Mon, Aug 02, 2021:On Demand
Background/Question/Methods
Widespread introductions of invasive fishes have led to continental-scale homogenization of fish assemblages and biodiversity loss. Once established in new habitats, invasive fishes are costly and difficult to remove. Salmonids are damaging introduced predators that can exert top-down controls on ecosystem function by altering food web dynamics and extirpating naïve prey fishes. Brown trout Salmo trutta were introduced into tributaries to the Colorado River in the Grand Canyon beginning in the 1920s and spread to the mainstem following the construction of Glen Canyon Dam. Trout and other invasive fishes threaten endangered humpback chub Gila cypha and other Colorado River endemic species through predation and competition, requiring the development of effective management strategies. We used linear mixed-effects models and a spatially-explicit stage-based population viability model to understand stochastic drivers of brown trout production in connected subpopulations in a spring-fed tributary to the Colorado River and in the tailwater of Glen Canyon Dam, and to simulate alternative suppression scenarios. Suppression scenarios included varying levels, frequency, and duration of electrofishing effort, and targeted different life-stages through removal of age-1+ trout, redd destruction, incentivized harvest, and a weir to remove migrants, in the context of variable environmental conditions.
Results/Conclusions We found brown trout to be resilient to suppression when optimum environmental conditions facilitated large successful recruitment events in the tributary population. Survival from egg to age-1 is driven by the magnitude of tributary discharge volume in April, which coincides with emergence of fry from gravel interstitial habitats – a sensitive period within the brown trout life cycle. In contrast, when spring discharge was higher than average, suppression effects targeting multiple life stages resulted in maintenance of low abundance. The tailwater brown trout subpopulation may vary with conditions related to basin-scale hydrology, whereas thermal regimes downstream of the dam are controlled by the upstream reservoir level – future drought may lessen the need for suppression as warmer dam discharge creates less suitable conditions. Nonetheless, experimental floods meant to conserve sediment in Grand Canyon may influence spawning movements between subpopulations, facilitating recolonization. Successful suppression scenarios may involve targeting multiple age-classes, and eradication through piscicides should be implemented where possible. Our modeling approach improved our understanding of the stochastic and anthropogenic drivers of the population dynamics of an introduced salmonid, which will lead to more focused suppression strategies, and ultimately, maintenance of populations of endemic fishes.
Results/Conclusions We found brown trout to be resilient to suppression when optimum environmental conditions facilitated large successful recruitment events in the tributary population. Survival from egg to age-1 is driven by the magnitude of tributary discharge volume in April, which coincides with emergence of fry from gravel interstitial habitats – a sensitive period within the brown trout life cycle. In contrast, when spring discharge was higher than average, suppression effects targeting multiple life stages resulted in maintenance of low abundance. The tailwater brown trout subpopulation may vary with conditions related to basin-scale hydrology, whereas thermal regimes downstream of the dam are controlled by the upstream reservoir level – future drought may lessen the need for suppression as warmer dam discharge creates less suitable conditions. Nonetheless, experimental floods meant to conserve sediment in Grand Canyon may influence spawning movements between subpopulations, facilitating recolonization. Successful suppression scenarios may involve targeting multiple age-classes, and eradication through piscicides should be implemented where possible. Our modeling approach improved our understanding of the stochastic and anthropogenic drivers of the population dynamics of an introduced salmonid, which will lead to more focused suppression strategies, and ultimately, maintenance of populations of endemic fishes.