Invasive predator diet plasticity has implications for native fish conservation and invasive species suppression

Abstract

Diet plasticity is a common behavior exhibited by piscivores to sustain predator biomass when preferred prey biomass is reduced. Invasive piscivore diet plasticity could complicate suppression success; thus, understanding invasive predator consumption is insightful to meeting conservation targets. Here, we determine if diet plasticity exists in an invasive apex piscivore and whether plasticity could influence native species recovery benchmarks and invasive species suppression goals. We compared diet and stable isotope signatures of invasive lake trout and native Yellowstone cutthroat trout (cutthroat trout) from Yellowstone Lake, Wyoming, U.S.A. as a function of no, low-, moderate-, and high-lake trout density states. Lake trout exhibited plasticity in relation to their density; consumption of cutthroat trout decreased 5-fold (diet proportion from 0.89 to 0.18) from low- to high-density state. During the high-density state, lake trout switched to amphipods, which were also consumed by cutthroat trout, resulting in high diet overlap (Schoener’s index value, D = 0.68) between the species. As suppression reduced lake trout densities (moderate-density state), more cutthroat trout were consumed (proportion of cutthroat trout = 0.42), and diet overlap was released between the species (D = 0.30). A shift in lake trout δ13C signatures from the high- to the moderate-density state also corroborated increased consumption of cutthroat trout and lake trout diet plasticity. Observed declines in lake trout are not commensurate with expected cutthroat trout recovery due to lake trout diet plasticity. The abundance of the native species in need of conservation may take longer to recover due to the diet plasticity of the invasive species. The changes observed in diet, diet overlap, and isotopes associated with predator suppression provides more insight into conservation and suppression dynamics than using predator and prey biomass alone. By understanding these dynamics, we can better prepare conservation programs for potential feedbacks caused by invasive species suppression.

Yellowstone Cutthroat Trout Recovery in Yellowstone Lake: Complex Interactions Among Invasive Species Suppression, Disease, and Climate Change

“…Nevertheless, the gill netting suppression program enacted by the NPS over the past 30 years has likely prevented the collapse of the Cutthroat Trout population in (Hostetler et al 2021) influenced predator-prey dynamics between Lake Trout and Cutthroat Trout, which may inhibit Cutthroat Trout recovery in the presence of Lake Trout. Lake Trout were forecasted to expand during climate change under the 50% effort scenario, potentially due to release from competition with Cutthroat Trout for a shared prey item, amphipods (Ruzycki et al 2003;Syslo et al 2016;Glassic et al 2023). This climate change scenario resulted in a 10-fold increase in Lake Trout predation mortality of amphipods compared to all other scenarios.…”

“…Cutthroat Trout are known to control abundance of amphipods (Wilmot et al 2016). Cutthroat Trout decline during the 50% effort scenario with extreme climate change may have been large enough to allow Lake Trout to exhibit diet plasticity (Glassic et al 2023) and increase consumption of amphipods. Though the conclusion that Cutthroat Trout may never reach primary benchmarks during extreme climate change may be discouraging, similar conclusions have been made for other lake ecosystems balancing hydrological changes and native fish conservation (Wang et al 2016).…”

“…In Ecosim, consumption by predators is modeled using vulnerability parameters, which are based on foraging arena theory (Ahrens et al 2012), where predation rates are limited by prey biomass "moving" from invulnerable states (resting, hiding) into the foraging arena and becoming The Ecopath model that we developed emphasized dynamics between Lake Trout and Cutthroat Trout in Yellowstone Lake (Table 2). Lake Trout were represented in the model by Fisheries | www.fisheries.org 59 five age-classes (≤6 months, 7 months to 1 year, 2 years, 3-4 years, ≥5 years) to capture application of suppression strategies (Koel et al 2020a;Syslo et al 2020) and ontogenetic diet shifts (Ruzycki et al 2003;Syslo et al 2016;Glassic et al 2023). Cutthroat Trout were represented by five age-classes (≤23 months, 2 years, 3 years, 4 years, ≥5 years) to capture ontogenetic diet shifts (Ruzycki et al 2003;Syslo et al 2016;Glassic et al 2023), vulnerability to Lake Trout predation (≤23 months, 2 years, 3 years; Ruzycki et al 2003;Syslo et al 2016;Glassic et al 2023), vulnerability to tributary disconnection or reduction in discharge (≤23 months; Kaeding 2020), and susceptibility to disease (≤23 months, 2 years; Koel et al 2006Koel et al , 2019.…”

“…Throughout the late 1990s and 2000s, abundance of Cutthroat Trout declined because of increasing abundance of Lake Trout Salvelinus namaycush, outbreaks of whirling disease Myxobolus cerebralis, and climate change (Koel et al 2006(Koel et al , 2020aKaeding 2020). Lake Trout, an invasive species in the western United States first discovered in Yellowstone Lake in 1994 (Kaeding et al 1996), caused the decline of the Cutthroat Trout population through predation (Ruzycki et al 2003;Syslo et al 2016;Glassic et al 2023). The decline of Cutthroat Trout in Yellowstone Lake caused a trophic cascade (Tronstad et al 2010) that affected terrestrial and aquatic food webs throughout the Greater Yellowstone Ecosystem (Koel et al 2019), threatening the integrity of one of the largest, mostly intact, temperate zone ecosystems in the world (National Park Service 2020).…”

“…Lake Trout exhibit diet plasticity in Yellowstone Lake-as Lake Trout density increases, consumption of Cutthroat Trout decreases, whereas consumption of amphipods increases (Glassic et al 2023). Cutthroat Trout have not recovered as expected in comparison with the reduction of predatory Lake Trout due to Lake Trout diet plasticity (Glassic et al 2023). Although recovery Fisheries | www.fisheries.org 57 benchmarks for Cutthroat Trout exist (Koel et al 2010;Koel et al 2020a), understanding how Cutthroat Trout respond to Lake Trout suppression simultaneously with disease prevalence, diet plasticity of Lake Trout, and climate change has not been evaluated.…”