Abstract
Increasingly, cold-water pollution (CWP) is being recognised as a significant threat to aquatic communities downstream of large, bottom-release dams. Cold water releases typically occur during summer when storage dams release unseasonably cold and anoxic hypolimnetic waters, which can decrease the temperature of downstream waters by up to 16°C. Depending on the release duration, these hypothermic conditions can persist for many months. The capacity of ectothermic species to tolerate or rapidly adjust to acute temperature changes may determine the nature and magnitude of the impact of CWP on affected species. This study assessed the impacts of an acute reduction in water temperature on the physiological function and locomotor performance of juvenile silver perch (Bidyanus bidyanus) and examined their capacity to thermally compensate for the depressive effects of low temperatures via phenotypic plasticity. Locomotor performance (Ucrit and Usprint) and energetic costs (routine and maximum metabolic rate) were measured at multiple points over a 10-week period following an abrupt 10°C drop in water temperature. We also measured the thermal sensitivity of metabolic enzymes from muscle samples taken from fish following the exposure period. Cold exposure had significant depressive effects on physiological traits, resulting in decreases in performance between 10% and 55%. Although there was partial acclimation of Ucrit (~35% increase in performance) and complete compensation of metabolic rate, this occurred late in the exposure period, meaning silver perch were unable to rapidly compensate for the depressive effects of thermal pollution. The results of this study have substantial implications for the management of cold water releases from large-scale dams and the conservation of native freshwater fish species, as this form of thermal pollution can act as a barrier to fish movement, cause reduced recruitment, ecological community shifts and disruptions to timing and success of reproduction.
Highlights
The role of temperature as a key environmental driver of physiological performance in ectotherms has been recognised for decades; the capacity for species to adjust and respond to temperature changes within their environment is increasingly acknowledged as an important trait that can be used to model the threats posed to organisms from anthropogenic climate change (Seebacher et al, 2015)
There was no significant interaction between acclimation. Both acute and prolonged exposure to cold temperatures, similar in magnitude to those occurring during cold-water pollution (CWP) events, had substantial negative effects on physiological performance in juvenile silver perch (B. bidyanus)
Juvenile silver perch had some capacity to compensate for the persistent effect of low temperature on Ucrit, Routine metabolic rate (RMR) and maximal metabolic rate (MMR), this capacity did not manifest until at least 7 weeks of exposure, suggesting that CWP-like reductions in water temperature prevent fish from rapidly compensating for the depressive effects of low temperature on performance
Summary
The role of temperature as a key environmental driver of physiological performance in ectotherms has been recognised for decades; the capacity for species to adjust and respond to temperature changes within their environment is increasingly acknowledged as an important trait that can be used to model the threats posed to organisms from anthropogenic climate change (Seebacher et al, 2015). The release of cold water from hypolimnetic layers of large dams, termed cold-water pollution (CWP), has been identified as a key threatening process in freshwater systems (Lugg and Copeland, 2014). The effects of CWP can extend for up to 350 km downstream from the release point of the dam causing reductions in water temperature of up to 16◦C (Preece, 2004; Lugg and Copeland, 2014) and are likely to have contributed substantially to the decline of a large number of native fish species (Boys et al, 2009)
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