Continuous light and elevated temperature can trigger maturation both during and immediately after smoltification in male Atlantic salmon (Salmo salar)

In the present study, two experiments were performed to examine if manipulation of photoperiod and temperature can trigger sexual maturation in male Atlantic salmon during smoltification and in the first period after smoltification. In experiment 1 (exp. 1), parr (83g) previously reared under LD12:12 were subjected to LD18:6 or LD24:0 photoperiods at 5, 10 or 16°C FW for six weeks (01 December to 14 January), and then evaluated for maturity status (GSI) and seawater survival (96h challenge tests in 35‰ salt water). After the six week period, 47% of the males reared under 16°C–LD24:0 were maturing (GSI 1.5±0.2%). No maturing males were found in the other groups. Survival in seawater was high (>92%) in all groups. After the challenge tests, all groups except the one held at 16°C–LD24:0 were terminated. This group was reared on for two months in seawater (8.9°C and LD24:0) to verify maturation. There was no mortality during the period in seawater, and the GSI of mature males increased to 7.3±0.5%. In experiment 2 (exp. 2), under-yearling smolts (60g) previously reared under LD24:0 and natural temperature in fresh water were subjected to LD24:0 and 16°C seawater or to natural temperature seawater and natural photoperiod (60° N, 5° E, Western Norway) for three months (17 October to 20 January), followed by natural temperature and photoperiod for three months (20 January to 29 April). At termination, 8.3% of the fish in the 16°C–LD24:0 group were fully mature males (running), while there were no mature males in the group reared under natural conditions. The mature males developed a ‘female mimicry’; their lower jaw lacked the kype (hook) typical of mature male adults, and their skin coloration was dark yellow-greenish. These two experiments demonstrate that a combination of elevated temperature and continuous light can trigger maturation both during and immediately after smoltification in male Atlantic salmon. The model presented here (exp. 1, 16°C–LD24:0) can be used to study potential conflicts between maturation and smoltification in male Atlantic salmon. This is also the first report (exp. 2) of fully mature Atlantic salmon in spring (April–May). In practical terms, the present study shows that the use of LD24:0 at elevated water temperatures increases the risk for post-smolt maturation, or ‘jacking’, in male Atlantic salmon.