Have Peak Fishery Production Levels beenPassed in Continental Shelf Area? Some Perspectives Arising from Historical Trends in Production per Shelf Area

Trends in landings from global shelf areas (excluding the Arctic/Antarctic) for 1950– 94 are expressed per shelf surface of FAO Statistical Areas in four categories: "shelfdependent" species (demersal fish plus commercial benthic invertebrates), "shelf-associated" species (small-medium sized pelagics and neritic squids), and "deep-water" and "oceanic" species; largely focusing on the first two categories. Geographical variation for shelf-dependent resources is explained by dividing FAO Statistical Areas into three categories: Arcto-boreal regions in the northern hemisphere, with five-year mean fishery production peaking at 2.1–2.7 tons per km2; from the Antarctic convergence to south-boreal latitudes, with production peaking at 0.5–2.2 tons per km2, and tropical/subtropical shelves, peaking at 0.4–0.9 tons per km (except for the Central East Atlantic). For zoogeographically similar areas, overall production levels for shelf-dependent resources have generally reached similar peak values and declined recently in areas with long histories of fishing. For several areas, especially in the tropics, production peaked in the last five-year period considered, 1990–94, but further potential for shelf-dependent species is considered limited. A low limit for shelf-dependent resources in the tropics of <1 tons per km2 reflects peak production well inside 200 m, despite local highly productive coastal ecosystems. Mediterranean shelf-dependent production was at tropical/sub-tropical levels in the 1960s but has increased through the 43-year time series to 2.4 tons per km2, similar to north-boreal shelves. As for other inland seas, this increase probably represents, at least in part, nutrient runoff effects on marine production. The upper limit for shelf-associated resources is more variable than for shelf-dependent resources and may be an index of biological productivity. Production of small-medium pelagic fish is considered loosely correlated with shelf-dependent production, and in Arctoboreal regions may be dependent on nutrients from shelf water mixing, while high shelf pelagic production is confined to upwelling regimes. Globally, the recent rate of production increase for oceanic and deep-water species exceeds that for shelf resources, pointing to the recent diversion of effort to offshore areas. The apparent difference in shelf-dependent production in similar North and South temperate areas is suggested to be due to a greater "land effect", roughly estimated at some 1.00–1.5 tons per km2 for comparable levels of production of small pelagics. This difference is postulated as due to increased nutrient runoff and/or larval retention on northern than the more exposed southern hemisphere shelves, and an analogy is drawn between effects of excessive "natural eutrophication" and that due to anthropogenic causes. Although nutrients may constrain local production, resources closely linked to continental shelves are overfished or potentially so, and recovery will only be achievable by better management. One estimate derived from this study suggests that the potential for recovery by improved management of shelf-dependent resources is of the order of 7 million tons globally (not including a reduction in discarding).