Changes in swimming depths of Atlantic salmon Salmo salar post-smolts relative to light intensity

Eight hatchery-reared Atlantic salmon Salmo salar post-smolts, implanted with acoustic depth sensing transmitters and manually tracked for 5-12 h in the Hardangerfjord (Norway), spent most of their time (49-99%) at 1-3 m depth during the day, whereas four of seven fish tracked were found close (<0.5 m) to the surface at night, with a strong negative cross-correlation between general swimming depth and surface light intensity. Hence, the actual swimming depth of post-smolts during their early marine migration may depend on the light conditions, although the individual variation in vertical movement pattern was large. No cross-correlations were found between light intensity and swimming depth during daytime periods with rapid changes in light intensity, indicating that other factors than light intensity were important in initiating the irregular dives that were recorded down to 6.5 m depth. Key words: acoustic telemetry; behaviour, salmonid; vertical movement. 2 The period of sea entry and the first weeks of the marine life of the Atlantic salmon Salmo salar L. is considered as a critical period for their survival, and thus the total production of this species (Montevecchi et al., 2002; Rikardsen et al., 2004). Several studies have shown that post-smolt