| Many species are isolated in small geographic areas due to habitat loss and
fragmentation. This is especially true of riverine fishes, where alteration of waterways has
restricted many species to low abundances in headwater streams, threatening loss of genetic
diversity where drift and inbreeding depression are prevalent. In addition, hybridization and
introgression between native and introduced populations can also impact the distribution of
genetic variation among stream fish populations. To assess the effects of habitat extent and
hybridization on the level of genetic diversity in fragmented populations of Redband Trout,
Oncorhynchus mykiss gairdneri, I conducted a study within the southernmost drainages of the
Snake River watershed. Introgression between Redband Trout and coastal Rainbow Trout
(Oncorhynchus mykiss irideus) was found in nearly half of the populations assessed, where high
levels of admixture corresponded to increased heterozygosity. Independent of hybridization, the
level of genetic diversity was found to be positively related to habitat extent, with populations
occupying over 10 km of stream exhibiting higher heterozygosity than those in under 5 km.
While high levels of genetic diversity in nonhybridized populations may correspond to elevated
adaptive potential, raised genetic diversity due to introgression represents a potential concern to
fitness due to the effects of outbreeding depression.
Keywords: Habitat Fragmentation, Genetic Diversity, Intraspecific Hybridization, Redband
Trout, Stream Extent. |