Mechanisms and significance of physiological plasticity in winter-dormant fishes: a model to understand constraints on performance in cold environments

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Low temperature in winter, and accompanying food scarcity, are key bottlenecks on the persistence of fishes at poleward latitudes, yet a detailed mechanistic understanding of winter responses in fishes remains elusive. Research on overwintering and cold adaptation of fishes has focused on winter-active species (eg. salmons, cods); much less is known about the many ecologically-important species that undergo winter dormancy (e.g. temperate wrasses, sunfishes). Winter dormancy is an inactive, sheltering, non-feeding, and low-metabolism state to cope with harsh winter conditions. Understanding winter dormancy is important: it is a key seasonal event that can comprise up to half the fish's lifespan and temperature constraints on activity and energy budgets can have major consequences for the health and persistence of fish populations. The objective of this grant is to reveal the significance and underlying mechanisms of winter dormancy in fishes. Based on the evolutionary patterns of winter dormancy, I propose that it is a novel strategy evolved by certain lineages of warm-adapted' fishes to extend their poleward geographic range, providing an ideal model to study physiological constraints on cold adaptation and geographic limits. To evaluate this overarching hypothesis, we will investigate 3 major questions. (1) What are the physiological & behavioural mechanisms underlying winter dormancy? We will use an integrative approach to elucidate the roles of behavioural activity and cellular function in setting the low metabolic rate that is characteristic of winter-dormant fishes. (2) Why does winter dormancy occur? First, we will test whether critical aerobic (cardiorespiratory & swimming) performance of winter dormant fishes is greatly constrained in the cold, thus obligating entry to dormancy possibly because of an inability to forage or flee predators. Then, by measuring in vivo feeding rates, growth processes, and underlying mechanisms in active and dormant fish fed variable ration or force-fed, we will also test whether winter dormancy is a facultative behaviour to avoid energetically-inefficient foraging when food is scarce, or an obligatory consequence of thermal limitations on food assimilation. (3) How have developmental conditions, phenotypic plasticity, or adaptation to the local environment set different seasonal temperature thresholds for winter dormancy among latitudinally-separated populations and is it a result of interpopulation variation in thermal performance? The proposed research will address fundamental knowledge gaps about how ecologically-relevant stressors (e.g. temperature, food) shape the seasonal physiology and behaviour of fishes. The information gained will improve our understanding of how physiology sets species distributions and will aid efforts to predict if fish populations will shift or expand ranges with climate change.