The timing of emergence of Atlantic salmon and rainbow trout, incubated on different substrates
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Atlantic salmon (Salmo salar) and rainbow trout (S. gairdneri) eggs were incubated in plexi-glass aquariums. After hatching, alevins were kept in darkness, two groups of each species without substrate, two groups in gravel and two groups in Astroturf artificial substrate. Every sixth day after hatching, individual swimming height above the bottom were monitored with a video recording system. The use of infrared light made it possible to observe the positions in the water column in darkness. Mean swimming height in a coordinate system and percentage occurrence in specific heights above the bottom were used as measures on emergence from the bottom. Emergence was more conspicuous among the rainbow trout alevins (196 - 226 day degrees) than Atlantic salmon alevins (335 - 400 day degrees). After emergence, the rainbow trout alevins swam close to the water surface, and 50 % of them became photopositive on day 39 (269 day degrees). The Atlantic salmon stayed closer to the bottom after emergence, and they became heedless to light at this stage. This reflect different evolutionary strategies. Neither substrates nor feeding made influence on overall timing of emergence.