Delayed photoperiod produces more uniform growth and greater seawater adaptability in underyearling coho salmon ( Oncorhynchus kisutch)

Abstract

Coho fry were distributed among tanks in a factorial design of three temperatures (8, 11, and 14°C) by three photoperiod phases (natural, 1- and 2-month delayed) in February. Growth rate in fresh water was about 30% greater at 14°C than at 8°C. After 24 weeks of rearing, the coho on natural photoperiod at 14°C had separated into two distinct size modes; the lower mode contained fish smaller than 25 g and the upper mode contained fish larger than 35 g. At 8 and 11°C under natural photoperiod, the weight distributions were unimodal but were skewed toward larger sizes. In contrast, the weight distributions of fish under delayed photoperiod were much less variable and were not skewed. Periodic seawater challenge tests revealed a greater and more sustained seawater adaptability in coho reared under delayed photoperiod compared with natural photoperiod. Growth in sea water was assessed for groups of 50 fish transferred from 8°C fresh water to 12°C sea water in late July or August. Coho reared under delayed photoperiods grew about five times as fast in sea water as did those exposed to natural photoperiod. These results indicate that bimodal growth in juvenile salmon is a function of photoperiod phase at the time of first feeding and demonstrate the feasibility of producing underyearling coho smolts at temperatures as low as 8°C.