It’s a nightmare for fish breeders, causes damage amounting to millions, and is also a growing problem for wild salmon: the salmon louse is high on the list of notorious fish parasites. In a new project, experts at the AWI, together with several partners, are developing a new procedure that will rid the fish of these pesky nuisances more effectively and more gently.
Whether as a steak, in a sandwich or in a lasagne: salmon is one of the most popular types of fish in Germany. According to statistics from the Fish Information Centre (FIZ), a total of more than 13 kilogrammes of fish are consumed per person per year there, and in 2019 salmon accounted for over 17 percent. And the figure is rising. To meet the demand, which is also high in other countries, the most popular fish are now cultivated at large sea farms. Especially in the Norwegian fjords, but also off the coasts of Chile, Scotland and Ireland, breeders have installed huge net pens where the salmon are raised.
But for those running these large-scale aquaculture installations, the salmon louse (Lepeophtheirus salmonis) is a veritable nightmare, since the tightly packed net pens offer ideal conditions for the parasite to spread rapidly. The eggs of the small crustaceans swim freely in the water, where they develop into larvae with just one goal: to find a salmon on whose body they can reach sexual maturity. “These larvae actively seek out suitable hosts,” explains Kai Lorkowski, who heads the Centre for Aquaculture Research at the AWI in Bremerhaven. “Although they can swim about for roughly 14 days, after that they need a salmon host.”
Pity the poor victim when their search is successful. The young of the mini crustaceans bite into the host’s body and feed on its tissues, its mucous and its blood. And the older they get, the bigger their appetite becomes. Often the larvae not only weaken the salmon, but also inflict wounds on them that can become infected. “An infestation with 20 to 30 salmon lice can be fatal,” says Lorkowski.
For the Norwegian aquaculture industry alone, in some years this means losses of several hundred million euros. According to estimates, roughly ten percent of salmon rearing costs are spent on fighting the parasite. Furthermore, infested farmed salmon can easily become a source of infection for their counterparts in the wild. After all, the mature crustaceans release their eggs into the water, and these can pass unhindered through the aquaculture nets into the sea. Then all it takes is for a few wild salmon to swim by for them to become infested.
As you can see, there are plenty of reasons to rid the fish of these troublesome and dangerous hangers-on. There are already effective procedures for doing so, but they have their drawbacks. One such method involves treating the water in the pens with hydrogen peroxide (H2O2), which kills the lice chemically. However, this has the disadvantage of burdening the environment with additional chemicals. Over time, the parasites can also become resistant to it.
Other approaches attempt to remove the lice from the fish mechanically. This involves first pumping the salmon from their pen onto a ship, where they pass through a treatment station. This can consist of briefly immersing the fish in a container filled with 30-degree water. This causes the parasites to leave their host, after which they can be filtered out of the water to be destroyed. But it’s an energy-intensive procedure, which is just one reason it is viewed critically by the Norwegian authorities; what’s more, salmon aren’t fond of such high temperatures. “This procedure is extremely stressful for the animals,” Lorkowski says. The same is true for a similar process, which instead of warm water, relies on mechanically spraying the infested salmon – essentially like a ‘car wash’ for fish.
Therefore a new method is called for, one that frees the salmon from the parasites more effectively and more gently. Guido Becker from the company Technische Innovations Leistungen (TIL) has already developed one idea based on a combination of ultrasound and infrared light. In the new project, Lorkowski and his team, together with other partners, will implement this idea in practice. As before, the infested fish will be treated at a ship-based installation, but this time using the new procedure.
The Technologie-Transfer-Zentrum (ttz) in Bremerhaven will coordinate the entire project, which also includes the development of a prototype treatment device. Also on board are infrared equipment specialists Micor GmbH, the company Purima, which will supply the ultrasound baths, and the aquaculture specialists glammeier+john aquakultur. The project, which will continue until May 2022, is funded by the Federal Ministry for Economic Affairs and Energy’s Central Innovation Programme for small and medium-sized enterprises (SMEs).
“The AWI will be responsible for the biological aspects of the research,” explains Lorkowski. It has long been known that bacteria and other small organisms can be killed using so-called cavitation effects. This involves using ultrasound to vibrate their cells, causing them to burst. This could also work with the salmon louse. At the very least, the acoustic radiation should force them to release their victim. Infrared light will reinforce the effect by heating the parasites, but not the entire fish. Which ultrasound frequencies affect the parasites without harming the fish? How long does the infrared light need in order to work? And what’s the best way to remove the lice from the water after they’ve been separated from the host? As part of the project, the Bremerhaven team will investigate these details, so that the salmon can finally have a rest from these greedy hangers-on.
