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
Fishing and Ecosystems
Ecosystems are characterized by their communities of plants, animals and microorganisms, and by the local physical, chemical and structural environments in which those communities reside. Fishing alters marine ecosystems both by modifying community composition and by altering local environments, as when trawls drag across the ocean floor. Because ecosystem components interact, the effects of fishing are more complex than the simple removal of a few species. Ecosystems can suddenly collapse when some of their components are damaged, just as houses can fall if their foundations rot away.
Marine community members interact in many ways: They feed on each other, compete for key resources and, as with kelps and corals, provide habitat structure. A species' importance in a community depends on its relative abundance and the strength of its interactions with other components. Some species have little impact on others, whereas other species greatly influence their fellow community members. A species' position in the food web determines its average trophic level. Primary producers are said to occupy the first trophic level because they derive energy from the sun or chemicals in seawater. Phytoplankton, seaweeds and sulfur bacteria are primary producers. Herbivores occupy the second trophic level because they eat primary producers. Carnivores occupy the third because they eat herbivores. The top species is called the apex predator.
Fishing can directly affect communities, as when it changes key life-history traits. Fisheries typically seek larger, more valuable fish, decreasing the average size of fish in target populations. Because reproductive ability relates to size in many species, this decrease has the potential to reduce the number of fecund fish, altering population dynamics. Moreover, large species often have long life cycles, and long-lived populations cannot rebound as quickly as can shorter-lived ones. Yet fishing has shifted many marine communities toward shorter-lived species.
Community structure is also altered by the practice of targeting organisms at lower and lower trophic levels as higher levels become depleted. Such a progression, called "fishing down food webs," is exemplified by the shift from harvesting large groundfish, such as Atlantic cod (Gadus morhua), in the Gulf of Maine, to the current reliance on herring, lobsters, sea urchins and shrimp. Work by Daniel Pauly from the University of British Columbia and others has demonstrated that the trophic level of fish landings worldwide has declined in recent decades, particularly in the Northern Hemisphere. (See "Fishing Down Aquatic Food Webs," January–February 2000.)
Many of fishing's effects on ecosystems are indirect. Trophic cascades, for example, are changes in biomass propagated across three or more trophic levels, as when depletion of a predator allows populations of prey to increase, which in turn suppresses the prey's own prey. If the cascade reaches primary producers, the ecosystem may become less productive. For example, the massive overfishing of coastal Atlantic cod and other predatory groundfish increased the abundance of the sea urchin Strongylocentrotus droebachiensis, which transformed forests of the kelp Laminaria, a primary producer and important habitat for young cod, to ecosystems dominated by coralline algae. Similarly, overhunting of sea otters in the Aleutian Islands allowed sea urchin populations to expand; the urchins then severely overgrazed kelp. Overharvesting played a key role in major shifts of entire communities in both these examples, as in the shift from corals to algae on Jamaica's reefs.
In most cases, we do not know how alterations in community composition affect ecosystem function, although they are generally believed to decrease stability. Because marine systems are immense, variable and affected by many abiotic and biological forces, they are inherently difficult to study. Qualities such as species diversity are easily measured but not explicitly linked to community function. More relevant qualities, such as stability (the ability of a community to maintain a state) and resilience (the ability of a community to return to a state after perturbation), are nearly impossible to assess in natural communities. Furthermore, communities are dynamic—some species are seasonal because they migrate, many populations naturally fluctuate in size, and all communities are at the mercy of natural disturbances, such as hurricanes. To compound these problems, we lack the comparisons in time or space that would allow us to quantify the effects of fishing or other disturbances. This lack of perspective severely limits our ability to predict how communities will respond to human pressures.
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