Using dietary dynamics to assess the efficacy of biocontrol and to predict the effects of warming water temperatures on salmonids

Abstract

Salmonids are coldwater fishes with substantial ecological and economic importance, particularly in the northern Rocky Mountains in Montana, USA, where fisheries are valued over US$750M annually. Georgetown Lake (Montana, USA) is renowned for its salmonid fishery. Although many anglers target kokanee (Oncorhynchus nerka) in Georgetown Lake, the body length of kokanee has typically been considered unsatisfactory. To reduce the density of kokanee and increase the average size, Montana Fish, Wildlife & Parks (MFWP) began stocking piscivorous Gerrard strain of rainbow trout (Oncorhynchus mykiss; hereafter Gerrard) in 2015 to consume kokanee. To assess the efficacy of biocontrol through the introduction of a piscivore to improve the size structure of kokanee, I used diet composition to determine the amount of predation on kokanee and to understand the feeding ecology of all potential predators. There was extremely low prevalence of piscivory and no evidence of Gerrards consuming kokanee. Gerrards exhibited a generalist feeding strategy and there was dietary and niche overlap and no difference in trophic position among Gerrards and trout. These findings highlight the unpredictability of predator-prey dynamics and the importance of evaluating management interventions, such as biocontrols. Additionally, this popular fishery could be in jeopardy because air temperatures in the region have warmed at twice the global average, leading to warmer water temperatures that could affect the thermal suitability for salmonids. Increased water temperatures can have sub-lethal effects, influencing growth, metabolism, and feeding rates of fish. Bioenergetics models were used to simulate the effects of warming water temperature on food consumption and growth for rainbow trout and kokanee within Georgetown Lake. My findings indicate that kokanee are more sensitive to warming than rainbow trout. While both species experience growth challenges as water temperatures exceed their optimal ranges, kokanee are particularly vulnerable, requiring higher food consumption to meet basic metabolic needs under elevated water temperatures. Thus, kokanee are likely to experience greater declines in growth compared to rainbow trout. Climate change will pose challenges for freshwater fisheries management, thus understanding how projected warming water temperatures may affect popular recreational fisheries can provide managers with information to establish reasonable expectations for fish growth.