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
The Baltic Sea
We have tried to predict the effects of fishing on marine communities by deriving a model of an ecosystem and varying the levels at which different species are harvested. For this task, we used the computer software Ecopath (available at www.ecopath.org), which simulates food-web structure by modeling the production and transfer of biomass among species. Ecopath can be tailored to any food web and has been applied to more than 130 marine communities. It handles data on abundance, production and feeding rates, diets and harvests of key food-web components (primary producers, invertebrates, fish, marine fishing pressure on a particular species). In addition, we used Ecosim, a dynamic model that simulates how a community would likely respond to changes in fishing intensity and other forces. Ideally, field studies are used to validate model parameters, thus improving model performance and our understanding of community dynamics.
We applied Ecopath/Ecosim to the Baltic Sea, a northern European arm of the Atlantic Ocean. This marine system is ideal for this type of application because it is relatively simple and well-studied. Because fisheries are a mainstay of the region's commerce, there is a rich body of data on local fish species and the prey, predators and fisheries that interact with them.
The most visible components of the Baltic Sea food web (Figure 3)—fish and seals—are currently in undesirable states: Sprat (Sprattus sprattus), a fish of low commercial value, dominates the fish biomass; herring (Clupea harengus) and cod (Gadus morhua) are near all-time lows; and seals are slowly recovering from near-total collapse during the past century. These conditions have resulted largely from overharvesting. But how has overfishing affected the rest of the community?
We do not know how the Baltic Sea community was structured before fisheries developed, so we simulated an unfished state by eliminating all fishing from the model for 100 years. Compared with the year 2000, the 2100 community featured much larger biomasses of herring and cod at the expense of sprat (Figure 4a). These changes cascaded through the rest of the community. For example, herring's increased abundance reduced the level of one of its main prey, pelagic invertebrates such as small shrimp (Figure 4b), to half the 2000 level. Similarly, the recovery of cod cut down the biomass of benthic macrofauna, allowing one of their major prey, benthic meiofauna, to double in biomass by 2100. The overall growth in total fish biomass in the unfished scenario tremendously increased seal biomass (Figure 4c).
We also determined what the system might look like if fishing pressure were maintained at the status quo. After 100 years of status quo fishing, sprat continued to dominate the fish community, cod biomass was low, and herring were totally extirpated. The rest of the community was profoundly different from the unfished state: The abundance of benthic macrofauna was the same as at present, pelagic invertebrates were more abundant because they were not being eaten by herring, but seal recovery was only modest.
We introduced fishing into the unfished system by incrementally increasing fishing pressure on cod, herring and sprat. As we moved from no fishing to 20 percent of the status quo, the three species and the overall community responded most strongly. Seals also responded dramatically: This relatively small increase in fishing allowed human competition for fish to greatly impede seal recovery.
We also used the model to predict the consequences of a more precautionary fisheries management plan, which some Baltic fisheries scientists advocate. Their approach would cut fishing pressure on cod and herring by about 35 percent and 65 percent, respectively, from current levels and would slightly increase sprat fishing. It aims to bolster cod and herring populations by protecting them against overfishing. We detected a desirable response from the fish community: Biomasses of cod and herring clearly were higher in 2100 than in 2000 or status quo conditions. Unfortunately, the rest of the community retained the characteristics of a heavily fished system, falling somewhere between the 40 percent and 60 percent status quo scenarios. These results suggest that this type of fishery rehabilitation would do little to shift community structure toward the unfished state.
An additional simulation looked at the effects of fishing pressure on community trophic structure. As seen above, fishing directly and indirectly changes the relative abundances of species in a food web. While others have focused on the change in average trophic level of the harvested species, we calculated the average trophic level (weighted by the abundance of each community component) of the entire Baltic community across a range of fishing pressures. As pressure increased from zero to the status quo level, the average trophic level decreased, following a curve shaped like a decay function (Figure 5). Much of that decrease resulted from a shift of biomass from relatively large fish to a community increasingly dominated by phytoplankton and benthic macrofauna. The dominant fish species changed from a major predator, cod, to one of its prey, sprat, and the biomass of the overall community shifted closer to the primary consumer level (trophic level 2). Our model therefore supports the idea that fishing can change an ecosystem's trophic structure.
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