- In an experiment carried out at The Institute of Marine Research’s research station at Matre, we observed that salmon leaped more frequently than usual after they were kept away from the surface for several days using a cage roof. This observation inspired us to retry an old and largely forgotten method of delousing in which the fish themselves help to remove the lice, says project manager Tore Kristiansen, a scientist at The Institute of Marine Research.
- Having a method for getting salmon to leap frequently and predictably make it possible to develop new, oil-based delousing treatments. Now we are looking for effective mixtures of oil and delousing agent, and once we have selected the mixtures we will carry out full-scale trials. If they are successful, the method can be further developed for use at commercial fish farms. Since the method is less stressful for the fish, and thus improves fish welfare in relation to current methods, we hope that in the long term this may lead to a gentler way of removing sea lice from farmed salmon, says Frode Oppedal, a researcher at The Institute of Marine Research.
Exploiting natural behaviour
Sea lice are one of the biggest challenges facing the aquaculture industry in Norway. Currently the main methods used to combat sea lice are medicated feed, chemical treatments and wrasse. However, there are a number of problems with these methods: some sea lice are becoming resistant to the chemicals used; the delousing agents are released into the environment; and it is unclear how the wrasse fisheries are affecting local populations. It is against this background that researchers at SINTEF Fisheries and Aquaculture, University of Melbourne and Institute of Marine Research have been testing whether the natural behaviour of salmon can be encouraged and exploited in order to combat sea lice.
Starting in 1989, attempts were made to delouse farmed salmon by getting them to leap through a film of oil. Although early trials were promising, full-scale experiments were not very effective, as the salmon didn’t leap often enough and the chemicals were broken down by sunlight. Now it appears that we may have found a way to change the leaping behaviour of salmon, thanks to the fact that the fish leap much more than usual for a short period after being allowed back to the surface.
- In 12 m x 12 m (2000 m3) test cages, we prevented several groups of salmon from reaching the surface for various periods of time, by putting a net across the top of the cage. When the nets were removed, we recorded how often the salmon broke the surface. After access to the surface was prevented for one to two days, 93 per cent of the salmon leaped two or three times in the first two hours after the net was removed. In the control groups that were not kept away from the surface, we did not record any increase in leaping, explains Tim Dempster of SINTEF Fisheries and Aquaculture and the University of Melbourne.
The salmon probably leap more than normal because when they lose air from their swim bladders as they are denied access to the surface for short periods of time. This means that they have to spend energy to remain at a specific water depth, instead of using the swim bladder to regulate their depth. When the salmon are given renewed access to the surface, they feel the urge to leap and swallow air in order to refill their swim bladders.
Reduced environmental impact and stress
Any delousing method involving an oil-based treatment will be more environmentally friendly than the chemical treatments currently used.
- Currently, when the fish are deloused using chemicals, the agents are released into the sea after the treatment. If the method of using an oil film is successful, it will for the first time be possible to remove the chemicals after the treatment, e.g. using practices designed for dealing with oil leaks in the offshore industry. Overall, developing a successful method would make the treatment of sea louse infestations less stressful for the fish, and would mean that delousing could be carried out without any risk of oxygen levels falling or other accidents, and it would also be cost-effective, says Tore Kristiansen.