EFFECT OF STOCKING DENSITY, OXYGEN LEVEL, LIGHT REGIME AND SWIMMING VELOCITY ON THE INCIDENCE OF SEXUAL-MATURATION IN ADULT ATLANTIC SALMON(SALMO-SALAR)

The effects of various environmental parameters on sexual maturation o f two sea-winter Atlantic salmon (Salmo salar) were tested in two sepa rate experiments, In the first experiment Atlantic salmon with initial mean individual weight 1.5 kg (smolt 13 months before) were reared fo r 8 months from June to February at different oxygen levels and stocki ng densities using continuous light. Oxygen levels of 5-7, 7.5-9.5 and 10-12 mg O-2 1(-1) and stocking densities starting at about 20, 30 an d 40 kg m(-3) and increasing as the fish grew to 80-90 kg m(-3) for th e highest densities were tested in a factorial design. Only male fish matured, and incidence of maturation among males varied from 4.1% to 2 5% between tanks. The highest percentage of mature males was found in the tanks with low stocking density. No clear effect on oxygen level w as found. The second experiment lasted 20 months from seawater transfe r in May until the fish weighed 3.3-3.5 kg. Two water current speeds ( 14-16 and 20-24 cm s(-1)) and two photoperiod regimes (LD 20:4) and co ntinuous light (LL) were tested in a factorial design. Neither swimmin g velocity nor photoperiod affected growth rate. Continuous light redu ced the incidence of sexual maturation. The average proportion of matu ration among males was 8% and 25% under the LL and LD 20:4 regimes res pectively, The fish reared under the LD 20:4 light regime had a signif icant lower condition factor and significant larger hearts than the fi sh reared under continuous light. Swimming velocity had no significant effect on the incidence of maturation. The results indicate that the swimming velocity must be higher than 0.5 BL s(-1) in order to influen ce the energy stores. An important finding in this study is that light cues are not required for gonadal growth. The results also indicate t hat environmental factors can affect maturation even after the first s ea-winter.