DISPERSAL OF STOCKED RIO GRANDE SILVERY MINNOW (HYBOGNATHUS AlVIARUS) IN THE MIDDLE RIO GRANDE, NEW MEXICO

Pelagic-broadcast spawning riverine fishes (pelagophils), species that produce eggs and larvae that drift laterally and downstream with the current, are declining throughout their native ranges in North America. Persistence and recolonization of pelagophils require upstream dispersal of later life stages; however, observations of dispersal are limited. We performed a mark-recapture study of stocked Rio Grande silvery minnow (Hybognathus amarus) in the Middle Rio Grande, New Mexico, during 2002 to assess dispersal of this imperiled pelagophil. Approximately 11,500 hatchery-reared Rio Grande silvery minnows marked with a fluorescent-colored visible implant elastomer were released by the Southwestern Native Aquatic and Technology Center (Dexter, New Mexico) at two locations in the 94.1-km San Acacia Reach of the Middle Rio Grande in January 2002. We recaptured 66 marked individuals (0.57%) through May 2002, upstream and downstream of both release locations. Distances traveled ranged from 0.0 to 25.2 km (0.3 +/- 5.3 km [mean +/- SD]), and movement rates ranged from 0.0 to 220 m/day (19 +/- 63 m/day). We recaptured two individuals >20 km upstream of their release location. Overall, stocked fish tended to disperse downstream. We often recaptured marked fish with wild conspecifics, implying repatriation of a portion of stocked fish. Gravid and spent marked female Rio Grande silvery minnows recaptured during April and May indicated that stocked fish were reproductively active concurrent with the wild population. Our study documented long-distance upstream dispersal of stocked Rio Grande silvery minnows and thus has conservation implications for restoring connectivity in the Middle Rio Grande to support the recovery of this federally listed endangered species.