ABRUPT CHANGES IN PHOTOPERIOD AFFECT AGE AT MATURITY, TIMING OF OVULATION AND PLASMA TESTOSTERONE AND ESTRADIOL-17-BETA PROFILES IN ATLANTIC SALMON, SALMO-SALAR

Atlantic salmon (Salmo salar L.), reared in sea cages for 18 months (a ge 36 months from hatching), were exposed to natural light (NL, 61 deg rees N), or continuous additional light from January (ALJ) or March (A LM) until July. On July 13, the fish were moved to indoor raceways wit h brackish water (2-19 parts per thousand) and ambient temperature (de clining from 13.0 to 5.6 degrees C). Fish from each treatment were sub jected to either simulated natural photoperiod (SNP), continuous light (24L), or short photoperiod (8L = 8 L:16D), creating a total of nine experimental groups with approx. 50 fish in each. The proportion of se xually maturing females was reduced from 91% in the NL. groups, to 67% and 9% in the ALM and ALJ groups, respectively (p less than or equal to 0.005). A similar reduction was observed among the males, from 74% in the NL groups, to 57% and 16% in the ALM and ALJ groups, respective ly (p less than or equal to 0.001). Ovulation commenced in late Octobe r in the control group (NL-SNP). Compared with control, median ovulati on time was advanced by 5, 4 and 3 weeks in the ALM-8L, NL-8L and ALM- SNP groups, respectively, whereas ovulation was delayed by 1 and 6 wee ks in the ALM-24L and NL-24L groups, respectively. The altered timing of ovulation among the groups was paralleled by similar shifts in the seasonal plasma oestradiol-17 beta and testosterone profiles. Survival of eggs to the eyed stage was lower in the ALM-8L group (mean = 64.2% ) compared with the NL-SNP group (mean = 92.5%), indicating a negative effect on egg quality in the most advanced group. Although abrupt cha nges in photoperiod can be used to control timing of ovulation in Atla ntic salmon to obtain off-season eggs, the decrease in egg survival an d proportion of maturing fish may set constraints on how much maturati on can be advanced by use of continuous light during winter and spring . However, the effect on age at maturity may also be exploited to redu ce the problem with unwanted early maturation in salmon farming. (C) 1 998 Elsevier Science B.V.