The polar regions are experiencing climate change at the fastest rates on Earth and serve as bellwethers for the profound threats facing species, ecosystems, and physical processes worldwide due to uncurbed anthropogenic greenhouse gas emissions. My dissertation research focuses on early life stages of Antarctic fishes, which are thought to be particularly vulnerable to climate change due to their unique evolutionary history and specialization to their stenothermal habitat. I used a comparative framework, examining four closely related species in the Nototheniidae family – Trematomus bernacchii, Trematomus pennellii, Trematomus nicolai, and Pagothenia borchgrevinki – to understand how subtle interspecific variation in traits may impact species-specific performance under projected future ocean conditions. I first measured basal characteristics across the four species, as very little is known about Antarctic fishes at young life stages, focusing on metabolic traits and the exploration-avoidance axis of behavior, two key dimensions of species fitness and drivers of niche differentiation. While basal metabolic demands appeared relatively conserved across species at the juvenile life stage, I found divergent behavioral strategies that could be a critical driver of niche differentiation in Antarctic fish assemblages. T. bernacchii and T. pennellii showed risk-prone behavior, T. nicolai showed avoidant behavior, and P. borchgrevinki showed cautious exploratory behavior. I also observed a potentially conserved freezing strategy in response to novelty, which, when paired with in situ observations, indicates that freezing may be an important predator avoidance strategy in these fishes. I then focused on the two ‘risky’ species – Trematomus bernacchii and Trematomus pennellii – to explore how acclimation to projected future ocean warming and ocean acidification conditions may impact their risk-prone behavior. While acclimation to warming and elevated pCO₂ affected behavior in both species, the effect sizes of pCO₂ were small, and warming was the driving force behind behavioral modifications. In both species, fishes acclimated to ocean warming conditions demonstrated reduced exploratory activity and showed indications of neophilia. These responses amplified over time, and T. pennellii demonstrated a stronger response (i.e., effect sizes) in both behaviors. Consistent with previous physiological and behavioral studies, while limited, our results support the inference that T. pennellii have a particularly risk-prone strategy when faced with novelty that is amplified when acclimated to warming. My final chapter proposes a novel ‘ice reef’ framework and emphasizes how three-dimensional ice habitat formed by platelet, anchor, and brinicle ice may function as critical nursery and refugia habitats for young polar fishes. Drawing on in situ observations and the literature, I discuss the recurring behavioral, physiological, and morphological features across a diversity of polar fishes, suggesting ice-associated and ice-obligate life history strategies may be much more widespread than previously acknowledged. As climate change rapidly alters ice phenology and stability, the loss of ice reefs could jeopardize fish recruitment, community resilience, and key ecosystem services. This perspective underscores the urgent need to study ice reefs before they disappear altogether.
Frazier A. J., 2025. Antarctic fishes in a changing climate: a comparative approach to predicting species-specific futures. PhD thesis, UC Davis. Thesis (embargoed).
