Response by trout populations in alpine lakes to an experimental halt to stocking

Abstract

Trout are often stocked into alpine lakes based on the generally untested assumption that resident populations would go extinct without stocking. The objectives of our study were to estimate the proportion of currently or formerly stocked alpine lakes in the Sierra Nevada, California, containing naturally reproducing trout populations (Oncorhynchus mykiss, Oncorhynchus mykiss aguabonita, Oncorhynchus clarki henshawi), identify characteristics of lakes associated with successful reproduction, and quantify the effects of stocking termination on trout density and individual growth rates in reproducing populations. We surveyed trout populations in 95 lakes in the John Muir Wilderness before and after a 4- to 8-year stocking halt and in 84 lakes in Sequoia–Kings Canyon National Park after a ≥20-year stocking hiatus. Based on recruitment during the no-stocking period, 70% and 68% of study lakes in the John Muir Wilderness and Sequoia-Kings Canyon National Park, respectively, contained reproducing populations. Results indicated that lakes with >2.1 m2 of spawning habitat and at elevations <3520 m nearly always showed evidence of reproduction. For reproducing populations, stocking termination did not result in significant changes in population density, but may have increased individual growth rates. We conclude that most trout stocking in Sierra Nevada alpine lakes could be permanently halted with only minimal impact on the recreational fishery.