15.2.3.3. Marine Fisheries

Climate-related variations in marine/coastal environments are now recognized as playing an important role in determining the productivity of several North American fisheries. For example, large changes in species abundance and ecosystem dynamics off the coast of California have been associated with changes in sea-surface temperatures (SSTs), nutrient supply, and circulation dynamics (Ebbesmeyer et al., 1991; Roemmich and McGowan, 1995; Bakun, 1996). Similar relationships have been observed in the Bering Sea, the northeastern Pacific, and the Gulf of Alaska (Polovina et al., 1995; Ware, 1995; Shuntov et al., 1996; Beamish et al., 1997; Downton and Miller, 1998; Francis et al., 1998; Beamish et al., 1999a, 2000) and in the North Atlantic (Atkinson et al., 1997; Sinclair et al., 1997; Hofmann and Powell, 1998). In the Gulf of Mexico, variations in freshwater discharge affect harvests of some commercially important species (Hofmann and Powell, 1998). Projected climate changes have the potential to affect coastal and marine ecosystems through changes in coastal habitats, upwelling, temperature, salinity, and current regimes. Such changes may affect the abundance and spatial distribution of species that are important to commercial and recreational fisheries (Boesch et al., 2000).

Fishery management involves the difficult task of maintaining viable fish populations in the presence of difficult-to-predict shifts in resource availability, while regulating competition among harvesters for access to publicly managed, common-property fishery resources (McKay, 1995; Fujita et al., 1998; Myers and Mertz, 1998; Roughgarden, 1998). Attainment of management objectives may be confounded by the fact that some fish stocks tend to fluctuate widely from year to year. These fluctuations may arise from natural causes that are unrelated to fishing pressure or be exacerbated by harvesting. The exact cause of a sudden shift in abundance often is poorly understood. Climate variations often play a role in natural fluctuations, although their role may be complex and indirect. For example, a climatic variation may affect phytoplankton and zooplankton abundance in some part of the ocean, with cascading effects through a chain of predator-prey relationships (Bakun, 1996). These processes may result in multiple and lagged impacts on the abundance of a harvested species. Because it is difficult to identify and predict such effects, climate variability constitutes a significant source of uncertainty for fishery managers.

The potential impacts of climate change on fish populations are equally difficult to predict. Some work has focused on the direct impacts of warmer temperatures on marine species (e.g., Wood and McDonald, 1997; Welch et al., 1998a,b). However, Bakun (1996) notes that climate variables that are important on land (e.g., temperature and precipitation) may be relatively unimportant for organisms that live in the ocean. He identifies three basic processes (enrichment, concentration, and transport /retention) that influence the productivity and spatial distribution of marine fish populations but notes that very little is known regarding how these will change with global climate change.

Efforts to assess the impacts of climate change on the U.S. fishery sector are severely hampered by our current lack of understanding of possible changes in fish populations. Markowski et al. (1999) performed a sensitivity analysis that examined the potential economic impacts of hypothetical changes in the abundance of selected fish populations, but the analysis is too hypothetical for use here.

Uncertainty regarding the magnitude and sources of variations in fish stocks also creates political stumbling blocks to effective fisheries management. Within single jurisdictions, competing harvesters and gear groups vie for shares of a "pie" whose dimensions are imperfectly known. In the case of international fisheries, cooperative harvesting agreements often have degenerated into mutually destructive fish wars when expectations have been upset by unforeseen changes in abundance or the spatial pattern of availability (McKelvey, 1997). For example, the Pacific Salmon Treaty foundered for several years because declining runs of southern coho and chinook salmon and increasing salmon abundance in Alaskan waters frustrated efforts to achieve a mutually acceptable balance of U.S. and Canadian interceptions of one another's salmon stocks (Munro et al., 1998; Miller, 2000a).

Accounts of the collapse of cod stocks off Newfoundland on Canada's east coast have cited the inability of governments to effectively control fishing pressure and a natural shift to less favorable environmental conditions (Hutchings and Myers, 1994; Sinclair et al., 1997; Hofmann and Powell, 1998). This case suggests that sustainable fisheries management will require timely and accurate scientific information on the environmental conditions that affect fish stocks and institutional flexibility to respond quickly to such information.

The western U.S.-Mexican border region is located between subtropical and mid-latitude ocean regions. Variations in temperature in this transition zone result in major fluctuations in fisheries productivity (Lluch et al., 1991). In recent decades, this region of the Pacific has shown a trend toward warming and changes in regional productivity, independent of overexploitation. In a global warming scenario, the sardine population may decrease along the U.S.-Mexican Pacific Ocean border region, whereas the shrimp population may increase. Interdecadal natural climate variability, however, appears to be the most important sardine population modulator (Lluch-Cota et al., 1997).

Available evidence suggests that there are likely to be impacts on fisheries arising, for example, from changes in current dynamics, temperature-dependent distribution, and food web dynamics. These impacts will be variable across species and locations and are difficult to forecast with any precision. Because the effects of exploitation and environmental change can be synergistic, it will be increasingly important to consider changing environmental conditions in future fisheries management (Boesch et al., 2000; see Chapter 6 for further discussion).

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