Performance of Diploid and Triploid Westslope Cutthroat Trout Fry Stocked into Idaho Alpine Lakes

Abstract

AbstractAnglers value alpine lakes as providing a scenic, backcountry fishing experience that is seldom found in other fisheries. However, trout introduced into alpine lakes often pose a risk to native salmonids in downstream habitats by establishing source populations in headwater locations. Although the stocking of sterile triploid trout can reduce this risk, poststocking performance should be equivalent to that of diploid fish before fisheries managers and anglers consider triploid stocking to be ideal. In this study, three different mechanical pressure treatments were tested to create mixed‐sex triploid Westslope Cutthroat Trout Oncorhynchus clarkii lewisi; at 65,500.19 kPa (9,500 psi) applied for 5 min at 300 Celsius‐minutes after fertilization, triploid induction rates were 100%, and egg survival to eye‐up was only 8% lower than that for diploids. Fry were stocked into 51 alpine lakes (25 with triploids and 26 with diploids), and fish were sampled 3 years later. Gill‐net catch rates, growth, and condition did not differ between stocked diploid and triploid fish. General linear models revealed that test fish were larger and in better condition in lakes with fewer total fish (indicating a density‐dependent influence on growth and condition) and were also in better condition in lower‐elevation lakes containing a higher percentage of shallow habitat. Ploidy level was not included in the most plausible model for any of the response variables. Our results indicate that triploid Westslope Cutthroat Trout can easily be created by using pressure treatment of fertilized eggs and appear to be a suitable alternative to diploids in alpine lake stocking programs. Our study suggests that the use of triploid Westslope Cutthroat Trout in alpine lakes similar to those we studied would prevent hybridization with native salmonids in adjacent habitats without reducing the abundance, size, or condition of fish available to anglers in the lakes where they are stocked.