Mid-winter activity and movement of Atlantic salmon parr during ice formation events in a Norwegian regulated river

A telemetry study in a Norwegian regulated river was conducted through a 12-day period in mid-winter 2003. The objective was to study activity (defined as number of movement per hour) and movement (defined as distance moved per hour) during different ice formation events. Twenty-four Atlantic salmon (Salmo salar L.) parr were radio tagged and continuously monitored by both manually tracking (N = 24) and by fixed recording stations (N = 15). Detailed data on climate, flow and ice formation and its spatial distribution were collected and used in the analyses. Fish activity was not found to be affected by their size (L (F)). There was a significant difference in activity between diel periods with highest activity during dusk (5-6 p.m.). Between high and low flow (mean +/- SD, 21.1 m(3) s(-1) +/- 1.7 SD and 11.1 m(3) s(-1) +/- 1.7 SD, respectively) no significant difference in activity was found. During the experiment extensive anchor ice growth occurred mainly in the riffle part with thickness up to 50 cm. Juveniles tend to avoid riffle section during anchor ice formation and exploited ice covered areas, indicating critical and preferable habitats respectively. Further, a significant difference in movement was found between five selected ice events with highest mean movement during an anchor ice event and lowest mean movement during an ice break up with no anchor ice formation. No significant difference in activity or movement between parr exposed to frazil ice and parr not exposed were found.