In the 1950s, Jamaica’s coral reefswere thriving, with branching formsof staghorn and elkhorn corals rulingthe shallows and plate-like corals dom-inating deeper waters. The reefs werepopular habitats for large predatoryfish such as sharks, snappers, groupersand jacks, and Jamaicans came to thereefs to trap these species.The scene looked much the same inthe 1970s—except that the large fishhad disappeared onto dinner plates.Struggling to feed the island’s swellingpopulation, Jamaicans began usingmotorized canoes to improve their effi-ciency in trapping smaller, herbivorousfish, such as surgeonfish and parrot-fish. The reefs still looked healthy,however, and sea urchins were doingwell. As herbivorous fish were re-moved, the urchins no longer had tocompete with them for the mainstay oftheir diet: algae. By the 1990s, Jamaica’s reefs weredepleted of both carnivorous and her-bivorous fish and smothered withfleshy algae, such as Sargassum. Seaurchins were uncommon, and the vastdiversity of the reefs was reduced. Be-cause the algae had taken over the realestate, there was little space for renew-al by larval corals. Back in the late1970s, surveys off the northern coast-line had found corals over half thereefs’ surface, with fleshy algae cover-ing only 4 percent. By the early 1990s,Terry Hughes from James Cook Uni-versity demonstrated that those figureswere more than reversed: 3 percent ofthe space occupied by corals, 92 per-cent by algae. Thus reefs that had exist-ed for thousands of years as one of themost diverse habitats on earth hadchanged into algal mats in a fewdecades.What tipped the balance? Most sci-entists point to overfishing, althoughpollution and natural disasters certain-ly also took their toll. (See “Mud, Ma-rine Snow and Coral Reefs,” Janu-ary–February.) When carnivorous fishbecame scarce, populations of one oftheir prey species, the sea urchin Diade-ma antillarum, exploded. These black,spiny creatures flourished even morewith the demise of their competitors,the herbivorous fish. But in 1983, amysterious pathogen sickened theurchins, reducing their number by 99percent. With few herbivorous fish andnow few urchins, the algae were freeto grow unchecked.In the past decade, Jamaican reefshave begun to revive—corals now cov-er about 10 percent of the surface. Butit is unclear whether they will regaintheir dominance or whether the algaewill continue to hold sway.This example makes an importantpoint: Harvesting a few types of fishaffects not only those species but dis-turbs whole ecosystems, which in turncan reduce target species. We shall ar-gue in this article that, although theprimary goal of “sustainable fisheries”is to preserve the long-term viability oftarget species, even harvest levels con-sidered sustainable can impact marineecosystems. Protection of the world’soceans will in the future require abroader and more integrated scientificview than one that focuses on one or afew species.Marine ResourcesA spectacular diversity of habitats liesin the world’s oceans, which cover 361million square kilometers and constitutemore than 99 percent of the biosphere’svolume. Some habitats, such as coralreefs, seagrass meadows and kelpforests, are relatively well known. Oth-ers, such as deep-sea plains and vastopen waters, are alien to most people. We are only beginning to appreciatethe depth and breadth of marine diver-sity, but it surely rivals that on land.Coral reefs, for instance, occupy only0.1 percent of the earth’s surface, yetthey may support as many as 9 millionspecies. There also could be millions ofspecies in the deep sea, once thought tobe a vast desert. At higher taxonomiclevels, marine systems are much morediverse than those on land. Of the 34recognized animal phyla, 33 can befound in the ocean, and 15—includingcomb jellies, echinoderms and lamp-shells—are exclusively marine. Be-cause certain marine groups, especiallybottom-dwelling invertebrates, arepoorly known, enormous diversity re-mains to be discovered. For instance,20 new families, 100 new genera and200 new species were recently found inor near hydrothermal vents.Humans have exploited marineecosystems for at least 10,000 years, butoverharvesting was not a significant is-sue when human populations were150 American Scientist, Volume 91Ecologically Sustainable YieldMarine conservation requires a new ecosystem-based concept for fisheriesmanagement that looks beyond sustainable yield for individual fish species Richard W. Zabel, Chris J. Harvey, Stephen L. Katz, Thomas P. Good and Phillip S. LevinThe authors are members of the Science for Ecosys-tem-based Management Initiative at the North-west Fisheries Science Center of the National Ma-rine Fisheries Service in Seattle. Richard W. Zabelreceived his Ph.D. in quantitative ecology and re-source management from the University of Wash-ington in 1994. Chris J. Harvey received his Ph.D.in limnology and marine science from the Univer-sity of Wisconsin in 2001. Steven L. Katz receivedhis Ph.D.in zoology from the University of BritishColumbia in 1991. Thomas P. Good received hisPh.D. in systematics and ecology from the Univer-sity of Kansas in 1998. Phillip S. Levin receivedhis Ph.D. in zoology from the University of NewHampshire in 1993. Address for Zabel: NationalMarine Fisheries Service, Northwest Fisheries Sci-ence Center, 2725 Montlake Blvd. E, Seattle, WA98112. Internet for Zabel: [email protected] and fishing gear was mostly linesand hooks. Nonetheless, the firstknown anthropogenically caused col-lapse of a marine stock occurred about3,000 years ago, when peoples on thePeruvian coast continued to harvestshellfish that had been depleted by anatural disturbance. With the techno-logical advances of the Industrial Rev-olution, such as the internal combus-tion engine and refrigeration, theability of humans to extract fish fromthe sea, particularly in offshore regions,increased dramatically. The resultingincreased frequency of stock collapsesled to the realization that the sea is notinfinitely bountiful. More recent devel-opments include factory trawlers,which harvest a disproportionate shareof the world’s fish. Commercial fishing now directly killsenormous numbers of fish: More than90 million metric tons are captured an-nually for consumption, and about 30million metric tons are discarded as by-catch. Worldwide, 25 to 30 percent of ex-ploited stocks experience overfishing,despite numerous incidents of stock col-lapse. Moreover,