(based on FAO data) that from 1980 to 1990 the number of overexploited fisheries increased by 250 percent, whereas the number of underexploited fisheries decreased by about 75 percent. Depleted species are being replaced in today's catches by species that were less heavily fished in the past.
For example, the Chilean Inca scad (Trachurus murphyi), Japanese pilchard (Sardinops sagax melanosticus), South American pilchard or Chilean sardine (Sardinops sagax sagax), and skipjack tuna (Katsuwonus pelamis) replaced chub mackerel (Scomber japonicus), Atlantic mackerel (Scomber scombrus), Atlantic cutlassfish (Trichiurus lepturus), and saithe (Atlantic pollock, Pollachius virens) in the top-10 species list between 1973 and 1993. In the United States, skates and dogfish have replaced more commercially valuable fish on Georges Bank. This process of depletion of one resource and replacement by another is limited by the number of potentially catchable and usable species; depleted species may not return to previous abundance levels. For example, Atlantic halibut (Hipploglossus hippoglossus) and spring-spawning Icelandic herring (Clupea harengus) have not recovered from overfishing, although they probably would if mortality were reduced (Myers et al. 1997). Many shark populations appear to be declining as well. For instance, Van der Elst (1979) described the impact of South African antishark nets on local populations of oceanic sharks, and the resultant and unanticipated decline in nearshore bony fishes. On a global scale, Manire and Gruber (1990) concluded that sharks were overfished by approximately 30 percent per year in U.S. waters; and cited domestic demand for shark meat; wasteful fisheries practices, especially discarded bycatch of sharks; irrational dread; and an increasing global demand for shark fins as major factors contributing to excess fishing mortality of sharks.
In addition, unexploited fish populations that are long lived and slow growing cannot support high exploitation rates, unlike populations of faster-growing, short-lived species. For example, the five species of the genus Sebastes, including the Pacific Ocean perch itself (S. alutus) and the northern (S. polyspinus), rougheye (S. aleutianus), sharpchin (S. zacentrus), and shortraker (S. borealis) rockfishes off the northwestern and Alaskan coasts of the United States and the coast of British Columbia, are all slow-growing and were severely overfished, although they have now largely recovered (NPFMC 1997, NMFS 1996b). The marbled rockcod (Notothenia rossi) in the Southern Ocean also has been severely overfished (Kock 1992).
Approximately 3.5 million vessels are engaged in fisheries worldwide; about two-thirds are small undocked vessels (FAO 1995b), but the total also includes about 24,000 high-seas fishing vessels of more than 500 gross tons (NMFS 1993). The gross tonnage of the world's fishing fleets (decked vessels only) increased by an average of 2.9 percent annually from 1970 to 1992. This rate of