Does thermal history influence thermal tolerance of the freshwater fish Galaxias zebratus in a global biodiversity hotspot?

Compared to other climate regions of the world, Mediterranean regions are likely to experience more severe effects of climate change as rainfall decreases and temperatures increase. Global climate change models predict a reduction in rainfall and rise in the temperature of rivers in South Africa's Cape Fold Ecoregion (CFE) – a Mediterranean region in the south-west corner of the country. In the climate change context and with the prediction that stream temperatures will increase, determining thermal thresholds of freshwater fish in relation to their thermal history is a key element in understanding the potential impacts climate change and a rise in stream temperatures will have on already threatened freshwater fish. The ability of aquatic species to withstand warming in the CFE will be determined by its upper tolerance limit (critical thermal maximum, CTmax). A fish's CTmax can be influenced by several factors, including the thermal characteristics of its environment (thermal history). In this study, we set out to examine whether the thermal tolerance of an endemic CFE fish species, the Cape Galaxias, Galaxias zebratus Castelnau, 1861, is influenced by its thermal history. We hypothesised that CTmax of G. zebratus individuals from warmer sites will have higher CTmax values than individuals from cooler sites. Hourly in situ water temperature data were collected and the CTmax values were determined for fish (n = 15 per site) from ten sites on six rivers on the Cape Peninsula of the CFE, and regression analyses revealed that G. zebratus upper thermal tolerance limits are indeed significantly influenced by its thermal history (as characterised by the seven day moving average of daily mean, Mean_7). CTmax was positively related to the thermal history of the rivers, indicating that G. zebratus, and potentially other stenothermic CFE freshwater fish species, is likely to be particularly vulnerable to climate warming. These findings broaden our understanding of thermal history patterns on fish thermal tolerances in Mediterranean rivers, and support G. zebratus conservation by determining its biological temperature thresholds and thermal requirements. Thermal data should be used to monitor and manage stream temperatures to ensure Mediterranean stenotherms can persist in their natural environment.