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 of two sea-winter Atlantic salmon (Salmo salar) were tested in two separate experiments, In the first experiment Atlantic salmon with initial mean individual weight 1.5 kg (smolt 13 months before) were reared for 8 months from June to February at different oxygen levels and stocking 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 and 40 kg m(-3) and increasing as the fish grew to 80-90 kg m(-3) for the highest densities were tested in a factorial design. Only male fish matured, and incidence of maturation among males varied from 4.1% to 25% between tanks. The highest percentage of mature males was found in the tanks with low stocking density. No clear effect on oxygen level was found. The second experiment lasted 20 months from seawater transfer 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 continuous light (LL) were tested in a factorial design. Neither swimming velocity nor photoperiod affected growth rate. Continuous light reduced the incidence of sexual maturation. The average proportion of maturation among males was 8% and 25% under the LL and LD 20:4 regimes respectively, The fish reared under the LD 20:4 light regime had a significant lower condition factor and significant larger hearts than the fish 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 influence the energy stores. An important finding in this study is that light cues are not required for gonadal growth. The results also indicate that environmental factors can affect maturation even after the first sea-winter.