Bluehead Sucker and the Complex Drainage History of the Upper Little Colorado River

Big River ecosystems and organisms that depend on them are increasingly imperiled and fishes of the Colorado River basin in western North America are no exception, with many native species listed under the Endangered Species Act (ESA). Currently, the Zuni Bluehead Sucker (Zuni BHS; Catostomus discobolus jarrovii) is a candidate for listing as endangered, yet disagreement exists regarding the extent of its distribution. We generated mitochondrial DNA (2 genes) and microsatellite data (17 loci) for 340 Bluehead Sucker (BHS: C. discobolus) from drainages straddling Arizona, New Mexico and Utah to assess degree of isolation among populations, and to identify distinct evolutionary lineages (=ESUs) and management units (=MUs). Based on our analyses, BHS in the Zuni River drainage is part of the main BHS lineage (the greater Colorado River ESU), but one population showed potential introgression by another species, the Rio Grande Sucker (C. plebeius). Population-level analyses indicated that BHS persists as distinct, demographically independent units among drainages (MUs). In conclusion, Bluehead Sucker populations in the Upper Little Colorado River and adjacent drainages reflect a complex evolutionary history, with periods of isolation punctuated by drainage re-arrangements and potential stream captures. Our data suggest that for current consideration for listing under the ESA, the Zuni BHS is restricted to the Zuni River (NM).