Population Characteristics of Greenback Cutthroat Trout in Streams: Their Relation to Model Predictions andRecovery Criteria

Abstract

Fishes listed under the U.S. Endangered Species Act receive intensive management, but evaluations of population characteristics are rare. We report population and habitat characteristics of federally threatened greenback cutthroat trout Oncorhynchus clarki stomias in 12 streams in north-central Colorado surveyed in 1998 and 1999. Our objectives were to assess population size, structure, and distribution, to evaluate a model developed by Harig and Fausch (2002) to assess translocation success, and to compare population characteristics to those reported in the recovery plan. Allopatric population sizes in 10 streams varied from 170 to 7,347 greenback cutthroat trout at least 75 mm long, whereas the 2 streams with brook trout throughout supported 0–142 greenback cutthroat trout. Populations displayed few upstream–downstream trends in fish abundance or mean length, and the coefficient of variation among sampled reaches within streams averaged 61%. As the density of fish at least 75 mm long increased, the percentage of juvenile (<75 mm) fish in a population increased, the percentage of adult (>125 mm) fish longer than 200 mm decreased, and the average weight of a 200-mm fish declined. The abundance of juvenile fish was positively correlated with summer water temperature. Population estimates were highly correlated with the translocation model index values, suggesting that the translocation model may apply to other cutthroat trout taxa in similar environments. Of the 12 streams for which our extensive field data are available, 4 satisfied the recovery criteria we studied and other criteria in the recovery plan; 8 did not. We conclude that many populations of greenback cutthroat trout, including some that meet recovery criteria, are still at risk of extinction and that a reliable monitoring protocol needs to be established.