Growth and maturation of Atlantic salmon (Salmo salar) populations with different grilse proportions under natural photoperiod and superimposed nighttime light

This study determined the influence of night illumination, superimposed on the natural photoperiod, upon growth performance in two different stocks of Atlantic salmon (Salmo salar). One stock had been selected to yield a low proportion of grilse (fish that mature after one winter in the sea) and the second a high proportion of grilse. Growth based on weight gain, gender, and sexual maturity of 250 individually tagged postsmolts in each of four pens was analyzed using five Atlantic salmon groups; mature females and males, immature females and males, and precocious males (the earliest mature males that produce sperm), identified during and at the end of culture. Both stocks under natural photoperiod generally showed that mature females and males grew faster, followed by immature females and males, and followed by precocious males. Mature males in both stocks did better than their respective mature females and growth of immature males in the high grilse stock was similar to mature males during the last two months of rearing. Precocious males had variable growth, depending on population origin. In contrast, salmon groups in stocks subjected to nighttime light did not show growth disparity, and each group grew faster when compared to the same group under natural photoperiod, except for mature females and males from the low grilse stock. Percentage of maturation in the low grilse stock was low and not significantly different between natural photoperiod (21%) and nighttime light (16%) treatments, whereas the high grilse stock reached 87% and was significantly reduced to 56%, respectively. This reduction occurred in mature females and males, except precocious males, whose proportion remained similar in both treatments.