biomass in the early and mid-1990s;3 the recommended exploitation levels based on optimal sustained yield were 13 and 22 percent, respectively. Spawning-stock biomasses were also very low, about 10 percent or less of their values in the 1950s for haddock, cod, and yellowtail flounder.

The depleted groundfish populations have largely been replaced by populations of small elasmobranchs, mainly spiny dogfish (Squalus acanthias), smooth dogfish (Mustelis canis), and skates (Raja spp.). Other changes in the pelagic fishes included initial declines of mackerel and herring and concomitant increases in American sand lance (Ammodytes americanus), their prey. Recently, fishing pressure has resulted in declines in the elasmobranch populations—the species are mostly long lived and slow growing compared to many commercially important teleosts—and mackerel and herring populations have increased as well. This has been accompanied by a decline in sand lance populations.

This example makes the ecosystem consequences of fishing clear: the groundfish community first became dominated by small sharks (e.g., dogfish) and rays, and then overfishing reduced those populations. It is not yet known whether this ecosystem will recover to its preexploitation structure in the absence of fishing or whether it will attain some other composition.

The Bering Sea

Recent declines of many populations of marine mammals and birds that live in and near the Bering Sea, a semienclosed basin of the North Pacific Ocean between Alaska and Russia, have attracted attention and have been attributed by many to the effects of fishing. The National Research Council recently reviewed the information (NRC 1996a) and concluded that fishing probably has affected the ecosystem but in a more complicated fashion than simple overfishing and in combination with environmental changes. Documented changes include changes in abundances of many fish species and changes in the physical environment. There also is persuasive (although not conclusive) evidence that marine mammals and birds are declining because the juveniles are short of food.

The NRC report concluded that the changes in the Bering Sea ecosystem were probably caused by a combination of changes in the physical environment coupled with heavy exploitation of components of the system (whales and fishes). Many sperm and baleen whales were removed from the Bering Sea and adjacent waters in the 1950s, 1960s, and 1970s. Various flatfish species, Pacific Ocean perch, and herring were also heavily fished in that period, with resulting population declines. Many of those species feed heavily on zooplankton and thus compete with walleye pollock; others of those species prey on pollock. In the late

3  

The exploitation rates are much lower following recent action by the New England Fishery Management Council (NEFMC; see NRC 1998b and the Northeast Multispecies Fishery Management Plan and amendments at NEFMC's web site, www1.shore.net/˜nefmc/.



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