Effect of Three Hatchery Lighting Schemes on Indices of Smoltification in Chinook Salmon

This experiment examined whether photoperiodic changes induce chinook salmon Oncorhynchus tshawytscha to alter various indices of smoltification. The effect of 24 h light, a constant photoperiod of 9 h light: 015 h dark (9L:15D), and a naturally increasing photoperiod on plasma levels of thyroxine and cortisol, hematocrit, condition factor (K), and hepatosomatic index (HSI) was tested during the period of smoltification in chinook salmon. The 24-h-light group grew faster than the other two groups, but was significantly larger than one of the two groups on only three sampling dates. Mean plasma thyroxine and cortisol levels were highest in the natural-photoperiod group and lowest in the 9L:15D group. Mean plasma cortisol levels increased significantly in the increasing and 9L:15D photoperiod groups but were unchanged in the continuous-light group. Mean HSI decreased faster in the natural-photoperiod group than in the other two groups. Mean hematocrit did not change in the natural-photoperiod group but decreased in the other two groups. Mean K decreased in the natural-photoperiod and 24-h-light groups, but not in the 9L:15D group. The natural-photoperiod group clearly showed a more coordinated and complete smoltification based on the indices measured: highest mean and peak levels of both thyroxine and cortisol, greatest decrease in HSI, greatest decrease in K (along with the 24-h-light group), and least decrease in hematocrit. These results show that a natural photoperiod is beneficial and that continuous light or a short, unchanging photoperiod are detrimental to smoltification in chinook salmon. Hatcheries should account for photoperiodic effects when raising chinook salmon, particularly indoors.