Micro- and macro-habitat selection of Atlantic salmon, Salmo salar, post-smolts in relation to marine environmental cues

Atlantic salmon is an economically and culturally important species. The species encounters several natural and man-made threats during its migration between fresh water and the ocean, which in combination may explain its ongoing decline. With the aim to better understand whether post-smolt behaviour is influenced by physical oceanographic conditions, the migratory behaviour of 173 post-smolts in a high-latitude Norwegian fjord was investigated, combining acoustic telemetry with site- and time-specific environmental variables from an oceanographic model. Most post-smolts (94%) performed a unidirectional migration out the fjord. Progression rates were relatively high (0.42-2.41 km h(-1); 0.84-3.78 BL s(-1)) and increased with distance from the river. While post-smolts had an affinity for lower salinities in the inner fjord, statistical models failed to detect any significant relationship between the small-scale (within arrays) migratory behaviour and salinity, temperature, or coastal surface currents within the fjord. In the outer part, the post-smolts predominantly exited the fjord system through the strait with the highest surface salinities and lowest temperatures, independently of the current direction. Our findings indicate that the macro-habitat selection of the Atlantic salmon post-smolts was influenced by environmental factors: the post-smolts directed their migration towards ocean cues. However, this was not confirmed on the micro-habitat level.

Automated summary

This study investigated the migratory behavior of Atlantic salmon post-smolts in a high-latitude Norwegian fjord. The majority of post-smolts migrated unidirectionally out of the fjord at relatively high speeds, with progression rates increasing with distance from the river. While post-smolts showed an affinity for lower salinities in the inner fjord, their exit from the fjord system in the outer part was influenced by higher surface salinities and lower temperatures. The findings indicate that macro-habitat selection of post-smolts is influenced by environmental factors, although micro-habitat level relationships were not confirmed.