Uncertainties regarding the natural mortality of fish can increase due global climate change

The increase in temperature resulting from global climate change can directly affect the survival of fish and therefore population parameters such as natural mortality (M). The estimation of this parameter and the understanding of the uncertainties in its estimates are enormous challenges for studies that evaluate fish stocks. In addition, the effects of increases in temperature may be associated with life strategies. Therefore, the fuzzy set theory was used to evaluate the effects of temperature increase on the natural mortality of fish, considering different life strategies. The model showed that the increase in temperature increased the uncertainties in M estimates for all species, regardless of the life strategy. However, opportunistic species present greater uncertainties in estimates of M compared to equilibrium species. The patterns found in uncertainties of M associated with species groupings by life strategies can be used in holistic approaches for the assessment and management of recently exploited fisheries resources or for those with limited biological data.

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, CT_max). A fish's CT_max 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 CT_max of G. zebratus individuals from warmer sites will have higher CT_max values than individuals from cooler sites. Hourly in situ water temperature data were collected and the CT_max 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). CT_max 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.