Farmed salmon contain less vitamin D than before. This is because the feed now contains fewer raw materials from the sea, and therefore also less omega-3 fatty acids.
Researchers and salmon farmers want to do something about this.
Vitamin D is something we humans – especially those of us living in the cold and dark north – should make sure to get enough of.
Researchers have now tested a new method to increase the vitamin D level in salmon. They gave them more light.
Important for reducing wounds
Annonse
The study was carried out by two researchers from Molde University College and two researchers employed by the aquaculture company Salmon Evolution.
Researchers simply installed UVB lights above the rearing tanks, where young salmon known as smolt swim.
The salmon exposed to light produced up to five times more vitamin D than those not exposed to light.
"This is important information for the production of farmed salmon – for the fish themselves, their welfare and health, and for us humans," says Øystein Klakegg.
He is a veterinarian and associate professor at Molde University College.
"A lack of vitamin D in salmon can weaken their immune system and increase the risk of infectious diseases. Evidence indicates that vitamin D is particularly important for skin health and for reducing the development of wounds and repairing wounds in salmon," he says.
Eating salmon can also make it easier for us humans to get enough vitamin D, according to Klakegg.
From the left: Vegard Langvatn, Øystein Klakegg, and Tobias Backström take vitamin D samples from the fish.(Photo: Inge Fossen / Salmon Evolution)
Tobias Backström (right) from Molde University College and Vegard Langvatn from Salmon Evolution look at a rearing tank with UVB lighting above it.(Photo: Øystein Klakegg / Molde University College)
Significant increase in vitamin D levels
The researchers analysed vitamin D3 in the muscle tissue of Atlantic salmon in two different experiments. The salmon were between one and two years old.
Each experiment included groups of fish exposed to UVB light and two control groups kept in standard lighting conditions.
Samples were taken at the start of the experiment, then after 4 weeks, and finally after 10 weeks.
After 4 weeks, the salmon exposed to UVB light had between 2.5 and 6 times more vitamin D3 in their muscles than the control fish – and more than 5 times as much after about 10 weeks.
Important findings for the industry
"The researchers have clearly shown that salmon produce vitamin D in their skin when exposed to UVB rays," says researcher Øystein Sæle at the Institute of Marine Research.
Sæle considers the findings reliable due to the number of fish tested and the fact that the researchers conducted multiple experiments.
"The study shows that light makes a significant contribution, knowledge that's important for salmon farms where natural light doesn't reach," he says.
However, not everyone is without concern.
Constant light – what are the consequences?
In the study, the fish were exposed to light 24 hours a day for several weeks.
Constant lighting is already common in modern aquaculture during the salmon’s first year, called the freshwater phase. This allows them to see their feed at night, eat more, and grow faster.
But what happens when the salmon never experience darkness?
"Exposing salmon to artificial light at night may disrupt their circadian rhythm," Mariann Eilertsen tells Science Norway.
Like humans and other animals, salmon rely on cycles of light and dark to maintain natural melatonin levels in their blood.
Melatonin also helps signals to the salmon what season it is, helping it to orient itself.
Eilertsen warns that it’s unclear how such unnatural conditions affect the salmon’s health and resilience.
"Our research shows that constant light exposure interferes with the regulation of daily and seasonal rhythms. I would therefore assume that the salmon's biology is negatively affected," she says.
A common practice
Øystein Klakegg tells Science Norway that this approach is standard in salmon production.
"It's an interesting question whether we should raise smolt in a way that's closer to nature. But this is the normal method used in the industry today," he tells Science Norway.
Even though salmon can provide vitamin D, researchers warn about environmental toxins in fatty fish. While fatty fish are excellent sources of vitamin D and omega-3 fatty acids, they also contain unwanted substances that can push people above the recommended limits.
