but become sexually mature when much older (e.g., bluefin tuna, some cod populations) are particularly vulnerable to overfishing.
This approach is not always as successful, as described above. For example, Pacific ocean perch, severely depleted by fishing in the 1960s, supported almost no directed fisheries in the 1970s and 1980s. Their stocks were considered to be rebuilt only in the mid-1990s (NMFS 1996b, North Pacific Fishery Management Council 1997). This is not a complete surprise, as Pacific ocean perch are very long lived, but, even so, 30 years is a long time to wait for positive results. Pacific sardine populations declined drastically off the U.S. west coast in the early 1950s and were unmeasurably low by the 1970s despite essentially zero landings from about 1960. Only after the late 1980s did their populations begin to recover, and they are still low (NMFS 1996a). However, as described in Chapter 3, many small pelagic marine species like sardines and anchovies are subject to large, environmentally influenced fluctuations, so cause-effect relationships are not clear in this case. In general, a large reduction of fishing effort is a biologically effective method of conserving or rebuilding many marine fish populations, however disruptive it might be socioeconomically.
Although it is often effective, a conservative single-species approach alone is probably insufficient to sustain fisheries or ecosystems at acceptable levels of productivity. One reason is that it, like many other approaches, is difficult to implement and enforce. A considerable amount of political energy was needed to implement moratoria on striped-bass fishing; the International Pacific Halibut Commission, which manages Pacific halibut, was established by international treaty. More important, however, is that continued adverse ecosystem effects can accrue even when the target species is not depleted (see Chapter 3).
Although single-species management can be effective for maintaining population levels of individual species (e.g., Bering Sea groundfish and striped bass in the Chesapeake Bay), other organisms in the ecosystems may be affected through bycatch and trophic interactions. For example, current fisheries in the Bering Sea apparently are stable under single-species management, although earlier fisheries coupled with changes in atmospheric and oceanic circulation patterns probably contributed to declines in marine mammals and birds (NRC 1996a). Nonetheless, universal application of conservative management on a single-species basis would go a long way toward reducing overexploitation of the world's marine fisheries.
Reducing bycatch and discards is clearly a high priority for management and has been made a specific goal in recent national policies and international agreements. The matter has been addressed recently by the U.S. Congress in the revised Magnuson-Stevens Fishery Conservation and Management Act (see pp 78–80). The National Marine Fisheries Service has drafted a national bycatch plan (NMFS 1998). These and other efforts appear to have produced results.