Cleaning system reduces feed waste in halibut farms
Anne Berit Bjørken
A newly developed cleaning system for fish farms is currently being tested at Ytterøy Sjøprodukter in Norway. The system will mean lowered emissions of "leftovers" and faeces. It will also monitor the feeding habits of the fish, which may reduce feeding costs.
Jan Henning Johansen raises halibut in tanks.
Scientists at SINTEF NHL are behind the cleaning system, which was installed at Ytterøy Sjøprodukter in March this year. The company is in the process of building up a commercial halibut farm. The cleaning system, which goes under the name of "Particle Trap," has been developed for use in closed aquaculture systems. The trap has two functions: it gathers food remains and faeces from the waste water so that these do not return, untreated, to the sea. It also continuously records the amount of feed present in the water, so that the quantity and timing of meals can be regulated.
"We are very pleased with the results so far. At first, we had a few problems with blocked hoses, but these have been fixed. The Particle Trap has already shown us that for a while, we were giving the fish too much food, while we subsequently had to increase the amount of food because virtually none of it was going to waste. Feed is a major cost element for us, and if this system enables us to accurately adjust the quantity we give the fish it will cut our costs," says Jan Henning Johansen, manager of Ytterøy Sjøprodukter.
The fish farm has a total of 8,500 halibut, and the Particle Trap is being tried out in three tanks that hold a total of about 2,000 fish. Two tanks has built-in shelves on which the halibut can lie, another development which is due to the NHL researchers. The idea has been to increase the effective area of the tank by allowing the fish to lie on both the bottom and the shelves.
"Halibut are bottom-living fish that prefer to lie on the floor of the tank. But it soon gets crowded down there. That is why we put in these shelves, and the fish seem to like them. The Particle Trap also functions well in this tank," says Johansen. The water in the tanks is changed continuously. So far, the waste is not recycled, but work is under way to develop disinfection processes that would allow it to be used as fertilizer, for example.
How it works
The principle of the Particle Trap is that the water outlet in the middle of the tank is divided in two. The main pipe goes vertically up in the tank with a strainer at the top, so that most of the water is discharged without carrying food remains or faeces with it. The other pipe lies right at the bottom of the tank and collect the particles."We remove so much water here that the bottom sludge is taken by a pipe to a collector where it is separated, so that the particles fall to the bottom and can be collected, while the water is released," says Arve Berg, a SINTEF NHL scientist.
Two types of particle trap have been developed. One of them requires a swirl or current eddy in the centre of the tank, and is intended for salmon, which prefer to swim in rapidly flowing water. This version has been patented by SINTEF, and has been industrialized by Rupro Plast of Løkken in the County of Sør-Trøndelag.
The other principle is based on gravity, in that the particles fall to the bottom of a sludge pot before they are sucked out into the collector. This is most suitable for flatfish such as halibut, which prefer water with a low current velocity.
"The Particle Trap is a tool that monitors the appetite of the fish in real time, offering a basis for controlling the amount and timing of meals. It is neither economic nor environmentally friendly to feed fish when they are not hungry, so that food is wasted," says Arve Berg. "We can say the same of giving fish too little food, because this means that they have to be kept for longer before they can be harvested."