Salmon maturing too soon? New research points to two fixable causes.
Can light and diet be used to control salmon maturation? That’s the question Vetle Skjold set out to answer in his doctoral research at Nofima. His findings could help improve animal welfare in salmon farming and address a long-standing industry challenge.
“I find it fascinating how something as fundamental as light is so influential on salmon maturation,” says Skjold, now a scientist at Nofima – the Norwegian Institute of Food, Fisheries and Aquaculture Research.
He recently defended his highly industry-relevant doctoral thesis at the Norwegian University of Life Sciences (NMBU). Some producers of large salmon smolt on land report challenges with early maturation. Skjold’s research may offer them some solutions.
Light as nature’s calendar
In the wild, salmon use daylight as a natural calendar. They perceive changes in day length to regulate key developmental processes. Increasing day length in spring signals both the young salmon in rivers that it is time to swim out to sea, and the larger fish when to return upriver to spawn.
In modern aquaculture, continuous light is often used to optimise production. While this may bring performance benefits, its biological implications are less clear.
“We deprive the salmon of its natural circadian rhythm, and this can be negative, especially if it leads to increased maturation,” says Skjold.
Vetle Skjold / Nofima.
Maturation at the wrong time
Early maturation in male salmon is a significant issue for the industry. Once a farmed salmon matures in brackish water, it begins adapting to freshwater conditions—despite being held in seawater. This mismatch affects welfare and can ultimately lead to mortality. Maturing salmon also stop eating, harming growth performance, and their flesh quality deteriorates, with pale fillets and discolouration at slaughter.
Traditionally, farmed salmon remain in freshwater tanks until ready for transfer to sea. However, the trend toward larger smolt and longer land-based phases aims to reduce time in sea cages and avoid exposure to pathogens and sea lice. This shift raises the question: how do you create the right environment on land for a fish biologically ready for the sea?
Results mirror salmon’s natural rhythms
Skjold and his colleagues studied how light exposure affected growth and maturation in land-based smolt production. They tested continuous light, increasing day length (spring simulation), and decreasing day length (autumn simulation). The experiment took place in brackish water tanks at Nofima’s Sunndalsøra facility, with fish grown from 100 to 1000 grams before transfer to seawater, where they grew to 2.5 kg.
Spring light produced the highest proportion of mature fish. Continuous light resulted in medium-high levels, while autumn light kept maturation to a minimum.
Vetle Skjold / Nofima.
“This aligns well with the salmon’s natural reproductive strategy,” says Skjold.
“In spring, maturation is initiated, while autumn is a period for either spawning or building energy reserves to prepare for spawning the following year. Continuous light, on the other hand, seems to facilitate early maturation after smoltification, although the exposure does not involve changes in light signals to the fish.”
Feed composition also matters
The researchers also tested whether feed composition influenced maturation. Two diets were trialled, with varying protein and fat content.
“We found that by feeding salmon a high protein and low fat diet, containing lower energy the testicular growth was reduced. Additionally, these fish stored less excess fat,” explains Skjold.
Salmon fed the lower-fat diet compensated by producing more short-chain saturated fatty acids to meet their energy needs.
Skjold’s team also studied gene expression in the testicles, pituitary gland, and fat tissue. They identified strong differences in the expression of puberty-regulating genes between mature and immature fish, and several previously unclassified genes that may be involved in the process.
The findings support previous results from trials with smaller smolt.
“These feed and light regimes may also be relevant for post-smolt production in facilities using recirculating aquaculture systems (RAS), even though the first part of the experiment was conducted using flow-through system. We maintained a relatively high temperature, comparable to typical RAS conditions,” says Jens-Erik Dessen, one of Skjold’s supervisors.
Advice for industry
For farmers considering longer land-based production:
“You could test a form of decreasing day length or reduced day length after smoltification of the fish, instead of continuous light, especially if you have challenges with maturation or poor growth,” says Skjold.
“It also seems that leaner diets may be used as a tool to reduce testicular growth and excess fat storage. However, this must be weighed against potential negative effects, such as increased feed conversion ratio and nitrogen emissions.”
Vetle Skjold, 33, is from Bergen. He defended his thesis on 27 February at NMBU, entitled “Sexual maturation, metabolism, and growth in Atlantic Salmon (Salmo salar L.): Effects of photoperiod and dietary protein-to-lipid ratio during post-smolt production.” His main supervisor was Bente Ruyter, with co-supervisors Jens-Erik Dessen, Kjell-Arne Rørvik, Lill Torunn Mydland, and Trine Ytrestøyl.
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