ABSTRACT
Culture of fish results in divergence from their wild phenotypes through developmental and eventually, evolutionary processes. Straying of such phenotypically altered fish presents a threat to the diversity of wild populations. This paper examines how the magnitude of genetic threats is closely linked to reduced reproductive capabilities of cultured relative to wild fish in nature. Introgression appears to be constrained by phenotypic divergence of cultured from wild fish, particularly in traits important for breeding such as competitive and breeding behavior, and morphology. Characteristics of the mating system and breeding environment also play important roles. Furthermore, while detrimental effects of introgression increase with genetic differentiation between cultured and wild populations, rates of introgression may decrease. The effect of a negative relationship between relative reproductive success and genetic differentiation of cultured from wild fish is examined using a model of unidirectional gene flow. The findings suggest that potential impacts on wild populations may be least when cultured fish are: (1) genetically similar to, or (2) highly, phenotypically divergent from wild fish. Domestication of cultured fish until they are unable to breed or survive in the wild may be an effective means of eliminating gene flow into wild populations. The greatest threat to wild populations may be posed by cultured fish of intermediate divergence from wild fish. The most effective long term means for rehabilitation of wild populations likely will be a reduction in human impacts and a revitalization of lost habitats.
QUOTES FROM TEXT
The intrusion of cultured fish into wild populations threatens the genetic and ecological integrity and persistence of these populations.
Culture of fish results in divergence from their wild phenotype. Culture exerts novel forms of selection for phenotypes which are best capable of succeeding in this new environment, as well as relaxing selection upon traits previously advantageous in the wild. An inevitable result of artificial propagation appears to be divergence of fish from their wild phenotypes through environmentally induced developmental and eventually, evolutionary processes.
raying of environmentally and genetically altered fish from culture facilities represents a threat to the genetic diversity of wild populations. The concern is that introgression (incorporation of genes from one population into another), may lead to the breakdown of locally adapted gene complexes and homogenization of genetic structure.
Evidence suggests that reproductive capabilities of cultured fish are reduced in nature relative to wild fish. Chilcote, 1986, Leider, 1990, Campton 1991, Jonsson, 1991, Fleming and Gross, 1993 found that sea-ranched coho salmon (coho released from hatcheries as smolts for rearing in the sea) in direct competition with wild coho are reproductively disadvantaged, averaging an estimated 72% of the breeding success of wild fish. Leider et al. (1990) records the lifetime reproductive success of sea-ranched steelhead trout from spawner to adult offspring to be 11-13% that of wild steelhead.
Fitness parameters, such as survival of cultured and hybrid offspring are commonly lower than that of native offspring. Depressed fitness of hybrid offspring may well continue into the F2 and later generations.
Common to all studies of introgression, either explicitly or implicitly, is an apparent reduced reproductive capability of cultured relative to the native, wild fish.
Establishment of hatchery broodstocks from local rather than foreign populations will not prevent the apparently inevitable divergence of the cultured population from its wild phenotype. Rehabilitation and maintenance of habitat will be vital to the long term recovery and survival of wild populations. However, it must be insured that the new habitat is exploited by native and not immigrant (foreign) fish. Hatcheries cannot compensate for loss of biodiversity resulting from loss of wild populations.