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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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