Effect of body size on heat tolerance of a freshwater catfish (Trichomycterus areolatus).

Rivers are under intense anthropogenic pressure, leading to increases in water temperature and changes in physicochemical properties, which threaten aquatic biota. Understanding how these environmental changes affect heat tolerance in freshwater organisms is critical for assessing the status of wild populations and predicting their vulnerability under global warming scenarios. Here, we studied how body mass and heat tolerance, measured by thermal death time (TDTs) curves under normoxic and hypoxic conditions, vary among populations of the Chilean pencil catfish Trichomycterus areolatus inhabiting a Mediterranean river in central Chile. We detected significant differences in fork length, body mass and Fulton's condition factor among populations, with fish from reference sites being significantly larger and in better condition. Although heat tolerance did not differ among populations, we found a strong effect of body mass under both normoxic and hypoxic experimental conditions. Simulations combining laboratory-derived TDTs with field-recorded water temperatures suggest that the window of vulnerability occurs at lower temperatures but over longer exposures, indicating that heat stress has chronic effects on T. areolatus. Accordingly, the cumulative survival simulation using the warmer season records is predicted to be lower in river sections with reduced levels of dissolved oxygen. While our results did not show population level differences in thermal tolerance per se, the significant effect of individual body mass may translate into varying vulnerability among populations, given their marked differences in body mass distribution. These findings highlight how the interplay between water quality, body condition and heat tolerance shapes the vulnerability of T. areolatus populations to warming. Thus, an integrated perspective is essential to properly assess the impact of global warming on wild freshwater populations.