Canadian fisheries, which encompass the Atlantic, Pacific and Arctic Oceans, as well as the world's largest freshwater system, are both economically and culturally important to Canada. Within each region, commercial, recreational and subsistence fisheries play a significant, though varying role. Shellfish are currently the most valuable commercial catch; salmon is a vital component of subsistence and recreational fisheries; and aquaculture is one of the fastest-growing food production activities in the country. Considerable shifts have been observed in marine ecosystems over recent decades, and much of the recent research has been dedicated to assessing the role of climate in these changes.

Climate change is expected to have significant impacts on fish populations and sustainable harvests. Fish have a distinct set of environmental conditions under which they experience optimal growth, reproduction and survival. As conditions change in response to a changing climate, fish would be impacted both directly and indirectly. Impacts would stem primarily from changes in water temperature, water levels, ice cover, extreme events, diseases and shifts in predator- prey dynamics. The key concerns for fisheries vary in different regions of the country.

Along the Pacific coast, drastic declines in the salmon catch during the 1980s and 1990s, as well as the importance of salmon to west coast fisheries, have resulted in research being focused primarily on salmon. Temperature changes affect salmon directly, through impacts on growth, survival and reproduction, as well as indirectly, through effects on predator-prey dynamics and habitat. Changes in river flows and extreme climate events have also been shown to affect salmon survival and production.

Marine ecosystems along the Atlantic coast also experienced significant changes in the 1990s, with shellfish replacing groundfish as the most valuable catch. Although this shift was driven primarily by fishing practices, climatic changes likely played a role. Future warming trends may impact the shellfish populations on which the region now relies. For example, water temperature has been shown to have a strong influence on snow crab reproduction and distribution. There is also concern that the frequency and intensity of toxic algal blooms, which can cause shellfish poisoning, may increase. Other important issues for the Atlantic region include the effects of climate change on salmon and aquaculture operations.

The most significant impacts of future climate change on Arctic marine ecosystems are expected to result from changes in sea-ice cover. A decrease in sea-ice cover would affect marine productivity, fish distribution and fishing practices (e.g., accessibility to sites, safety), as well as marine mammals. In fact, there is growing evidence that climate change has already begun to affect fisheries and marine mammals along the Arctic coast. For example, declines in polar bear condition and births in the western Hudson Bay region have been associated with warmer temperatures and earlier ice break-up, while capture of types of salmon outside of known species ranges may be early evidence that distributions are shifting. The opening of the Northwest Passage to international shipping would also affect Arctic fisheries, through the increase in traffic, pollution and noise in the region.

Key climate change impacts for freshwater fisheries are expected to result from higher water temperatures, lower water levels, shifts in seasonal ice cover and the invasion of new and exotic species. Overall, some fish (e.g., warm-water species) would likely benefit, while others (e.g., cold-water species) would suffer. For example, higher water temperatures have been shown to decrease the growth rate and survival of rainbow trout, yet increase the population sizes of lake sturgeon. Northward migration of fish species and local extinctions are expected, and would lead to changes in sustainable harvests (Figure 5). Higher temperatures and lower water levels would also exacerbate water quality problems, which would increase fish contamination and impair fish health.

There is growing awareness of the need to anticipate and prepare for climate change in the fisheries sector. One challenge for the fishing industry would be to adjust policies and practices in an appropriate and timely manner to deal with shifts in fish distribution and relative abundance. Recommendations for adaptation include monitoring for changes; enhancing the adaptive capacity of fish species by reducing non-climatic stresses and maintaining genetic diversity; and improving research and communication. Careful consideration of the role of regulatory regimes and programs in facilitating or constraining adaptation is also important.

Figure 5: Relative changes in maximum sustained yield of walleye in Ontario under a 2xCO2 climate change scenario. Note the general decrease in maximum sustained yield in the southern part of the province, and the increase in the central and northern regions. Courtesy of Fisheries and Oceans Canada.