Marine aquaculture production more than doubled from 4.96 Mt in 1990 to 11.14 Mt in 1997. Similar trends were exhibited in freshwater aquaculture, with increases from 8.17 Mt in 1990 to 17.13 Mt in 1997, while yields from marine and freshwater fisheries remained relatively constant. The net result was that aquaculture production represented approximately 30% of total fish and shellfish production for human consumption in 1997. Aquaculture production is expected to continue its upward trend in the foreseeable future, although in many areas (such as in Thailand) there is a boom and bust pattern to aquaculture.

About 30% (29.5 Mt) of the world fish catch is used for nonhuman consumption, including the production of fishmeal and fish oils that are employed in agriculture, in aquaculture, and for industrial purposes. Fishmeal and fish oils are key diet components for aquaculture production; depending on the species being cultured, they may constitute more than 50% of the feed. Climate change could have dramatic impacts on fish production, which would affect the supply of fishmeal and fish oils. Unless alternative sources of protein are found, future aquaculture production could be limited by the supply of fishmeal or fish oils if stocks of species used in the production of fishmeal are negatively affected by climate change and live-fish production. The precise impacts on future aquaculture production also will depend in part on other competing uses for fishmeal and fish oils. Usage of other sources of protein or developments in synthetic oils for industrial applications could reduce demands on fishmeal and fish oils, thereby reducing potential impacts on aquaculture.

Climate change is expected to have physical and ecosystem impacts in the freshwater and marine environments in which aquaculture is situated. Water and air temperatures in mid- to high latitudes are expected to rise, with a consequent lengthening of the growing season for cultured fish and shellfish. These changes could have beneficial impacts with respect to growth rate and feed conversion efficiency (Lehtonen, 1996). However, increased water temperatures and other associated physical changes, such as shifts in dissolved oxygen levels, have been linked to increases in the intensity and frequency of disease outbreaks and may result in more frequent algal blooms in coastal areas (Kent and Poppe, 1998). Any increases in the intensity and frequency of extreme climatic events such as storms, floods, and droughts will negatively impact aquaculture production and may result in significant infrastructure damage. Sea-level rise can be expected to have a negative effect on pond walls and defenses.

Elevated temperatures of coastal waters also could lead to increased production of aquaculture species by expanding their range. These species could be cultivated in higher latitudes as well as in existing aquafarms as a result of a longer warm season during which water temperature will be near optimal. A decrease in sea-ice cover could widen the geographical boundaries, allowing cultivation of commercially valuable species in areas hitherto not suitable for such developments.

Ocean ranching is used to increase the production of several fish species. The primary difference between ocean ranching and aquaculture is that in ocean ranching the fish are cultured for only a portion of their life cycle and then released prior to maturity. The cultured fish are then captured as "common property" in a variety of fisheries. These cultured or "enhanced" fish interact with the wild fish in the ecosystem and compete for the finite food and habitat resources available (often referred to as "carrying capacity"). Climate change will alter carrying capacity, but the impacts associated with ocean ranching or stock enhancement activities also need to be considered when examining overall changes to the ecosystem.

Although the precise impacts are species-dependent, the addition of billions of enhanced fish into the marine ecosystem may have significant consequences from genetic and ecological perspectives. In the Pacific, for example, large numbers of salmon are released from hatcheries in Russia, Japan, Canada, and the United States (Mahnken et al., 1998). Beamish et al. (1997) estimate that 83% of the catch of chum salmon by all countries is from hatchery production. If climate change increases SST and reduces winter wind mixing in the upper layers of the ocean, the feeding areas for salmon may be less productive because of increased surface layer stability. Reductions in overall production and catches of salmon—and likely other species—would result (Gargett, 1997).

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