Abstract:

Northern organisms exhibit a range of physiological, behavioural, and ecological strategies to both survive and exploit seasonal cycles in biotic and abiotic conditions. However, our baseline understanding of these seasonally motivated strategies in northern freshwater fishes remains quite limited, regardless of its potential utility for predicting responses to shifting seasonal environments due to anthropogenic disturbance. In this dissertation, I investigated how fish respond to the highly seasonal energetic demands associated with reproduction across populations and environmental conditions. Specifically, I asked: How does the seasonal phenology of abiotic cues: (i) shape the allocation of energy to reproduction, (ii) combine with biotic factors to motivate flexible digestive capacity, (iii) facilitate the availability of novel and potentially critical resources, and (iv) motivate individuals to express coordinated phenotypic flexibility/plasticity across multiple organ systems and populations? First, in Ch. 1, I used existing data from field and experimental studies on pumpkinseed sunfish (Lepomis gibbosus) to investigate how early spring warming affects the reproductive process. I identified a novel regulatory mechanism involving pre-reproductive preparation in the liver (termed the “hepato-gonadal shunt”), which was largely inhibited by earlier warming and putatively present in 20 other north-temperate fish species. Then, I conducted biweekly field sampling across five pumpkinseed populations to investigate how seasonal patterning in foraging, structural modifications, and energy accumulation across multiple somatic tissues shape reproductive outcomes. In Ch. 2, I identified seasonal digestive flexibility as a consistent pattern across pumpkinseed populations that responded to elevated energetic demands associated with reproduction. In Ch. 3, I used diet data to evaluate the potential for invasive terrestrial invertebrates to temporally subsidize fish populations, identifying novel population- and water level-dependent trophic interactions. Finally, I evaluated the contributions of seasonal somatic flexibility to reproductive outcomes. Here, the extent of observed flexibility, especially in the digestive tract, mediated population-level reproductive allocation. Altogether, the capacity for flexibility and reproductive investment varied among fish populations and appeared sensitive to the timing of spring warming. These new findings contribute to our understanding of both recruitment variation among northern freshwater fish species and their sensitivity to changes in seasonality due to climate change.