Isolation Management with Artificial Barriers as a Conservation Strategy for Cutthroat Trout in Headwater Streams

Abstract: We evaluated the effectiveness of isolation management and stocking to meet protection and enhancement goals for native Colorado River cutthroat trout ( Oncorhynchus clarki pleuriticus ) in Wyoming ( U.S.A. ). As a management strategy of the Wyoming Game and Fish Department, cutthroat trout were isolated upstream of artificial barriers in small headwater streams. Non‐native trout that might have hybridized, competed with, or preyed upon cutthroat trout were removed from the isolated reaches, and then cutthroat trout of hatchery origin were stocked to augment populations. We monitored the abundance and body condition of cutthroat trout for 4–7 years following isolation in four streams with barriers and in two reference streams without barriers. Barriers limited new invasions by non‐native trout, and removals of non‐native trout greatly reduced their abundance but did not eliminate them ( mainly brook trout Salvelinus fontinalis ). Wild cutthroat trout persisted in low numbers upstream of barriers, but there was no evidence of enhancement of populations. Stocked cutthroat trout did not persist upstream of barriers, and many moved downstream over barriers. The body condition of wild cutthroat trout was comparable among populations upstream and downstream of barriers and in reference streams. Isolation management provided only short‐term benefits by minimizing the risks of hybridization and allowed populations to persist during the study. Removal of non‐native trout and stocking did not enhance wild cutthroat trout populations, however, likely because the isolated reaches lacked critical habitat such as the deep pools necessary to sustain large fish. Also, barriers disrupt migratory patterns and prevent seasonal use of headwater reaches by adult cutthroat trout. Longer‐term consequences of isolation include vulnerability to stochastic processes and loss of genetic diversity. Where non‐native species pose an immediate threat to the survival of native fishes, isolation in headwater streams may be the only conservation alternative. In such situations, isolated reaches should be as large and diverse as possible, and improvements should be implemented to ensure that habitat requirements are met.