ABSTRACT
Fish are mainly farmed for two purposes: to rehabilitate populations through controlled releases into natural waters or for the table market. Breeding ornamental fish for the pet trade is another area of culture but is not included in this chapter. The world's fish consumption has increased from 45 million tonnes in 1973 to more than 130 millions in 2000 and the demand is expected to increase further in the future. At the same time, about 75% of the world's most valuable marine fish stocks are either over-fished or fished to the limit. To provide the consumer with fish and for restocking the depleted populations, farming fish is a fast-growing industry. The current global production of Atlantic salmon, Salmo salar, exceeds 1 million tonnes, and is ranked as the top money maker at
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612 Fish Behaviour
However, there is a big step from showing that environmental enrichment improves performance in the laboratory or hatchery to demonstrate an effect on post-release growth and survival in the wild. Presently, the few studies conducted to evaluate effects of enhanced rearing methods on post-release survival have indicated mixed results. In most cases, neither structural enrichment, anti-predator conditioning or prior experience with live food have been found to increase post-release survival over conventional rearing methods, although a few studies have indicated positive effects (Berejikian et al., 1999; Mirza and Chivers, 2000). It is, however, clear that there is scope to further improve the environmental conditions for hatchery fish. For example, there is evidence that reduced density will reduce stress in hatchery fish (Ellis et al., 2002) and recent data suggests that reduced density can increase growth rate, reduce fin damage and increase seawater survival in supplementary released salmon smolts (Brockmark et al., 2007). It is important to state that these fish reared for release are not fed 'optimally' to reduce interactions and maximize growth as the fish reared for the table market, and the effect of high densities will generally be negative (see above). To summarize, hatchery conditions can be improved to produce a more wild-like fish, but it will never be possible to completely avoid environmental and genetic changes (Waples, 1999). Therefore, it is unlikely that the hatcheries will ever be able to produce fish that are fully ecologically exchangeable to wild fish.
