While extrusion methods are good at preserving the nutrition in feed pellets there is a limit to size. Above 17 mm in diameter pellets become unstable and tend to fall apart. But a 17-mm pellet just isn’t large enough to attract the attention of the somewhat laid back halibut. The answer according to Skretting is to make lots of smaller pellets and then stick them together into a larger block. The same method is proving successful in wrasse farming.
“The halibut is not a hunter like a salmon,” says Nils Tore Hølsbø, Sales Manager Marine Feeds in Skretting Norway. “They have large mouths and when something attractive comes near, they just suck it in. A well grown salmon will chase a 13 mm pellet. A halibut of 5–6 kg or more will just let it drift by. Our solution was the same AquaSoft technology we are using with tuna. We extrude pellets of 5 mm diameter then bind them together in blocks up 60 mm in diameter and even up to 600 g in weight and they do attract the halibut’s attention. This way we retain the benefits of extrusion, with preserved nutritional value, high fat content and feed that is stable in water. Also we can adjust the sinking speed and we add some attractants into the mix when making the blocks. At present the halibut blocks are at prototype stage. They certainly perform well in the water and now we are assessing whether they increase feed intake, especially in marginal periods for example when temperatures are low.”
Figures quoted at a recent conference in Trondheim indicated that Norwegian salmon farmers had around 20 million Ballan wrasse (Labrus bergylta) in their cages in 2010. They also suggested the number could double in three to five years with eight of the 40 million wrasse coming from the expanding wrasse farming enterprises. Wrasse hatcheries can help offset the criticism coming from the fishing fraternity that catching wrasse for the farmers endangers the wild salmon. They say that, as wrasse have a relatively high winter mortality, if too many are caught there are too few the following season for effective natural control of the sea lice. Hølsbø adds, “We are supporting the wrasse farmers by developing these feed blocks for the broodstock. We also have feeds for the hatchery and the young growing wrasse. The challenge there is to make it good enough for them to eat but not so good that they do not want to eat sea lice when released into the salmon pens.”
