Effects of 5 different pkotoregimes on otolith increment formation were observed in embryonic and larval rainbow trout. Fertilized eggs were incubated within 4h under photoperiods of 6-h light and 6-h dark (LD 6:6), LD 12:12, and LD 24:24 at about 10.3°C Other eggs at l week post-fertilization were incubated under constant light (LL) or constant darkness (DD). They were sampled at hatching and swim-up, and the number of otolith (sagitta) increments were counted under transmitted light. Sagittae first appeared as multiprimordia 1 or 2 days before the initia-tion of eye pigmentation and the initial increment was laid down at the eyed stage. The mean counts of otolith increments were highly correlated with each event of the light-dark cycles and expressed by the regression lines: Y=1.96X+29.25 for LD 6:6, Y=1.04X+14.39 for LD 12:12, and Y=0.53X+6.83 for LD 24:24 groups, where Y signifies the count of increments and X re-presents the number of days elapsed (X=O at hatching). Increments were also found under LL and DD, but the counts were variable for each individual. The mean rate of increment deposi-fion from hatching to swim-up was about 1.2 and 1.4 per day in the LL and DD groups, respec-tively. These results indicate that photoperiod works as a potent entrainer for the rhythmic formation of otolith increments in embryonic and larval rainbow trout. Some stimuli other than photoperiod may also be capable of inducing otolith increment formation.
The Japanese Society of Fisheries Science