Effects of photoperiod on growth and smolting in juvenile Atlantic salmon (Salmo salar)

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Juvenile Atlantic salmon were reared under simulated natural photoperiod (LDN) from first feeding in May until late August. Three 16-h daylength regimes were started in August or September and continued until November or December (LD16:8 A-N; LD16:8 S-N; LD16:8 S-D). A control group remained on LDN. The fish reared under LD16:8 regimes became significantly larger soon after the start of photoperiod manipulation; these differences increased during autumn and persisted until completion of smolting in May. The LD16:8 A-N regime had a significantly higher proportion of fish in the upper-modal group than all other treatment groups. Based on length > 13.5 cm, all LD16:8 regimes produced significantly more smolt-sized fish than LDN. Thyroid hormone (T3, T4) levels rose in October-November in all treatment groups and declined in December-January. T3 and T4 levels rose slightly in February. T4, but not T3, continued to rise in March. Pituitary somatotrop and thyrotrop activity (mitotic figures and granulation) was observed in all groups in September-October, with most activity in LD16:8 groups. There was within-group variation in somatotrop activity, possibly in relation to faster and slower growing fish. All groups had transitory, high levels of salinity tolerance in early December. Although gill Na+K+-ATPase activity and salinity tolerance rose earlier and to a greater extent in LDN than in LD16:8 treatments, all groups appeared to reach smolt status in May as judged by high survival and growth rates during 6 months in seawater. The LD16:8 S-D group appeared to complete smolting later than the LD16:8 A-N and S-N groups and LDN. Thus, the application of LD16:8 daily light cycles to salmon during their first autumn increases the proportion of fish in the uppermodal group (presumptive smolts) and has only minor effects on the timing of completion of smolting the following spring. © 1989.

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