ABSTRACT:
Genetic variation can be partitioned into differences between individuals within populations, differences between populations within species, and differences between species or higher taxa. Although species-level differences have received the most attention with respect to biodiversity, the hierarchical levels are not independent. For example, just as population viability depends on maintaining genetic variation among individuals, so to may the long-term viability of a species depend on conserving multiple, semi-independent populations (or stocks). In addition to contributing to the erosion of between-population genetic diversity, stock transfers may lead to reduced population fitness through outbreeding depression. These risks are discussed with examples involving freshwater and anadromous fishes in North America. Studies of largemouth bass provide perhaps the clearest evidence of the negative effects of stock transfers on the fitness of local populations. Further more, genetic markers used in these studies demonstrate substantial introgression of foreign genes into local populations, indicating that the effects of stock transfers may be long lasting, if not permanent. In Pacific salmon, local populations exhibit considerable diversity in life history traits such as age structure; ocean migration patterns; and time of spawning, emergence, and outmigration. This diversity can be expected to buffer total productivity for the species against periodic or unpredictable changes in the environment. Extensive stock transfers, involving the exchange of eggs among hatcheries and intentional or unintentional release of hatchery fish into the wild, have contributed to a decline in between-population genetic diversity of these species.