Abstract
Chum ( Oncorhynchus keta), coho ( O. kisutch), ocean and stream-type chinook ( O. tshawytscha) salmon fry were reared from first feeding under constant 9.5 h or 14.5 h photoperiod for 2 months and then under simulated natural photoperiod for 4 months. The coho and stream-type chinook grew more rapidly and more uniformly both in fresh and in salt water after short than after long day treatment. In contrast, the chum and ocean-type chinook salmon grew well and adapted readily to sea water after either treatment. Plasma growth hormone levels in June or in August were not significantly different between photoperiod treatments in any of the species. It is concluded that juvenile coho and stream-type chinook require a period of exposure to short day photoperiod from first feeding in order to develop uniformly as zero-age smolts. This experiment showed that zero-age coho or stream-type chinook smolts can be produced using a 2-month constant short day photoperiod under simple lighting systems.
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