Evaluation of suppression methods targeting non-native lake trout embryos in Yellowstone Lake, Yellowstone National Park, Wyoming, USA

Abstract

Non-native Lake Trout Salvelinus namaycush threaten to extirpate native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in Yellowstone Lake, Yellowstone National Park. Suppression of Lake Trout in Yellowstone Lake has been ongoing since 1995, primarily by gillnetting. Unfortunately, bycatch of Yellowstone Cutthroat Trout is associated with this removal method, which targets adult and subadult Lake Trout. Alternative methods effective at causing mortality in Lake Trout early life stage(s) could be used simultaneously with gillnetting to improve suppression effectiveness. The vulnerability of salmonid embryos suggest increasing Lake Trout embryo mortality is feasible and because population growth rates are sensitive to age-0 survival an effective embryo suppression method could have population-level effects. Thus, the primary objective of this study was to evaluate the efficacy of methods to increase mortality of Lake Trout embryos. In situ experiments tested the effect of suction dredging, electroshocking, tarping, and Lake Trout carcass deposition on embryo mortality. The secondary objective of this study was to evaluate interstitial water flow using NaCl dry injection to better understand the feasibility of using chemicals for embryo suppression. Concurrent laboratory studies have shown that Lake Trout embryos experience high mortality after chemical exposure. Tarping and suction dredging were not effective at increasing embryo mortality. Electroshocking caused 0.92 mortality of embryos at the substrate surface but only 0.38 at 20 cm depth in the substrate. Lake Trout carcass deposition caused 0.99 mortality of embryos, both at the surface and at 20 cm in the substrate. The direction and rate of interstitial water flow was not consistent and future work on a larger scale is needed to inform the feasibility of chemical suppression. Hypoxic conditions within the carcass sites may have caused embryo mortality. In addition, carbon dioxide CO 2 and hydrogen sulfide H 2S are byproducts of organic matter decomposition that harm developing salmonid embryos. Embryo suppression methods are unlikely to replace traditional Lake Trout suppression methods. However, the success of Lake Trout carcass deposition shows potential for the development of an effective additional suppression method that could be implemented on a large scale.