This paper explores my shifting understandings of interactions primarily between salmonid fish culture and fish conservation during the latter half of the 20th century. The idea that conspecific natural and cultured fish were largely interchangeable among phenotypically similar populations began to change with the advent of molecular genetic markers. With the gradual clarification of major geographic lineages beginning in the 1970s came awareness that translocations among anadromous lineages were generally destined for failure; in contrast, gene flow more readily occurred among non-anadromous lineages and sometimes, species. Within lineages, data concurrently were accumulating that showed adaptations to their respective environments distinguished cultured and wild populations. Reduced obstacles to gene flow at this level often resulted in homogenizations among wild and cultured fish in areas where widespread hatchery releases occurred; conversely, adaptive radiations in vacant habitats sometimes occurred over a few decades from single source hatchery releases. Current ideas relating to salmonid interbreeding, population sub- structure and culture evolved from these observations. Among lineages, resistance to gene flow is much greater between anadromous than purely freshwater populations or species. Within lineages, ease of gene flow in both anadromous and freshwater populations is problematical with regard to cultured and wild populations because large-scale supplementation programs erode local adaptations and fine-scale popu- lation substructures. At this level, a potential ability to regenerate natural substructure upon relaxation of supplementation is offset by uncertainties of time scales and intrinsic capabilities of homogenized popu- lations. However, management that separates harvest and reproduction of wild and cultured subpopula- tions can minimize these losses. Some generality of this strategy to other fishes is supported by losses of local adaptations and outbreeding depression in black basses following population admixtures that parallel those observed in salmonids.
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