Evidence That Wild Salmonids Seek Cool Water Refuges to Reduce Parasite Virulence: The Proliferative Kidney Disease Case

Abstract

Conservation of cold-stenothermic freshwater fish populations relies on local management to mitigate effects of climate change. With limited opportunity to migrate, freshwater fish are increasingly exposed to excessive warmth during summer heat waves and are susceptible to temperature-associated diseases. As a case example, a significant proportion of salmonids are infected with a salmonid parasite that causes proliferative kidney disease (PKD), which is associated with high mortality when water temperatures remain above 15°C for several weeks. We hypothesised that wild trout would actively migrate to cooler water when temperatures exceeded such temperature levels. Such ‘chill behaviour’ would allow infected fish to survive periods of intense heat by providing a means of controlling the virulence of PKD in diseased fish. Three locations were selected at the confluence of a stream with summer water temperatures of 15°C–24°C and a 1°C–7°C cooler tributary. The selected streams are known to harbour PKD. In these streams, wild brown trout were individually tagged with passive integrated transponders (PIT). Fish movements at the confluence were monitored from summer to autumn. Fish were recaptured in late autumn and checked for PKD and its causative parasite using inspection of the kidney and qPCR. Movement tracking revealed that the greater the temperature difference between the main stream and the cooler tributary, the more likely fish were to migrate to the cooler water downstream of the tributary and/or into the cooler tributary. Independent of this temperature effect, the level of PKD infection was found to be positively associated with trout migration from warm into cooler water. The study represents the first report of a ‘behavioural chill’ response in diseased wild fish. Such a response is in contrast and opposite to the more widely known ‘behavioural fever’ response. We suggest that this chill behavioural response is important for surviving PKD during periods of warm weather. Our results strongly underscore the importance of protecting accessible cool-water refuges in river systems for the conservation of healthy cold-stenothermic fish populations in the face of progressing climate change.