Variations in distribution and mobility of Atlantic salmon parr during winter in a small, steep river

In many rivers, natural variations in water discharge are affected by flow regulation related to hydropower production. Due to increased energy demand, there has been increased development of mini hydropower stations on small, steep rivers. As the majority of previous research has focused on experimental streams or larger river systems, knowledge on the effects of hydro regulation on small rivers, in particular during winter, are scarce. In order to increase our understanding on the winter ecology of Atlantic salmon parr (Salmo salar L.), we studied parr distribution and mobility in a small (average wetted width: 10.7 m), steep (average riverbed gradient: 1.3%) and unregulated river throughout winter. We tagged 145 parr using Passive Integrated Transponder (PIT) technology. During autumn, freeze-up, mid winter and late winter, parr were tracked using manual tracking devices. Each time an individual was found, its position was geo-referenced. Water discharge and water temperature were recorded continuously throughout the study period. Forty-eight parr were observed at least once during all four tracking surveys. Of these, only thirty-one were observed more than once during every survey. Our results indicate that (1) in comparison to autumn, parr moved closer to the shoreline during winter; (2) parr performed longer movements during autumn and freeze-up than during mid winter and late winter; and (3) within the range of naturally occurring fluctuations, no differences in the mobility pattern of parr was observed in relation to water discharge. However, parr moved less at low water temperatures. Further, we discuss the results on distribution and mobility of parr during winter in perspective to the potential effects of hydro regulation.