ABSTRACT
Habitat destruction, over-exploitation and species introductions are causing declines in salmon populations and resulting in a growing need for enhancement. Enhancement may occur through supplementation of wild populations with hatchery reared fish, as well as by improvements in spawning and rearing habitats. Management regulations also may be used to protect habitats and populations against destruction and over exploitation. Although the fitness of hatchery salmon in nature is lower than that of wild salmon, supplementation has been used to enhance populations successfully, at least in the short term. However, supplementation is not without problems and conflicts. It can result in the spread of contagious diseases, in ecological interference with wild populations, and in the disruption of the genetic structure of wild populations through introgression, genetic drift, and unintentional changes in selection regimes. Thus, the use of supplementation to enhance populations should be carefully considered, even when only a single generation boost to a population seems warranted. Protection and restoration of habitat, combined with adequate management regulation, are likely to provide the only truly long term means to enhancing populations.
QUOTES FROM TEXT.
The combined effects of over-exploitation, habitat destruction, and species introductions have resulted in a worldwide trend of decline in native fish populations.
...hatchery programs appeared to be successful initially but, as time wore on, success declined. Since 1976, there has been in general an inverse relationship between the number of coho smolts released in Oregon and the size of the adult production. In British Columbia, a rapid expansion of chinook salmon hatcheries increased smolt releases ten fold during the 1980s, but resulted in lower catches of hatchery salmon. This pattern of declining survival of hatchery reared salmon is common to most hatchery programs in North America.
Salmon exhibit considerable local adaptation to their natal freshwater environments. This genetic and ecological diversity among populations is the ultimate source of biological diversity for the species. Yet the genetics of their populations, in contrast to populations of other vertebrates, have been largely ignored until recently.
Fitness parameters, such as survival, are often lower for hybrid than native offspring. Hybridization may also result in breakdown and loss of diversity among populations, reducing overall productivity and increasing the vulnerability of the species to environmental change.
Even supportive breeding (supplementation), where a fraction of the wild population is brought into the hatchery for artificial reproduction and offspring released back into their parentsā natal river, can be problematic. By increasing reproductive success and survival of one segment of a population above other segments, the effective population size can be reduced far below what it would have been without supportive breeding. Hence, supportive breeding may influence genetic structure, by reducing genetic variability of populations it was meant to assist. There is a tradeoff between increased production and decreased effective population size and loss of genetic variability. When supportive releases seem most warranted, as in the case of small populations with high probability of extinction, supportive breeding should be carefully considered.
Supplementation to increase or maintain high harvest rates threatens the existence of small, unsupplemented populations and the unique population genetic structure of species.
Poorly managed hatchery programs thus may alter or even destroy the biological diversity of species.
Hatchery rearing exposes salmon to new environmental and evolutionary forces that may alter phenotypes. The body shape of Atlantic salmon can change extensively form the wild form within a single generation of culture.
The morphological divergence of hatchery from wild juveniles likely affects their success in the wild and may explain, in part, their reduced survival in the sea.
Both rate of recapture in the fishery and of return to freshwater are twice as great for wild as for hatchery smolts having common genetic backgrounds. Hatchery salmon of the River Imsa have half of the marine survival, on average, of their wild counterparts.
Differences were evident for hatchery Atlantic salmon relative to wild salmon, with common genetic backgrounds, in breeding success after a single generation in the hatchery. Hatchery females averaged approximately 80% the breeding success of wild females. Hatchery males had significantly reduced breeding success, averaging approximately 65% of the success of wild males.
We know that if we use hatcheries for supportive breeding of wild populations, we are likely to proliferate certain genotypes more than others and thus alter the genetic structure of the population. We must be careful not to fall into the technological fix syndrome whereby hatcheries are viewed as the solution to the majority of our salmon management problems.To increase population abundance, habitat improvement is likely, in many cases, to be a better management tool than hatcheries.