Evaluating Gila Topminnow Poeciliopsis occidentalis Survival During Transport

Authors: Elizabeth Grube, Brian Hickerson, Anthony Robinson

Year: 2021

Abstract

Gila Topminnow (Poeciliopsis occidentalis) were historically widespread throughout the Gila River Basin, however, by the time the first recovery plan was drafted in 1984, Gila Topminnow were restricted to ten naturally occurring localities. The recovery plan for Gila Topminnow specified reintroduction as one of the primary recovery actions for the species. Many reintroduction sites are in remote locations with limited access. Fish were typically transported to these remote sites in insulated buckets (with battery-powered aerators) attached to pack frames carried by individual hikers. Mortality was occasionally high during the hiking portion of the transportation process which prompted our exploration of alternative methods that could be employed to reduce fish mortality. Sensitive trout species are often hiked into remote locations in buckets attached to pack frames containing sealed bags filled with water and pure oxygen. Similar methods are also used by fish hobbyists when shipping fish, and we believed these methods could be adapted to reduce Gila Topminnow mortality during backcountry transport. The objective of our study is to compare transportation mortality of Gila Topminnow during translocation to backcountry sites using the standard method (battery operated aerators) to an alternative method (sealed bags with oxygen). A sub-objective of our study is to evaluate the relationship between mortality and factors that may influence mortality during transportation like, density of fish in containers, gender, duration of transport, water temperature, dissolved oxygen, pH and acclimation time, and temperature difference at stocking. We evaluated both transportation methods during two backcountry Gila Topminnow stocking events with four paired trials. Treatments with the battery-operated aerator had an average mortality of 4.05% (range 0.00-13.60%) and treatments with the pure oxygen bags had an average mortality of 0.61% (range 0.00-0.82%). We evaluated factors contributing to topminnow transportation mortality using a dataset with Gila Topminnow stockings occurring from 2007-2021 and found temperature and density of fish had the largest influence on topminnow mortality. More data is needed to fully understand the influence of each transportation method on topminnow mortality, however these preliminary results suggest that oxygenated bags could improve upon existing methods.