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
Global Consequences
The Baltic Sea ecosystem is but one of hundreds of marine ecosystems that provide commercially useful fish and other resources. Taken together, these systems operate within the larger global system, which is closed and therefore sets important limits on exploitation rates. Because harvestable biomass depends on assimilated solar and chemical energy, as well as on individual animals capable of reproduction, the absolute limit on the sustainable rate of harvest is determined by the rate at which primary producers can assimilate the energy that supports higher trophic levels. If fisheries remove a substantial proportion of the energy acquired from primary producers, the system becomes potentially unstable.
Daniel Pauly and Villy Christensen estimated that 8 percent of the ocean's primary productivity is necessary to sustain global fisheries, which is four times greater than previous estimates. However, this figure was dominated by the open ocean, which represents most of the ocean's surface area but relatively little of its productivity. For commercially important regions, the primary production required to sustain fisheries was much greater: 25 percent for upwelling zones, 24 percent for tropical coastal shelves and an extraordinary 35 percent for temperate coastal shelves.
It is apparent that we are close to the limit on sustainable harvests and that simply improving fishing efficiency or management practices will not allow us to significantly increase global marine exploitation. In fact, since Pauly and Christensen published their study, global landings have leveled off (Figure 6), suggesting that we might already have reached some sort of threshold. Humans are now the apex predators in many marine communities, but, unlike other predators, we generally do not recycle energy back into the appropriate ecosystems. We also have little understanding of the long-term effects of removing huge amounts of energy and biomass from the sea.
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