Use of Genetic Data to Guide Management of An Historically Admixed Species Complex

Over the last century, humans have drastically altered freshwater systems in southwestern North America and thus provoked conservation and management issues for endemic fishes. Depleted/ extirpated populations are prime candidates for ‘supplementation programs’ that will augment numbers, but a clear understanding of genetically and demographically distinct lineages is required so as to avoid potentially deleterious admixtures. This is particularly true in the Colorado River for the endangered Humpback Chub (Gila cypha) and Bonytail (G. elegans), and Roundtail Chub (G. robusta), a species of concern. Sequence analyses of mitochondrial DNA (1,869 base pairs/ 336 specimens) was unable to discriminate G. cypha from G. robusta, although both separated from G. elegans at 4.8% sequence divergence. This broad admixture across two species is unusual, particularly given their fossil history and distinct morphologies. In contrast, microsatellite DNA analyses of both species (16 loci/ 643 specimens) identified six populations that are demographically independent and which segregate by geomorphology and phenotype. A seventh reflected broad morphological and genetic admixture. Supplementation can be a risky management tool within riverine systems confounded by historic admixture, and genetic data derived at several hierarchical levels are essential to parse effective conservation.