Ecologically Sustainable Yield
Marine conservation requires a new ecosystem-based concept for fisheries management that looks beyond sustainable yield for individual fish species
In the 1950s, Jamaica's coral reefs were thriving, with branching forms of staghorn and elkhorn corals ruling the shallows and plate-like corals dominating deeper waters. The reefs were popular habitats for large predatory fish such as sharks, snappers, groupers and jacks, and Jamaicans came to the reefs to trap these species.
The scene looked much the same in the 1970s—except that the large fish had disappeared onto dinner plates. Struggling to feed the island's swelling population, Jamaicans began using motorized canoes to improve their efficiency in trapping smaller, herbivorous fish, such as surgeonfish and parrotfish. The reefs still looked healthy, however, and sea urchins were doing well. As herbivorous fish were removed, the urchins no longer had to compete with them for the mainstay of their diet: algae.
By the 1990s, Jamaica's reefs were depleted of both carnivorous and herbivorous fish and smothered with fleshy algae, such as Sargassum. Sea urchins were uncommon, and the vast diversity of the reefs was reduced. Because the algae had taken over the real estate, there was little space for renewal by larval corals. Back in the late 1970s, surveys off the northern coastline had found corals over half the reefs' surface, with fleshy algae covering only 4 percent. By the early 1990s, Terry Hughes from James Cook University demonstrated that those figures were more than reversed: 3 percent of the space occupied by corals, 92 percent by algae. Thus reefs that had existed for thousands of years as one of the most diverse habitats on earth had changed into algal mats in a few decades.
What tipped the balance? Most scientists point to overfishing, although pollution and natural disasters certainly also took their toll. (See "Mud, Marine Snow and Coral Reefs," January–February.) When carnivorous fish became scarce, populations of one of their prey species, the sea urchin Diadema antillarum, exploded. These black, spiny creatures flourished even more with the demise of their competitors, the herbivorous fish. But in 1983, a mysterious pathogen sickened the urchins, reducing their number by 99 percent. With few herbivorous fish and now few urchins, the algae were free to grow unchecked.
In the past decade, Jamaican reefs have begun to revive—corals now cover about 10 percent of the surface. But it is unclear whether they will regain their dominance or whether the algae will continue to hold sway.
This example makes an important point: Harvesting a few types of fish affects not only those species but disturbs whole ecosystems, which in turn can reduce target species. We shall argue in this article that, although the primary goal of "sustainable fisheries" is to preserve the long-term viability of target species, even harvest levels considered sustainable can impact marine ecosystems. Protection of the world's oceans will in the future require a broader and more integrated scientific view than one that focuses on one or a few species.