Genetic characterization of big-river fish populations as an adaptive management tool

The implementation of the 1922 Colorado River Compact created serious management issues for indigenous big-river minnows, with Gila cypha and elegans now federally endangered, and G. robusta a candidate. Propagation and translocation are adaptive management tools for these species that can turn deleterious if their gene flow and population structure are unknown. We evaluated four mtDNA regions (1,869 bp, 336 specimens) in an attempt to identify ESUs among upper and lower basin G. cypha and G. robusta. Neither species could be discriminated from one another using mtDNA, yet both separated from G. elegans at 4.8% sd. This broad admixture across basins, populations and species is unusual given fossil histories and distinct morphologies. To identify potential MUs, 643 specimens were genotyped across 16 msat loci, with species discriminated in all but one locale and with genetically distinct populations in different regions of the basin. Six basin-wide MUs were identified for G. cypha. Grand Canyon linked via downstream gene flow from the largest Canyon tributary, yet clearly separated from Upper Basin populations. However, translocation/ augmentation are inadvisable as management tools for they would complicate the historic admixture between these species while concomitantly homogenizing those remaining populations that are genetically distinct.