Advancing Yellowstone cutthroat trout conservation in Yellowstone Lake: assessing tributary use and population dynamics to develop contemporary conservation benchmarks

Abstract

Yellowstone cutthroat trout (Oncorhynchus virginalis bouvieri) in Yellowstone Lake are threatened by invasive lake trout (Salvelinus namaycush), which predate on Yellowstone cutthroat trout. To conserve Yellowstone cutthroat trout, the National Park Service suppresses lake trout by gillnetting. Intensive suppression effort over the past three decades has reduced the abundance of piscivorous lake trout, and Yellowstone cutthroat trout are responding positively to lake trout suppression. Despite indications of recovery, research has not comprehensively assessed the influence of lake trout invasion and suppression on the Yellowstone cutthroat trout population, and additional research is critical for the development of updated, comprehensive conservation benchmarks for Yellowstone cutthroat trout in Yellowstone Lake. We used long-term monitoring data and contemporary surveys to assess changes in tributary use by spawning Yellowstone cutthroat trout. Tributary use declined following lake trout invasion but has increased since the initiation of intensive lake trout suppression, highlighting the ongoing recovery of Yellowstone cutthroat trout. Predation by invasive lake trout was a strong driver of Yellowstone cutthroat trout population dynamics. Suppression of predatory lake trout has reduced predation mortality, and the proportion of Yellowstone cutthroat trout biomass consumed by lake trout has declined. Population projections indicate that Yellowstone cutthroat trout will increase in abundance in response to levels of lake trout suppression that are similar to or higher than current suppression netting effort. Reductions in suppression netting effort are predicted to result in declines in the Yellowstone cutthroat trout population. We synthesized contemporary research and long-term monitoring data to develop conservation benchmarks for Yellowstone cutthroat trout and lake trout in Yellowstone Lake. We used a scenario-based approach to define benchmarks for seven metrics for Yellowstone cutthroat trout and three metrics for lake trout. The scenario-based approach ensures benchmarks are feasible. The scenario-based benchmarks provide a pragmatic and science-based guide to ensure that conservation efforts are efficient, influential, and promote the long-term persistence of Yellowstone cutthroat trout in Yellowstone Lake.