Intraspecific Variation: Sources and Implications for Fishes in a Changing Environment

Abstract

Freshwater fishes are facing increasing environmental stressors due to climate change. Among the growing list of impacts are temperature changes that significantly affect ectothermic species, such as fish, by influencing their physiological and ecological processes. Historically, research and management have primarily focused on species-level mean values for biological traits, often overlooking intraspecific variation (differences among and within populations). This oversight ignores the potential role of intraspecific variation in determining a species' adaptive capacity (its ability to cope with or adjust to changes) and sensitivity (its degree of vulnerability) to climate change, raising key questions: (1) How does intraspecific variation impact a species' response to climate change?, (2) What are the mechanisms driving variation in traits among populations?, (3) How can understanding intraspecific variation inform management and conservation decisions?

I adopted an interdisciplinary approach incorporating energetic, physiological, and molecular perspectives to answer these questions using walleye (Sander vitreus) as a focal organism. Walleye is a socially, economically, and ecologically important fish species in North America that is experiencing population declines attributed to climate change. I first approached intraspecific variation through a bioenergetics lens, considering the impacts of variation in physiological parameters on fish growth and consumption. Through a comprehensive literature review, I identified several gaps in current bioenergetics modeling approaches to assess variation in physiological traits. I then used simulations of the walleye bioenergetics model to demonstrate the importance of considering intraspecific variation in bioenergetic approaches and the utility of sensitivity analysis. I also experimentally assessed variation in the responses of juvenile walleye to rising temperatures through a series of metabolic experiments to determine their physiological sensitivity to acute and chronic temperature alterations. I found significant differences among walleye from three Michigan Department of Natural Resources rearing ponds across a latitudinal gradient, with those from the northernmost pond exhibiting significantly higher metabolic rates for acute and chronic increases in temperature. Additionally, I discovered significant differences in metabolic phenotypes between walleye from different ponds originating from the same wild population broodstock. Next, I investigated body composition as a potential mechanism driving variation in metabolic rates. By measuring the proximate composition of the walleye used in my respirometry experiments, I demonstrated that body composition and metabolic rates have a complex, interdependent relationship: body composition influences metabolic rates, and metabolic rates, in turn, affect body composition. Finally, I used a transcriptomics approach and analyzed differential gene expression in walleye exposed to chronic temperature increases. I found significant differences in gene expression across increasing temperatures and among the assessed populations, indicating varying sensitivity levels to chronic temperature exposures. By functionally enriching the differentially expressed genes, I identified pathways associated with metabolic processes and protein synthesis potentially indicative of chronic heat exposure.

My findings have important implications for fisheries management and broader conservation and management considerations across species, as they demonstrate that intraspecific variation leads to markedly different responses to climate change stressors. Additionally, my research provides insights that can guide fish stocking practices to produce fish that are more resilient to the projected impacts of climate change, thereby enhancing the survival rates of stocked fish. Ultimately, my research demonstrates the necessity of considering intraspecific variation across multiple traits when evaluating a species' adaptive capacity and sensitivity to climate change.