Detailed analyses of fish physiological response to water temperature have shown that the potential impact of climate change on freshwater and marine fish is large (Wood and McDonald, 1997). Unfortunately, current knowledge appears to be limited mostly to single key species, abstracted from the wider ecosystem context that supports fisheries production. It is likely that extrapolation from biological first principles will provide only limited foresight at a fisheries level in that context. However, it is likely that in the short term, fish will move to new habitats to find conditions to which they have adapted.
Two first-order effects-changes in biodiversity and changes in fisheries and aquaculture productivity-are examined. For each of these, key impacts in the European region are discussed with respect to temperature rise and other factors that are linked to climate change. For marine systems, these key factors are sea-level rise and changes in ice cover, salinity, and ocean currents; for inland waters, factors such as hydrological changes (e.g., dams, water abstraction), hydrochemical changes (including anoxia, water acidity, pollution and toxicity events), and eutrophication are key.
It has been possible, for the better-known species, to obtain a rough first picture of likely faunal movements and range shifts that would result from the temperature rise currently forecast for the European region.
This has been done, for example, for wild Atlantic salmon (Salmo salar), which sustains important recreational and commercial fisheries over most of northern Europe and has a high conservation indicator value over its entire range. Atlantic salmon spends its early and juvenile life in freshwater slowly moving away from the headwater spawning grounds in rivers out to sea, to grow and mature. Looking at the direct and indirect influence of temperature on protein synthesis on all life-cycle stages, McCarthy and Houlihan (1997) suggest that there will be a northward shift in the geographic distribution of Atlantic salmon in Europe, with likely local extinction at the southern edge of the current range and new habitats colonized in the north. The influence of temperature on overwintering survival of 1- and 2-year-old salmon and the distribution of post-smolts in the North Sea area already is apparent (Friedland et al., 1998).
High sensitivity to water temperature of fish larval and juvenile stages, combined with the higher susceptibility of headwaters and smaller rivers to air temperature rise, implies important effects of climate change on cold and temperate anadromous species such as the sea trout, alewife (Alosa alosa Atlantic and Mediterranean), and sturgeon (Atlantic, Black Sea, Caspian Sea). Similar impacts are likely for all salmonid species in Europe, including those that do not migrate to sea. Fish species are likely to extend their range northward (e.g., Brander, 1997). To identify impacts on recreational fisheries and conservation efforts that can be attributed to climate change, changes in local species presence will need to be assessed at a pan-European level, beyond watershed and national levels.
Global environmental change and introduction of species make it difficult to identify impacts of climate change in freshwater systems. Direct evidence of species shift and long-range movement often is physically limited between watersheds; introductions of alien species and genotypes-either voluntary or accidentally-have been very extensive. The influence of global warming on marine fish species and migratory species that reproduce and spend their early life history in marine waters (catadromous fish-e.g., eel) is more complex to foresee because of the various spatial and long temporal scales involved, as well as the feedback loop between air and ocean temperatures.
Some insight may be gained by reviewing historical shifts in the geographical distribution of European fish species to gain a better estimation of how fast and how far species may change their distributions. For example, the period of warming between 1920 and 1940 resulted in widespread changes in the distribution of terrestrial and marine species from Greenland to the Barents Sea.
Future impacts on aquatic systems productivity from climate change are mostly uncertain because of other, related or independent, pressures. Resource overexploitation appears to be the single most important factor directly threatening the sustainability of many commercial fisheries in Organisation for Economic Cooperation and Development (OECD) countries (OECD, 1997). Overexploitation increases the vulnerability of fisheries to climate variability because so few fish are left in the stock to grow and multiply in a year of poor recruitment. The North Sea cod fishing industry, for example, now relies on only 1- or 2-year classes (Cook et al.,1997) and therefore is vulnerable to a year or two of poor recruitment caused by adverse climatic conditions.
Productivity of some fish stocks may benefit from warming trends. For example, recruitment of cod in the Barents Sea is higher in warm years. This probably comes about as a result of indirect effects (on capelin and zooplankton) as much as through direct effects.
Cod in the North Sea are at the warm end of their thermal range, and their recruitment therefore seems to benefit during cold periods, such as the 1960s and 1970s (Dippner, 1997; Brander, 2000). The temperature and wind regime of the North Sea are strongly influenced by the NAO, records of which go back more than a century. Some of the variability in catches of North Sea cod may be explained by trends in the NAO, but changes in exploitation are a major factor.
Other factors are likely to combine with changes in temperature and decrease fish and shellfish productivity. Chronic levels of pollution are known to reduce marine and freshwater fish fecundity (Kime, 1995), decrease freshwater supply (which exacerbates low dissolved-oxygen concentrations), increase solid transport from erosion, and increase habitat fragmentation in inland waters. Development of marine aquaculture may be slowed by a decreasing availability of sites with cool enough surface water temperature and by increased susceptibility to disease.
The consequences of fisheries collapse are complex. For example, the North Sea herring fishery collapsed in 1977 and was closed for 5 years. Although the rapid recovery of the resource surprised fisheries biologists, the most dramatic effect on the industry resulted from a permanent change of consumer preferences (Bailey and Steele, 1992) away from kippers and fresh and pickled herring. The fishery recovered, but the market did not.
Finally, the dramatic effect of climate change on fisheries production is well documented (Durand, 1998) for the world's four main upwelling systems: the California current, the Peru current, the Canary current (off northwest Africa and southern Spain and Portugal), and the Benguela current (off the Atlantic coast of southern Africa). Although these currents are mostly outside European waters, the effects on Europe's distant water fishing fleet, fishing sector investments, and import prices for human consumption and aquaculture feed cannot be ignored.
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