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.

Variation in thermal tolerances of native freshwater fishes in South Africa's Cape Fold Ecoregion: examining the east-west gradient in species' sensitivity to climate warming.

The Cape Fold Ecoregion (CFE) is a biodiversity hotspot with high levels of endemism in its freshwater fish fauna. This study examined inter and intra-specific variation in critical thermal maxima (TCmax ) for eight native species of freshwater fish from the CFE. Cape galaxias Galaxias zebratus, Breede River redfin Pseudobarbus burchelli, Berg River redfin Pseudobarbus burgi, Clanwilliam redfin Pseudobarbus calidus and fiery redfin Pseudobarbus phlegethon were the most thermally sensitive (TCmax = 29.8-32.8°C). Clanwilliam rock-catfish Austroglanis gilli, Eastern Cape redfin Pseudobarbus afer and Cape kurper Sandelia capensis were moderately sensitive (TCmax = 33.0-36.8°C). An increase in intra-specific thermal sensitivity of S. capensis was observed from east to west. The results were related to in situ water temperature, which influenced TCmax for all species, suggesting that thermal history is a major driver of variation in thermal tolerance amongst populations. These thermal tolerance data for freshwater fishes in the CFE demonstrate that resilience to climate warming follows a geographical cline and that the more sensitive western species and regions are conservation priorities.

Towards setting environmental water temperature guidelines: a South African example.

Water temperature is a primary factor affecting the number and kinds of species in a stream. A key step towards including water temperatures in environmental flow assessments is to develop metrics which describe natural variability in a river's thermal regime. This is best achieved using time series analyses, where metrics are defined based either on time series disaggregation, or shapes of regimes defined using agglomerative techniques. The aim of this paper was to refine approaches in setting environmental water temperature guidelines for inclusion in defining environmental flows assessments. Annual water temperature series from 82 sites sampled across 48 rivers (mainstems and tributaries) in ten catchments in the southern Cape region of South Africa were described using 39 metrics based on the magnitude, frequency, duration and timing of thermal events. Sites were classified into thermal groups using their similarity in multivariate temperature regime and variation amongst groups along important temperature gradients examined. Deviation from a natural range of variability using a thermal confidence envelope is a suitable approach for broad evaluation of thermal guidelines. The approach presented can be applied at multiple levels of complexity to assess which elements of a thermal time series fall outside of reference conditions. Further steps in this approach are to link thermal patterns to biotic metrics, and gain a clearer understanding of interactions between flows, temperatures and biota, particularly below impoundments. Research on improving approaches in defining thermal regions is recommended.