SUMMARY
1.Escape of cultured organisms into natural ecosystems may threaten wild populations both ecologically and genetically. In the aquaculture industry, farmed Atlantic salmon often escape and enter the spawning grounds of wild salmon. We report experiments to assess the competitive and reproductive abilities of fifth generation farmed salmon and their potential impacts upon wild salmon.
2.The farmed and wild females had similar levels of competitive behavior, however, they differed in reproductive behavior and success. Farmed females displayed less breeding behavior, constructed fewer nests, retained a greater weight of eggs unspawned, were less efficient at nest covering, incurred more nest destruction, and suffered greater egg mortality than wild females. As a result, farmed females had less than on-third of the reproductive success of wild females.
3.The farmed males were even less successful than the farmed females in competition with the wild fish. They were less aggressive, courted less, partook in fewer spawnings, and achieved only an estimated one to three percentage of the reproductive success of the wild males.
4.The farmed males exhibited inappropriate mating behavior, that led to poor fertilization success, even in the absence of competition with wild males.
5.Adult farmed fish are thus likely to be relatively unsuccessful in natural environments due to a competitive and reproductive inferiority apparently resulting from domestication.
QUOTES FROM TEXT ö
Captive rearing conditions combined with artificial selection, both intentional and unintentional, cause farmed Atlantic salmon to diverge from their wild phenotype through environmental and eventually evolutionary processes.
Some species have been shown to rapidly adapt genetically to captivity and behavioral traits may be among the first traits to respond.
In salmon culture, where matings are determined artificially, adult aggression would afford no apparent reproductive advantage and directed selection for rapid growth may result in a correlated response for reduced aggressiveness. Thus, a combination of artificial and domestication selection may, in part, contribute to differences in aggressiveness between farmed and wild males. Reduced aggressiveness is also likely to be environmentally induced and may reflect the rapid deterioration in body condition of the farmed fish, particularly the males, which incurred high mortality, wounding, and fungus infection. (In farmed females it was noted by the authors that these fish had smaller hearts and reduced caudal fin size compared to wild females.) Inefficient nest covering probably resulted in greater egg loss, as fewer eggs of farmed females were recovered per nest.
The results of this study agree with other evidence that suggests captive breeding and artificial culture reduce natural reproductive ability of fish.
Our results suggest that farmed salmonids, artificially reared to maturity, will have an inferior reproductive ability relative to their wild counterparts. The extent of this inferiority is likely to be affected by the proportion of a fish’s life, as well as the number of generations in culture. The reproductive inferiority shown by sea-ranched salmonids relative to their wild counterparts is less than that shown by farmed salmon in this study. The extent of residency in the natural environment following escaped (or release) is thus likely to be an important determinant of reproductive ability once on the spawning grounds.
Even when reared to maturity, hybridization is likely to occur between escaped farmed females and wild males as found in the present study. This would result in sex-biased gene flow between cultured and wild fish. The potential for gene flow is great. Long term effects of such gene flow are unclear, as little is known about success of off spring from such spawnings, although in most cases we would expect it to be lower than that of wild offspring due to lack of local adaptation. Evidence of lower fitness in cultured/foreign and hybrid offspring relative to that of native offspring supports this contention. (See Reisenbichler and McIntyre 1977; Leider et al. 1990; Phillip 1991)
It might be speculated that the productivity of wild populations could be depressed by intrusions of cultured salmon that resulted in ecological interference, including mate and territorial competition.
Our results also have application to captive breeding programs for conservation and reintroduction of species, e.g Sacramento River winter-run chinook salmon (and Snake River spring chinook). To increase the success of such programs, detrimental effects of captive-rearing on an organisms phenotype and genotype, including its behavioral, morphological, and physiological traits, must be minimized. This may be accomplished by keeping the number of generations a species needs to be in captivity low and exposing it to naturalistic experiences and selection during this time.