Effects of Photoperiods on the Formation of Otolith Increments in the Embryonic and Larval Rainbow Trout Salmo gairdneri

Abstract

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.