The common management practice of introducing artificially produced fish into wild populations has raised concerns among fishery biologists. In part, these concerns arise from the observation that hatchery-produced fish commonly differ from wild fish in ways that may influence ecological interactions between them. In this review, we use a meta-analytical approach to provide quantitative tests for such differences and show that the hatchery rearing of salmonids results in increased pre-adult aggression, decreased response to predators, and decreased survival. Changes in growth rates are common, but less consistent. Changes in other fitness related traits such as migration, feeding, habitat use and morphology also occur. Based on the presented evidence we conclude that differences between hatchery-reared and wild fish may have negative implications for the success of stocking programs. A number of studies reporting population responses to stocking support this, suggesting that the performance of hatchery fish and their interactions with wild fish is of such a character that many of the current stocking practices may be detrimental to the recipient populations.
Genetic changes due to relaxed and/or altered selection are likely to accumulate in stocks being cultured over multiple generations (e.g., when brood stock is consistently chosen from adults originating from hatchery produced smolts). Multi-generation hatchery stocks are thus likely to differ more from wild fish than first generation stocks where most of the changes are likely to be of environmental origin.
Studies suggest that hatchery fish differ from wild fish in levels of aggression and predator avoidance behavior. In most studies, the effect of artificial rearing appears to result in an increase in levels of aggression (5 out of 9 studies). In the three studies where the origin of the difference was predominately environmental, hatchery fish were consistently more aggressive than wild fish.
Hatchery fish do differ from wild fish in levels of anti-predator behavior. Hatchery fish are observed to differ from wild fish in their timing of migration, which may influence both their susceptibility to predation and their energetic costs (i.e. due to different temperature and flow regimes). If this effect on timing of migration also influences breeding time, offspring survival may be compromised due to inappropriate emergence timing from nests.
Hatchery populations may also differ from wild populations in feeding behavior and habitat use.
Salmonid populations exhibit differences in morphological traits, and these differences have been suggested to result form local adaptations to environmental conditions. Furthermore, morphological traits are important determinants of breeding success. Thus any deviation in morphology from the local population may be expected in decreased fitness.
If hatchery fish differ from wild fish in so many respects, how successful are the released fish likely to be in the wild? Assuming that the wild populations have undergone natural selection for ten thousand years (since the end of the last ice age) to become adapted to the local environment, one would predict that these changes in fitness-related traits are a potential problem for released fish, and may influence their ability to survive and reproduce. Their performance in the wild should therefore be expected to be inferior to that of wild fish, a pattern that is commonly observed.
The success of hatchery-produced fish after release appears to be constrained by phenotypic divergence from their wild conspecifics. This is not surprising given the potential importance of local differences among wild salmonid populations in fitness-related traits and the evidence we have presented concerning the effects of hatchery environments on development and selection.
One might speculate that hatchery fish are to some degree able to displace naturally produced fish (in streams), but that they are unable to cope with the high cost associated with this behavior in terms of risk of starvation or predation. If so, net fish production may actually decrease as a result of stocking.
Releases of hatchery fish can also attract predators and thus may cause the intensity of predation on naturally produced fish to increase.
The effects that released hatchery fish can impose on naturally produced fish should make us cautious toward implementing stocking programs to compensate for habitat degradation and to increase fisheries. Indeed, under certain scenarios, theoretical models suggest that long-term stocking may lead to extinction of the native population. Existing empirical studies clearly show that fish density in stocked streams may not show the desired positive response to releases. In fact, in some cases a negative trend in population density has been associated with releases. Perhaps the best evidence for such an effect comes from a controlled study where populations of coho salmon were monitored for five years in 15 stocked and 15 unstocked streams. Stocked streams had higher densities of juveniles after stocking, but the number of adults returning to the two types of streams did not differ. Furthermore, spawning success of released fish was reduced, causing a lower density of juveniles in the stock streams than in the unstocked ones one generation later.
The performance of hatchery fish and their interactions with wild fish appear to be of such a character as to suggest that many of the current stocking practices may be detrimental to the recipient population. The present synthesis should incite caution in our attempts to mitigate negative effects of habitat degradation by releasing hatchery ÷produced fish.
A critical question we might ask ourselves is whether something can be done to avoid negative ecological effects of stocking. The answer to this question is yes and no. Better broodstock collection and mating protocols, more-natural rearing conditions, wild-fish-friendly release strategies and more focus on local broodstocks can improve the quality of hatchery fish released and reduce their impacts on wild fish. Released juveniles should be within the size range of wild juveniles, if not of a similar size distribution. The number of fish released should not exceed the carrying capacity of the environment, which varies spatially within the river and through time.
However, as Waples points out, it is a myth to believe that these changes will make the problems disappear altogether. This is because (1) environmental and genetic changes to fish in hatcheries cannot be avoided entirely; and (2) many of the risks are negatively correlated, so efforts to reduce one risk simultaneously increases another. Clearly we need to, first and foremost, be cautious in our use of hatcheries, particularly when releases are to be used in supplementing wild populations.