Post-stocking fate of bonytail Gila elegans in a lower Colorado River reservoir

Authors: Brian Kesner, Kristen Humphrey, Paul Marsh

Year: 2016 (xlviii)

Abstract

Bonytail Gila elegans, federally listed as endangered, is considered functionally extirpated from its historical range, and its presence in the Colorado River now relies entirely on stocking programs. Lake Havasu, Arizona, California, and Nevada, is one of the few release locations for hatchery fish where individuals are occasionally captured. Only a handful of these captures occurred more than a year after release. A multi-year research project was implemented on Lake Havasu in which the post-stocking distribution, habitat use, and mortality of bonytail were assessed using acoustic telemetry and remote passive integrated transponder (PIT) scanning. Out of a total of 85 telemetry-tagged bonytail throughout the entirety of the 3-year study, 44 were determined mortalities, and 35 were permanently lost to the study (never contacted again). The majority of these lost fish (83%) were last located within the study area and were not contacted by submersible ultrasonic receivers specifically placed in locations to detect fish leaving the study area. Most remote PIT scanning contacts (at least 55%) occurred within the first 2 weeks post- stocking. Trammel netting efforts during the multi-agency Native Fish Routine Monitoring ‘Roundup’ resulted in the capture of eight bonytail in February 2014, one of which was inside the digestive tract of a largemouth bass, and no bonytail were captured in 2015 or 2016. Predation was a major threat to bonytail survival at all release sites, and piscivorous birds likely accounted for a large proportion of mortality in telemetry-tagged fish. Tags recovered on land and under roosting sites and observed capture events provided direct evidence of bird predation. Few fish survived long enough after release to determine habitat selectivity, alt hough fish were documented to utilize bulrush Scirpus. Off-channel locations where the potential for fish to leave the study area is minimized, such as Regional Park Moabi and Laughlin Lagoon, are ideal sites to track survival through both telemetry and remo te PIT scanning efforts. Survival of stocked fish may be increased by choosing stocking locations based on availability of cover s uch as submergent, emergent and shoreline vegetation, large boulders, man-made structures, and turbidity to reduce the impact of piscivorous birds.