Designing a naturalized rearing system for Gila trout Oncorhynchus gilae recovery

Authors: Angela P. James, James E. Brooks

Year: 2009 (xli)

Abstract

The Gila trout Oncorhynchus gilae is native to New Mexico and Arizona and is listed as a threatened species. The U.S. Fish and Wildlife Service was tasked with recovering this species by protecting four distinct genetic lineages. A captive rearing program is currently in place, but high mortality rates for incoming wild fish reduced hatchery numbers and threatened unique genetic stocks. We conducted a literature review to develop a naturalized rearing system for the purposes of acclimating wild Gila trout to captive propagation environments while minimizing mortality. We modified a fully functioning recirculation system to enhance habitat complexity consisting of cover, tank color, substrate, light, and underwater cameras to monitor fish behavior. Physical structures were placed in the system to mimic the structural complexity of a natural stream system; these structures were cinder block, artificial plants, aquamats, and polyvinyl chloride (pvc) pipe structures. To mimic cryptic coloration we painted tanks a brown/bronze color and added natural colored substrate (gravel and cobble) to the recirculation system. A true-light lighting system was set-up with timers to mimic natural day-night cycles to improve growth conditions and maintain a natural diel-cycle. To complement the tanks appearance, additional methods such as maintaining near wild densities, varied natural feed diet, and polyculture (rearing multiple species) were utilized. We collected aquatic invertebrates and introduced them into the system to provide a natural food base for the incoming wild fish. Immediately after constructing the system, it was determined the ability to actively conduct fish observations had been diminished and an underwater camera system was used to monitor fish behavior. The naturalized rearing system successfully held aquatic invertebrates (Ephemeroptera sp. , Plecoptera sp. , and Diptera sp. ) for three months with larvae of all three orders emerging to adults. In addition, 15 Sonora suckers Catostomus insignis (Average TL = 101.5mm) and 15 Desert suckers Catostomus clarkii (Average TL = 75.1mm) were introduced to the system. The suckers were observed feeding and using the various habitat structures with the underwater camera system. The natural rearing system is expected to be a beneficial tool in acclimating wild Gila trout to captivity because of the encouraging results of the invertebrates and suckers.