© Terje Aamodt, Nofima
Zinc is a mineral that fish require to stay healthy, however, a drawback with today’s salmon feed is that the salmon retain a rather small fraction of the zinc, while large amounts are lost straight to the aquatic environment. The EU wants to reduce zinc emissions to the environment, but the salmon farming industry across Europe is already struggling with stricter limits on how much zinc is permitted in the feed, according to Anthony Philip, a researcher specialised in mineral nutrition at the Norwegian research institute, Nofima.
The current limit is 180 mg zinc per kilo of feed for salmonids and 150 mg/kg for other fish species. The European Food Safety Authority (EFSA) has proposed lowering the level to 150 mg/kg to all feeds, including salmon feed.
“The solution is to increase the digestibility of zinc in the feed, so the salmon still gets enough zinc, even if the content in the feed is reduced,” commented Philip in a press release.
The coating breakthrough
In standard production, zinc is added to the feed blend before it goes through an extruder, where high temperature and high pressure turns it into pellets. One the pellet comes out of the extruder, it is coated under vacuum with oils. The oil is drawn into the pellet, and this allows additives that cannot tolerate high temperature to be included in the coating. As minerals can tolerate high temperatures well, they are usually not included in this coating step. However, they can still become trapped in rigid protein structures formed during heat treatment.
“When zinc binds in these structures, it hampers release and uptake in the salmon gut,” explains Philip.“This is why he wants to move zinc supplementation in feed from the premix to the coating.”
After decades of working building knowledge on mineral nutrition in Salmon, Philip became convinced that the solution to low zinc uptake in salmon had to lie in the supplementation method. At Nofima, he therefore launched an internal project to develop the method and document the effects on post-smolt, weighing between 200 and 280 grams, kept in land-based tanks with seawater. He confirmed that when zinc was added in the coating, zinc digestibility increased by up to 15 percentage points and zinc in faeces was reduced by up to 25 percent.
The researchers describe the outcome as a "triple win": improved fish health through better absorption, reduced environmental leakage and lower zinc concentrations in aquaculture sludge, which enhances its value for circular economy applications.
Industry impacts
The project, titled TOP-zinc, is now moving into long-term sea-cage trials in collaboration with partners including the Institute of Marine Research, MOWI Feed AS and Huvepharma NV. The work is funded by the Norwegian Seafood Research Fund (FHF).
Renate Johansen, R&D director at FHF, confirms that excess zinc in the environment can have a negative impact on the microscopic life around fish farms.
“New EU requirements, which will probably mean that zinc levels in the feed must be limited, could cause deficiency diseases in salmon unless we find a good solution that gets more of the zinc from the feed into the fish and less out into the environment. The industry therefore has high expectations that the TOP-zinc project can help solve this challenge,” said Johansen.
While the new EU regulations are not yet in force, and the researchers still have a lot of documentation work ahead of them, Philip believes that feed companies and farmers stand to gain a lot by beginning to test the method.
“By changing the method, feed companies can help increase uptake, cut emissions and reduce feed costs. In TOP-zinc we are exploring options that can support decisions on zinc supplementation strategies in salmon feed, also in the grow-out phase at sea,” added Philip. “In the past, poor zinc digestibility was ‘only’ a biological issue, but now it is also becoming an operational challenge, and that makes the interest in finding solutions much greater.”
