population dynamics that aim to sustain harvests of commercial species. Such an approach ignores a broad suite of interactions among exploited species and between exploited species and other members of their communities. Widespread recognition of this shortcoming has spawned considerable interest in ecosystem-based fisheries management, and scientists are beginning to identify suitable metrics for monitoring marine ecosystems. Ecosystem-based management requires a long-term commitment to monitor all trophic levels of marine organisms and the physical forces that influence their communities. The well-studied Baltic Sea community and a few others, such as the Bering Sea community, lend themselves to such analyses, but these studies must be extended to include all impacts of fishing, including habitat destruction and bycatch, which in some cases depletes more biomass than does harvest.
Achieving the goal of sustainable harvest of fish, however, does not mean that the impact on marine ecosystems of fisheries is eliminated. The Baltic Sea example shows us that careful assessment of fish stocks in concert with limited levels of fishing effort allow sustainable fisheries. At the same time, prosecution of the Baltic Sea fishery, even at a limited level, fundamentally alters community structure at all trophic levels. In our hypothetical scenarios, fishing above intermediate levels (approximately 40 percent of the status quo) elicited no appreciable positive response from the marine community. This inertia implies that even precautionary levels of harvest can diminish the ecological importance of target species by significantly weakening their linkages to other community members.
Thus, the goal of sustainable fisheries is simply different from, and in some cases may be incompatible with, the goal of maintaining natural marine communities. This should not come as a shock. Sustainable fishing will lower the size of fish populations well below the size of unfished populations. Thus we might expect community shifts in systems where fish strongly influence the population size of their prey or predators. We argue, however, that the cost of mismanaging a community might be far greater than the cost of mismanaging a fishery. Although overfished stocks have been known to recover, revival of communities that have changed states can be excruciatingly slow or even impossible. Such potential impacts are rarely considered in cost-benefit or risk analyses.
Fisheries agencies around the globe focus on the management of natural "resources." To the extent that ecosystems affect exploited resources, these agencies have embraced ecosystem research that allows better resource management. However, a shift in the species composition of benthic invertebrates or seaweeds has not historically been the concern of agencies unless such changes affected fish production. We suggest the time has come to consider the status of marine ecosystems, in addition to the status of fisheries.
One solution to the environmental crisis in our oceans might be to redefine sustainability. If we can shift our way of thinking about fisheries and ecosystems we may prevent the shifting of the ecosystems themselves. We propose a change from maximum sustainable yield to ecologically sustainable yield (ESY): the yield an ecosystem can sustain without shifting to an undesirable state.
To determine ESY, one will have to simultaneously consider the impacts of all harvested species on an ecosystem, quantifying important qualities such as community stability or resilience. This will be challenging because of the uncertainty and indeterminacy inherent in ecological systems and because ecosystems change in response to natural processes in ways that we do not fully understand. However, we can and should improve our understanding of the bounds of expected ecosystem behavior and define ESY within the limits of predictability. Maximum sustainable yield or other more conservative fisheries targets are critical tools in achieving the goal of sustainable fisheries. We submit that a concept such as ESY is a necessary instrument in achieving the objective of marine conservation.
Widespread implementation of ESY is obviously an issue of policy, rather than science. For many ecosystems, harvesting at ESY levels will require even stricter limits than those used to achieve maximum sustainable yields. But we believe that until we fully incorporate the notion of ecological sustainability into our philosophy of fisheries management, we will continue to degrade the communities that nurture commercially valuable fish.
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