The impacts of stress on sodium metabolism and copper accumulation in a freshwater fish

In freshwater fish, stress can often result in significant modifications to Na⁺ metabolism and may be an important aspect to consider in conservation efforts; as maintaining ion balance is critical to survival and ion transport is also a key determinant of metal toxicity. In order to better quantify the response of stress, Na⁺ influx, Na⁺ efflux, and copper accumulation were measured as a result of handling stress in inanga (Galaxias maculatus). This species is a culturally and economically important fish in New Zealand as one of the major species in the local 'whitebait' fishery. Na⁺ influx rates in inanga were found to be 2-3 times greater after handling than in 'recovered' fish, and Na⁺ efflux rates increased in the range of 5-6 times. Both influx and efflux rates quickly returned to resting levels within 24h. Increases in Na⁺ efflux were strongly correlated with opercular beat frequency. This suggests an increas in ventilation, and subsequent enhanced diffusive loss of Na⁺, as the mechanism of increased Na⁺ efflux. Total body copper levels were also measured under similar treatments. Fish had significantly higher levels of copper directly after handling than following a 24h recovery; likely due to a shared Na⁺/copper uptake pathway. As accumulation is linked to toxicity, fish exposed to elevated copper levels in stressful environments will consequently be more at risk to metal toxicity. In a natural environment, stress can come from many different sources; among which, anthropogenic disturbances can often be a cause. Given that inanga must migrate through metal-contaminated coastal regions to reach breeding habitats, they will be exposed to toxicants under conditions where perfusion and ventilation of the gill is increased. As such, ion loss would be exacerbated, leading to an enhanced compensatory ion uptake and an increase in accumulation of ion-mimicking toxicants such as copper, exacerbating toxicity. This is a concern as conservation efforts in more disruptive environments may not be adequately protected.