Population Changes after 14 Years of Harvest Closure on a Migratory Population of Bull Trout in Idaho

Abstract

Primary data (size and age structure, abundance) and derived data (growth, mortality, recruitment) were used to assess the status and trends of Bull Trout Salvelinus confluentus in the North Fork of the Clearwater River, Idaho, under a 14-year harvest closure. From 2000 (6 years after harvest closure) to 2008, an increase in the number of larger and older migratory Bull Trout occurred, as evidenced by the rightward shift in the cumulative length-frequency distribution, increases in mean total length and weight, and increases in age. The stability in growth rates over an 8-year interval indicated that the increases in size structure were age related (e.g., recruitment and mortality changes) rather than growth related. The abundance of migratory spawning adults also steadily increased over the period 1994–2008, as indicated by the increases in redd counts. A logistic model fitted to population estimates (not including unsampled portions of the drainage where migratory Bull Trout are known to exist) indicated that the rate of population growth as of 2005 was beginning to slow and that a carrying capacity of 5,215 total migratory adults will be asymptotically approached, surpassing the U.S. Fish and Wildlife Service's drainage-wide recovery goal of 5,000 total adults. The results from an age-structured population model (model 1) indicated a carrying capacity (5,716 migratory adults) similar to that of the logistic model. If the results under model 1 are achieved, the migratory adult population (including nonspawning adults) will surpass 5,000 adults in 2019. The results under a second model depicted a lower carrying capacity (3,592 fish). This analytical approach has promise for application in situations where harvest restrictions or the elimination of fishing are part of Bull Trout restoration programs.

Received August 8, 2013; accepted December 11, 2013

ACKNOWLEDGMENTS

We thank colleagues at the University of Idaho Department of Fish and Wildlife Resources for their efforts, which contributed to this study. We acknowledge the Idaho Department of Fish and Game for providing funding and much assistance. Comments by D. Schiff, E. Schriever, and three anonymous peer reviewers greatly improved this manuscript.