Effects of continuous additional light on growth and sexual maturity in Atlantic salmon, Salmo salar, reared in sea cages

Individually tagged Atlantic salmon postsmolts (body weight 243 +/- 0.9 g [mean +/- SE], n = 1800) were distributed randomly among four sea cages, two cages received continuous additional light (AL) from November to July, and two received natural light (NL) only (controls). Equal numbers of fish (150 from each cage) were moved between the AL and NL cages in December and January, creating a total of six experimental groups. The fish were fed to excess during the hours of NL. Compared with the control group, exposure to AL from November, December or January until July (Nov-Jul, Dec-Jul and Jan-Jul groups, respectively) resulted in a 48-52% reduction in specific growth rate (SGR) during the subsequent 6 weeks, followed by a higher SGR during the next 4 to 5 months. A similar growth response as in the Nov-Jul group occurred in the groups receiving only 6 or 12 weeks of AL from November (Nov-Dec and Nov-Jan groups, respectively). The SGR of the Nov-Jul group was higher than in the Nov-Jan and Nov-Dec groups at the end of the experiment. Between May and July, groups exposed to AL from November (Nov-Jul, Nov-Jan and Nov-Dec) grew significantly less than the groups initially receiving NL (Control, Jan-Jul and Dec-Jul). In July, the body weight (mean +/- SE) of the fish depended on the duration and timing of AL exposure; Nov-Jul: 1072 +/- 26 g, Dec-Jul: 995 +/- 23 g, Jan-Jul: 977 +/- 20 g, Nov-Dec: 930 +/- 23 g, Nov-Jan: 870 +/- 22 g and Control: 815 +/- 17 g. The proportion of sexually maturing males increased with early exposure and duration of AL; Control: 6.5%, Jan-Jul: 8.0%, Dec-Jul: 14.7%, Nov-Dec: 15.5%, Nov-Jan: 21.7% and Nov-Jul: 37.6%. The study provides evidence that AL superimposed on NL enhances growth of Atlantic salmon in sea cages during winter and spring, with timing and duration of the exposure affecting growth and the proportion of early maturing males. (C) 2000 Elsevier Science B.V. All rights reserved.