The basic limiting factors for fish production in polar and subpolar regions are light and temperature. Warming in high latitudes should lead to longer growing periods, increased growth rates, and ultimately, perhaps, increases in the general productivity of these regions (IPCC 1996, WG II, Section 188.8.131.52). On the other hand, the probability of nutrient loss resulting from reduced deep-water exchange could result in reduced productivity in the long term. Again, this complexity highlights the importance of changes in temperature for patterns of circulation. Global warming could have especially strong impacts on the regions of oceanic subpolar fronts, where temperature increases in deep water could lead to a substantial redistribution of pelagic and benthic communities, including commercially important fish species (IPCC 1996, WG II, Section 8.3.2).
In polar regions, the number of dominant fish species is small; many species of low abundance are typical of tropical regions, with the exception of upwelling areas. Only 15-20 commercially important species in the Arctic or Antarctic Oceans are recorded, whereas the numbers increase to about 50 and 16-450 in the boreal and tropic areas, respectively (Laevastu, et al., 1996). The poleward distribution of fish due to climate warming generally expands fishing areas. This expansion might produce better yields of fish production. In the higher latitudes, however, spawning grounds of cold-water species that are very sensitive to the temperature change might be destroyed by changes in water properties.
In some cases, fisheries on the margin of profitability could prosper or decline. For example, if there is a retreat of sea ice in Antarctica, the krill fishery-which is regulated by the current ice-free period-could become more attractive to nations not already involved (IPCC 1996, WG II, Section 184.108.40.206). Fishery statistics may be more valuable for the analyses of interannual and long-term fluctuations of marine populations than was previously thought. Time series of catch-per-unit-effort (CPUE) statistics from the commercial krill fishery operating around South Georgia during 1973-1993 have been used for considering the hypothesis that fluctuations in the abundance of krill in the Scotia Sea area are related to environmental changes. A consistent correlation has been found between the various CPUE indices and ice-edge positions: The further south the ice-edge occurred during the winter-spring season, the lower the CPUE values in the following fishing season. The most extreme expression of this relationship was the lack of a krill fishery in 1978 and 1984, when the ice did not extend far north during the previous winter. By contrast, in 1978 and 1984 the March ice-edge reached its northern limit at 50ŮS, preceding high CPUE values in 1979 and 1985. A consistent relationship also exists between CPUE and water temperature. Warm-water temperature in the South Georgia shelf area in January-February corresponded to lower CPUE values in the same year. There also is significant correlation between air transport in late spring and CPUE in the next year. For example, a prevalence of southerly meridional air transport precedes high CPUE values (Fedoulov et al., 1996). It must be emphasized, however, that the physical regimes of the sub-Antarctic region in the vicinity of South Georgia are very complicated, and this model may not be applicable to the entire Antarctic.
Fedoulov et al. (1996) proposed the following mechanism as a hypothesis to explain how ice, ocean, and atmospheric components of the Southern Ocean affect krill distribution. Krill usually are more abundant in the southern Scotia Sea along the Weddell Scotia Confluence (WSC), so it is likely that the currents play a key role in krill transport to South Georgia from the Antarctic Peninsula. The WSC zone extends northward in the eastern Scotia Sea, and this colder water penetrates along the southeastern shelf of South Georgia. The position of the WSC is thought to be determined by the intensity of the Weddell gyre, which in turn is driven by the formation of dense and cold Weddell water. The main factor in the creation of the cold Weddell water is increased salinity resulting from ice formation. Hence, the dominance of a warm or cold year reflects the intensity of the Weddell gyre and consequently the general position of the WSC. It is reasonable to suppose that ice can start to influence krill distribution when it is close to or covers the area of the WSC. Ice cover modifies the mechanism of drift current formation and creates favorable (northern ice-edge position) or unfavorable (southern ice-edge position) conditions for krill transport to South Georgia.
In a recent study, Loeb et al. (1997) are documenting a more complex relationship between krill and salpa, a pelagic tunicate. In essence, extensive seasonal ice cover promotes early krill spawning, inhibits population blooms of pelagic salps, and favors the survival of krill larvae through their first winter. Salpa blooms affect adult krill reproduction and the survival of krill larvae. If a decrease in the frequency of winters with extensive sea-ice development accompanies the warming trend in the Antarctic Peninsula area, the frequency of krill recruitment failures would be expected to increase, and the krill population would decline. An increase in salpa blooms would further depress krill numbers. This codependency of competing species on changing climate variables has implications for the management of the krill fishery and for populations of vertebrate predators such as penguins, fish, and whales, which depend on krill.
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