A new method to control when and how much salmon jump out of water makes possible the effective treatment of sea lice, say scientists. Pictured here is a salmon affected by sea lice.
Credit: Tim Dempster
SYDNEY: A new method to control when and how much salmon jump out of water could potentially save the industry more than A$500 million a year, say Australian scientists.
This deceivingly straightforward method, which uses a simple net barrier, makes possible the effective treatment of sea lice, which are blood-sucking parasites that hinder fish growth.
Scientists have long known how to treat sea lice by depositing a layer of oil mixed with a treatment chemical on the surface of the water. But because it requires salmon to jump out of the water and coat themselves in the mixture before the anti-lice chemical degrades in sunlight, the treatment was unreliable.
"Our study has demonstrated a way to induce salmon jumping behaviour so that it is frequent and predictable, ensuring the surface treatment method is effective in de-lousing salmon," said zoologist Tim Dempster from University of Melbourne and lead author of the study published in the Journal of Animal Science.
How to de-louse salmon
Dempster discovered that blocking the water's surface with a net barrier for a day would make more than 90% of salmon jump out of the water within two hours of the net's removal. Salmon blocked from the water surface jumped earlier and more frequently than those allowed constant surface access.
"That we could create such a strong behavioural driver for salmon to jump with a simple and non-stressful method was a great surprise," Dempster said.
The authors of the study suggested that an empty swim bladder - an internal air pocket that some fish use to regulate their buoyancy - may be behind the behavioural changes they observed. While most fish have what is called a closed bladder, to which gas can be transported through blood vessels wrapped around the organ, salmon have an open bladder, which can only be filled up by swallowing air above the water surface.
Eco-friendly solution to a costly infection
Unrelated to human lice, sea lice are small parasitic crustaceans that cause painful skin lesions by feeding on the mucous and blood of salmon and other fish. Because sea lice populations can be amplified by salmon farms in coastal ecosystems and wild salmon can become infected by the release of larvae from those farmed salmon, lice have led to the decline of wild salmon and trout populations worldwide.
Although there is no problem with eating salmon infected by sea lice, Dempster said, the ailment is so costly to the industry because infected salmon "grow more slowly and are believed to be more susceptible to other diseases (for example, viruses) when they are heavily infected by sea lice, due to poorer condition and poorer immunity."
Though multiple chemical treatments exist, the oil mixture is the most environmentally friendly because it can be recollected from water after use, unlike others that dissolve in the water. "Compared to present-day techniques which are expensive and manual labour intensive, this method has the potential to be both cheaper and easier for farmers," Dempster said.
Boost profitability, improve health
"Compared to present-day techniques which are expensive and manual labour intensive, this method has the potential to be both cheaper and easier for farmers," Dempster said.
But practical issues, such as designing nets that can be easily raised and lowered by farmers, need to be addressed before salmon can be induced to jump on actual farms. "Our work now opens up possibilities to investigate how this technique can be adapted so it is practical," Dempster said.
"This study highlights how research can synergistically help both the environment and the industry," said University of Sydney veterinary scientist Joy Becker, who was not involved in the study. "As such it has potential to boost profitability and improved animal welfare in the salmon aquaculture worldwide."
