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Figure 1. Marine fish are a major food source for humans—as well as for gulls. Moreover, fisheries, especially small operations, provide employment for more than 200 million people worldwide. This continuing need for protein and income, however, will likely be impossible to meet in the future without the development of new ecosystem-based fisheries management practices. The authors have used data from one of best understood marine ecosystems, the Baltic Sea, to model how various approaches to managing catch might influence the future health of the world’s fisheries. They conclude that a model based on ecologically sustainable yield will be most effective in balancing the needs of current and future generations. This herring boat was photographed near Ahvenanmaa (the Åland Islands), Finland. Nik Wheeler/Corbis Figure 2. Based on the logistic population growth model, populations have a modest capacity for growth (top) when they are small and attain the greatest capacity when they reach an intermediate size. When the number of organisms in a population becomes so large that the level of resources available to each member becomes limiting, the capacity for growth is reduced once more. In turn, because modeling suggests that the sustainable yield of a fish species is maximal when that species’ population is intermediate in size (bottom), optimal population size can be maintained by harvesting the species at a rate equal to the annual growth rate. The same general result holds for more complex models. These models, which have been widely applied for more than half a century, do not take into account the ecologically important effects that impacts on nontarget populations can have on target populations. Figure 3. Marine organisms interact in numerous ways, transferring energy through various trophic levels in the marine food web. Thus fishing can affect entire ecosystems as well as target species, often disturbing several trophic levels. By removing cod, sprat and herring, for example, fishing decreases the amount of food available to seals. Conversely, it favors organisms that normally serve as prey for cod, sprat and herring. When populations of organisms at this second trophic level increase, they consume more organisms at the first trophic level, reducing the number of primary producers, such as phytoplankton, that bring energy into the ecosystem. Figure 4. Computer modeling with Ecopath/Ecosim predicts how populations of sprat, herring and cod (a) might respond if fishing in the Baltic Sea were prohibited between the years 2000 and 2100 (unfished) or permitted at 20 percent, 40 percent, 60 percent, 80 percent or 100 percent of the year-2000 level (the status quo level, or SQ). One current recommendation, precautionary fishery management (PFM), would reduce cod fishing by 35 percent and herring fishing by 65 percent. Various scenarios for fishing levels also influence the prevalence of species at lower trophic levels (b), particularly benthic macrofauna, which profit from reduced numbers of predators at higher fishing levels. These models suggest that changes in predator populations affect the rest of the ecosystem. They also predict that changes in fishing pressure affect seal biomass (c). Because humans compete directly with seals for fish, even a low level of fishing (20 percent of SQ) reduces seal populations far below the predicted unfished level. Figure 5. Models suggest that fishing decreases an ecosystem’s average trophic level by removing predatory secondary consumers, such as cod, and enabling populations of primary consumers, such as herbivorous fish, to increase. This shift occurs at low fishing pressures, resulting in a curve shaped like a decay function. Barbara Aulicino Figure 6. Data demonstrate that marine fisheries landings have, at best, remained level since the late 1980s. Further, the amount of primary productivity required to sustain these yields (as much as 35 percent on temperate coastal shelves) is quite high. It is unlikely, therefore, that improvements in efficiency or management will increase harvests. Instead, the authors propose that future fisheries management be based on ecologically sustainable yield, a concept that will support marine ecosystems and therefore maintain productivity for generations to come. Barbara Aulicino