Researchers at the Norwegian Institute of Food, Fisheries and Agriculture Research (Nofima), were investigating whether phosphorus hydrolyzed from fish bones could be used as an efficient source of the mineral in salmon feed, when they stumbled upon the discovery that this ingredient appeared to influence astaxanthin uptake.
The color level of the salmon muscle increased considerably – an effect that was visible to the naked eye and was confirmed by chemical analyses, they said.
Astaxanthin is the pigment that lends its distinctive red color to salmon fillets – perceived by consumers as an indicator of quality. Neither salmon nor any other animals can make astaxanthin themselves. In the sea, algae that make astaxanthin are eaten by small crustaceans, which in turn are eaten by fish, such as salmon. In fish farming, a synthetic pigment is added to salmon feed to achieve a red color in fillets.
Only a limited portion – usually less than 10 % – of the astaxanthin in salmon feed is absorbed in the muscle of the farmed salmon. This can be due to limitations in absorption and transport via blood and liver or limited absorption and pigmentation in muscle tissue. Therefore, if uptake of astaxanthin can be increased, this is a positive side effect that has the potential to add to the market value of this fish.
In the initial experiment with smaller salmon, it was found that the astaxanthin levels in blood, liver and whole fish were 55, 29 and 22 per cent higher respectively, compared with fish that were fed an ordinary source of commercial phosphorus.
Sissel Albrektsen, lead author and senior scientist at Nofima, told FeedNavigator the aim of this latest trial was to confirm these findings in larger fish.
“The trial was targeted to understand the impacts of astaxanthin utilization,” she explained. “In the previous trials on smolt, we just observed that as well as acting as a mineral supplement, the mineral-rich fish bone ingredients appeared to have additional effects on astaxanthin.”
Feed with the phosphorus-rich mineral ingredient was tested on salmon during the growth period from 1.7 to 2.5kg. Fish were reared in 12 net-pens and fed with one of four formulated diets for 78 days.
The control diet (D1) used CaPO4 as the phosphorus source, whereas diets D2 and D3 were supplemented with a fish bone hydrolysate at a dose of 2.1% and 4.2% respectively. Diet D2 was also further balanced with CaPO4.
Higher astaxanthin retention
Retention of astaxanthin in the muscle (which translates to a redder color) was found to be 35% higher in fish on the D3 diet. The researchers believe this is because the salmon digest more of the astaxanthin with the mineral ingredient present in the feed.
“The results suggest that the increased astaxanthin utilization in the fish was explained by slightly improved astaxanthin digestibility, possibly combined with lower metabolic loss of astaxanthin,” wrote the researchers.
They added that confirmation of the physiological impacts of the fish bone compounds on flesh quality and texture would be needed to claim such properties.
“Dietary impacts of sulphuric acid extracted fish bone compounds on astaxanthin utilization and muscle quality in Atlantic salmon (Salmo salar)”
Authors: Albrektsen S, Østbye T.-K, Pedersen M, Ytteborg E, Ruyter B, Ytrestøy lT