Will I survive? Assessing the thermal tolerance of an estuary-dependent mugilid, Chelon dumerili, in a changing climate

Abstract

Determining the thermal tolerance limits of fishes is crucial for understanding their persistence in the face of anthropogenic climate change. This study used dynamic critical thermal assays to determine the upper (CT_max) and lower (CT_min) critical thermal limits of an estuary-dependent mugilid, the grooved mullet Chelon dumerili, through laboratory experiments with individuals captured from the Kowie Estuary on the south coast of South Africa. Fish were exposed to simulated heating and cooling events, and their ventilation rate (opercular beats per minute) was evaluated at every 2 °C of incremental change to assess the thermal stress response of this species prior to reaching the equilibrium endpoint. The CT_max range of the individuals was 35.1–37.9 °C (mean ± standard deviation: 36.45 ± 0.56 °C), and the CT_min range was 5.4–7.6 °C (6.63 ± 0.64 °C), resulting in a broad thermal range (difference between CT_max and CT_min) of 28.7–30.4 °C (29.4 ± 0.5 °C). Despite a broad thermal range, break-point analysis of the linear relationship between ventilation rate and water temperature indicated that the species has a narrower thermal range because the experimental fish experienced thermal stress prior to reaching the critical thermal limits (mean upper and lower break points of 30.8 °C and 10.7 °C, respectively). Our findings suggest that water temperature changes driven by climate change may reduce fish performance near a species’ thermal break points, potentially impacting its population dynamics.