Synergistic effects of climate and invasions: a case study of juvenile Pacific salmon and their introduced freshwater predator (Esox lucius) in a changing Alaska river

Abstract

Climate change and invasive species are major individual drivers of declines in freshwater productivity and diversity, yet the potential for these factors to interact is poorly understood. I explored the synergistic effects of climate change and invasion to better understand observed changes in a freshwater community comprised of socially valuable salmonid fishes. I modeled daily predation and annual consumption of juvenile salmon by an introduced apex predator, Northern Pike (Esox lucius), as a function of time, streamflow, water temperature, and Northern Pike demography in a rapidly warming system in Southcentral Alaska. I hypothesized that Northern Pike predation would shift from juvenile salmon to alternative prey as a function of progressed invasion and that salmon predation would peak during species-specific out migrations. I also hypothesized that predation on juvenile salmon would be positively correlated with streamflow because high water would cause juvenile salmon to seek refuge in off-channel habitats instigating interaction with Northern Pike. Daily predation and annual consumption of Northern Pike in this system have shifted away from salmon-dominated to dominated by resident fishes over the past decade (2011 to 2022). Streamflow was positively correlated with daily predation on juvenile Chinook Salmon and total Oncorhynchus species in diets. Between 2011-2012 and 2021-2022 the population of Northern Pike shifted to younger individuals with increased growth. Estimates revealed that annual consumption of Chinook Salmon and Coho Salmon by Northern Pike decreased between time periods. Bioenergetic simulations revealed increases of 6 to 12.5% in total per capita consumption of prey by Northern Pike under end of century scenarios. These findings collectively suggest that Northern Pike have switched to alternative fish prey as salmon declined. Seasonal high streamflows and increased water temperature consistent with observed and predicted climate change in Southcentral Alaska appear to increase predation rates on juvenile Chinook Salmon by Northern Pike, while patterns in predation for juvenile Coho Salmon are more uncertain. Predicted future temperatures may increase total consumption of Northern Pike enough to appreciably increase mortality of salmon exacerbating changes in species composition and community structure and further jeopardizing struggling salmon runs. These findings suggest that invasive predators may also become more impactful in other systems undergoing rapid climate change.