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Any effects of climate change on fisheries will occur in a sector that already is characterized on a global scale by full utilization, massive overcapacities of usage, and sharp conflicts between fleets and among competing uses of aquatic ecosystems. Climate change impacts are likely to exacerbate existing stresses on fish stocks-notably overfishing, diminishing wetlands and nursery areas (perhaps aggravated by sea-level rise), pollution, and UV-B radiation.
The fishing industry is a significant income earner in Europe, but in recent decades the industry has experienced numerous problems related to fishing rights and quotas, dwindling stocks due to overfishing, and disputes as part of the European fishing fleet encroaches upon areas traditionally exploited by other nations (e.g., eastern Canada, the western African coast). Evidence in support of the view that environmental changes drive many changes in fish stocks has been accumulating in recent years (Mann and Lazier, 1991; Mann, 1992, 1993; Mann and Drinkwater, 1994; Polovina et al., 1995). The question of whether overfishing, environmental change, or a combination of the two is responsible for major declines in fish stocks is still a matter for debate and is situation-specific.
About 70% of global fish resources depend on near-shore or estuarine habitats at some point in their life cycle (IPCC, 1990; Chambers, 1991). The growing rate of human occupation (with associated pollution) and the high property values of littoral areas, especially in western countries, will severely constrain the inland displacement of wetlands and other habitats as sea level rises. Fish production will suffer when wetlands and other habitats that serve as nurseries are lost (Costa et al., 1994). Fish habitats are downstream of many impacts, and fish integrate the effects. Fish are symbolic of the health of ecosystems and our ability to manage our resources.
Growing demands for fish, water, and space; encroachment by large-scale fishing and aquaculture operations; population concentrations; urban expansion; pollution; and tourism already have harmed small-scale fishing communities in shallow marine waters, lakes, and rivers. Persons engaged in this economic sector have limited occupational or geographic mobility. With climate change, global and regional problems of disparity between catching power and the abundance of fish stocks will worsen-particularly with regard to the interaction between large mobile fleets and localized fishing communities. Aquaculture will develop in new areas-sometimes assisting, sometimes disrupting existing traditional fishing techniques.
Many studies have related historical changes in the abundance and distribution of aquatic organisms to climate changes. These studies reveal that relatively small changes in climate often produce dramatic changes in the abundance of species-sometimes of many orders of magnitude-because of impacts on water masses and hydrodynamics (e.g., Sharp and Csirke, 1983; Beukema et al., 1990; Kawasaki et al., 1991). The impacts of warming can be inferred from past displacements of transition zones-for example, the Russell cycle in the western English Channel (Southward, 1980) and simultaneous discontinuities in the local occurrence of pilchard and herring (Cushing, 1957, 1982). Opposing fluctuations in these fisheries in the Channel and the Bay of Biscay have followed long-term changes in climate for three centuries (Binet, 1988b).
A general poleward extension of habitats and range of species is likely, but an extension toward the Equator may occur in eastern boundary currents. For example, the range of Sardina pilchardus prior to World War II was from south Brittany to Morocco. In warm years following the war, sardines were fished up to the North Sea. During the 1970s, new fisheries developed off the Sahara and Mauritania, with small amounts landed as far south as Senegal-perhaps as a consequence of upwelling and ecological processes related to tradewind acceleration (Binet, 1988a). Changes in the circulation pattern are likely to induce changes in the larval advection/retention rates in and out of favorable areas and may explain changes in abundance and distribution (Binet, 1988a; Binet and Marchal, 1992). Areal overlaps in closely related species may change in unpredictable directions (Ntiba and Harding, 1993).
A poleward movement of species in response to climate warming is predictable on intuitive grounds (Shuter and Post, 1990). Habitat, food supply, predators, pathogens, and competitors, however, also constrain the distributions of species. Furthermore, there must be a suitable dispersion route, not blocked by land or some property of the water such as temperature, salinity, structure, currents, or oxygen availability. Movement of animals without a natural dispersal path may require human intervention; in the absence of intervention, such movement may take hundreds or thousands of years (Kennedy, 1990).
As stresses intensify, impacts that for a long time were limited to freshwaters and littoral areas now are observed in closed and semi-enclosed seas (FAO, 1989; Caddy, 1993). Some semi-enclosed seas-such as the Mediterranean, Black, Aegean, and northern Adriatic-already are eutrophic. The diversity of uses in these areas also has introduced some of the most pervasive and damaging anthropogenic impacts on the world's ecosystems in the form of the spread of nonindigenous species (Mills et al., 1994).
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