The causes and consequences of sex differences and body size variation in fish

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Energy links physiology, behaviour, growth, and reproductive output in all animals. In ectotherms like fish, variation in temperature is a key driver of energy use (metabolism), which can affect how animals take in and transfer nutrients through their ecosystem. The recent proliferation of telemetry tracking, animal-borne data loggers, and respirometry provides an opportunity to make our understanding of animal energetics more powerful in ways that can help us manage fisheries in a warming climate. One knowledge gap in fish energetics is how energy use and its relationship with temperature differs between male and female fish, and among fish of different size/age classes. In many animals including some fishes, sex and body size are connected because of sexual size dimorphism (SSD). In fishes, SSD can occur in either direction, but more commonly there is evolutionary pressure for females to grow larger than males because large females have disproportionately higher reproductive output (i.e., reproductive hyperallometry). For example, in walleye-among the most important fishes in North America-females reach sizes about 20% larger than those of males. While there is a widely accepted evolutionary explanation for SSD, we know surprisingly little about how females achieve higher growth. The `mystery' of SSD is relevant to ecological processes but also to unknowns about fish growth and body size variation, including the problem reproductive hyperallometry poses for existing growth models and a lack of clarity about what mechanisms are causing fish to `shrink' with climate warming. Collectively, there remains considerable room for discovery about processes driving growth, variation in body size, and their connection to sex differences and thermal tolerance in fishes. My program has a focus on wild freshwater fishes and will quantify patterns of thermal tolerance and preference, growth, feeding ecology/physiology, and metabolic rate in wild fishes in field settings and in laboratory experiments. The program will focus partly on walleye and yellow perch as models (two species that exhibit SSD), in addition to the invasive round goby (SSD in the reverse direction; males grow to be larger) and stream-resident brook trout (no SSD, and a contrasting coldwater life history). The program consists of three overall projects. The first project will investigate the energetic mechanisms underlying sexual size dimorphism in walleye and yellow perch, using a comparative field- and lab-based approach. The second project focuses on using bioenergetic models, updated based on the data obtained in the first project, to predict changes to walleye populations with climate warming and explore the impacts of SSD on food webs. The third project focuses in on the question of how thermal performance windows (also called thermal envelopes) change across sizes, life stages, and between sexes in species with and without SSD.