Early light regime may have long-term effects
A light regime of 21 hours of light and three hours of darkness early in a salmon's life can produce physiological effects that persist long after the fish have been transferred to natural light conditions. This is shown by a new study from the Institute of Marine Research and the University of Bergen.
Researchers in Norway investigated whether the lighting regimes traditionally used for the production of smaller smolts are also suitable for large smolts. They compared three regimes: no winter, six-week winter and 20-week winter.
In addition, they investigated whether three hours of darkness per day, instead of continuous light, could affect growth and the risk of early sexual maturation.
The trial initially included 1,200 salmon divided into six groups. The fish were produced at 10°C in fresh water before 976 fish were transferred into sea cages in April.
Clearer smolt response
When released into the sea, the fish that had been given a long winter weighed an average of 485 grams. The groups with six weeks of winter and without winter weighed 767 and 851 grams, respectively.
In contrast, only the fish that had received a 20-week winter showed a clear increase in Na⁺/K⁺-ATPase activity in the gills before sea release. The activity was 48 and 70% higher than in the short winter and no winter groups, respectively.
"The fish that had a long winter were the only groups that showed some of the classic indicators of smoltification," the researchers write.
The fact that six weeks of winter did not produce a clear increase in gill enzyme activity was more surprising.
The researchers discuss whether larger smolts may have already developed sufficient ability to adapt to sea water, and therefore respond less to changes in the light regime.
Grows faster in the sea
After release into the sea, the fish with a long winter grew 15% faster than the group with a six-week winter. The latter in turn grew 5% faster than the fish that had not received a winter signal.
At the end of the experiment, the three groups weighed 2.34, 2.55 and 2.60 kilograms, respectively.
"The groups that had a long winter performed better than the groups with a standard winter at sea, both in terms of length growth and weight gain," the researchers write.
However, they emphasise that they cannot determine whether the differences were due to the length of the winter period, different fish sizes when released into the sea, or other physiological conditions.
Three hours of darkness could reduce sexual maturation
The researchers found relatively few significant effects of using 21 hours of light and three hours of darkness instead of continuous light.
But among the fish that had had six weeks of winter, there was a marked difference in signs of sexual maturation upon release into the sea.
In the group that then received continuous light, 42% showed signs of sexual maturation. With three hours of darkness, the proportion was only 2%.
However, the difference was not statistically significant.
"Although the difference was not significant, the incidence of sexual maturation in post-smolt at sea release was significantly lower after using six weeks of winter and 21 hours of light than after the same winter period with continuous light," the researchers write.
Almost all the fish reached sexual maturity
The experiment had an unexpected end. In June, two months after release into the sea, no fish showed signs of sexual maturity according to the researchers' criteria. When the experiment ended in August, 98% of the male fish were sexually mature.
"Here we report an example of farmed male fish in sea cages that did not show clear signs of sexual maturation in June, but had reached a very advanced stage of maturation three months later," the researchers write.
They believe that the extensive sexual maturation was likely primarily due to the genetics of the clonal male fish line used.
Sexual maturation meant that the experiment had to be ended before the fish reached the planned harvest weight.
The researchers therefore conclude that more studies are needed to clarify whether the improved sea growth after a long winter persists throughout an entire production cycle, and whether short periods of darkness can reduce early sexual maturation over time.