Vitamin E helps overheated salmon to keep their cool
Ernst Hevrøy’s research findings have already led to a special new feed: for farmed salmon, a diet with more vitamin E counteracts the stress and slow growth associated with rising summertime ocean temperatures.
According to data from the Intergovernmental Panel on Climate Change (IPCC), ocean temperatures along the Norwegian coastline may rise by up to 1.5°C in coming decades. That would present some significant challenges to Norway’s aquaculture industry, since salmon lose their appetite and subsequently grow far more slowly when temperatures exceed 18°C.
In the past decade, Norwegian ocean temperatures in the summer and early autumn have remained elevated for prolonged periods, particularly in the fjords of Western Norway.
Ernst Hevrøy has received project funding under the HAVBRUK programme’s “Project for top young investigators in aquaculture research” (TOPPFORSK). He heads a project to study the impacts of global warming on aquaculture.
“Salmon living in 19°C seawater reduce their feed intake by half compared to salmon in 14°C water. This makes them grow more slowly,” says Dr Hevrøy, a scientist at Norway’s National Institute of Nutrition and Seafood Research (Nifes).
His analysis has revealed that temperature increases create a hormonal imbalance in salmon; elevated temperatures suppress the appetite-regulating hormone ghrelin.
“This inhibits the stimuli from the salmon brain’s neurotransmitters that regulate appetite, so the fish reduces its feed intake,” he explains.
Dr Hevrøy has studied two groups of salmon weighing roughly 2 kg after 56 days in the ocean: one group lived in 19°C water, the other in 14°C water. A variety of diets with differing fat content were administered. The salmon in the warmer water ate less and took up less fat, using instead their own fat reserves, particularly polyunsaturated marine omega-3 fatty acids. Growth slowed, commensurate with their reduced feed intake. During the test period, the colder-water control group gained a full kilogram more weight than the overheated salmon at 19°C.
“Elevated ocean temperatures also lead to lower oxygen content in the water. Wild salmon will swim to cooler, more oxygen-rich waters. But farmed salmon cannot, which turns off a number of survival functions – including appetite and digestion.”
“We have also observed that the salmon in warm seawater take much longer to recover after being handled, for example for weighing. Salmon can tolerate short-term temperature increases fairly well but have a far more difficult time if water temperatures remain high for an extended period.”
Vitamin E prevents anorexia
Dr Hevrøy and his research team have discovered that salmon mobilise large amounts of vitamin E in higher ocean temperatures. Feed producers have already made use of the findings, launching a special feed with more vitamin E. Feeding this to production fish before and during the summer months should reduce problems of stress and slower growth.
“Adult salmon do not tolerate high temperatures as well as younger ones,” Dr Hevrøy continues. “In post-smolts it appears that temperature increases have less of a negative effect on growth.”
“Looking ahead, we also want to investigate how well rainbow trout tolerate higher temperatures compared to salmon. If it turns out rainbow trout cope better, one response to rising ocean temperatures may be to replace salmon with rainbow trout as a production species in the fjords.”
Dr Hevrøy’s project is based on knowledge gleaned from original experiments previously carried out at the environmental laboratory at the Institute of Marine Research’s Matre Aquaculture Station.
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