Assessing the thermal tolerance of a cosmopolitan eurythermal species, the flathead mullet Mugil cephalus

Abstract

Water temperature is a key abiotic factor affecting fish physiology, behaviour and distribution, with fluctuations impacting species assemblages and habitat use. The flathead mullet Mugil cephalus is a eurythermal species with a cosmopolitan distribution across tropical, subtropical and temperate coastal environments, and is prevalent in South African estuaries. However, little is known about its thermal limits in temperate regions, where future climate change is expected to increase the frequency and intensity of temperature extremes. This study quantified the lower and upper critical thermal limits (CT_min and CT_max) of M. cephalus from the warm-temperate Kowie Estuary, Eastern Cape Province, South Africa. Using the critical thermal methodology, 14 juveniles and 1 sub-adult M. cephalus were exposed to simulated heating and cooling events in a controlled laboratory setting. The results showed that M. cephalus that were acclimated in seawater (salinity 35) at 18 °C had a broad thermal breadth of 34.4 °C, with a CT_min of 3.2 °C and CT_max of 37.7 °C. Despite this wide thermal tolerance, physiological stress was observed at 13 °C and 29 °C—temperatures that align with seasonal extremes (the minimum winter and maximum summer water temperatures recorded) at the estuarine–riverine interface of the in situ environment. These findings suggest that while M. cephalus is unlikely to face its thermal limits in this estuary, some individuals may experience thermal stress, potentially influencing their seasonal habitat use. Global rising temperatures are projected to negatively impact estuarine species through increased intensity and duration of thermal stress. Therefore, understanding species’ physiological thresholds is crucial for predicting how individual species and entire ecosystems will respond to climate change, and for developing appropriate management strategies under shifting environmental conditions.