Age-specific aggregation of wild juvenile Atlantic salmon Salmo salar at cool water sources during high temperature events

This study quantified the use of cool water sources by wild 0+, 1+ and 2+ year Atlantic salmon Salmo salar during high water temperatures (i.e. >23° C) in summer 1995 and 2004. During these events, 0+ year Atlantic salmon did not aggregate or increase in abundance in cool water sites. Interestingly, 1+ and 2+ year Atlantic salmon numbers increased in cool water sites. In addition, these older juveniles formed numerous, discrete aggregations along the plume created by a tributary with aggregation locations being similar between years. Aged 2+ year fish aggregations were at the coolest sources whereas 1+ year aggregations were in locations cooler than the main river. Fish in aggregations on average used deeper sections (average depth: 380 mm) compared with the coolest available habitat in the thermal plume (average depth: 230 mm). Hence, during high temperature events, older juvenile Atlantic salmon moved to cooler water sites and then aggregated in deeper microhabitats.