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The lice trap being tested on the Hardanger coast.

Cost-effective monitoring of salmon lice larvae

Environmental DNA and a new lice trap are teaming up to count salmon lice larvae.

Salmon lice are one of the biggest challenges for today’s Norwegian aquaculture industry and pose a threat to wild salmon. The authorities have therefore introduced several measures to reduce the lice levels in production facilities and the infection pressure on wild fish.

The traffic light system is designed to ensure predictable and sustainable growth in the aquaculture industry. The system uses the infection pressure from salmon lice on wild salmonids as an environmental indicator to determine whether production in an area can increase or must be reduced.

Here you can see a salmon infected with lice.

However, monitoring salmon lice is a demanding task. The current management system uses a combination of statistical models that calculate the spread of infectious salmon lice larvae from fish farms and manual counts in the field to monitor the lice on fish.

The field monitoring is done either by catching wild salmon in trawls and purse seines, or by putting farmed salmon into so-called ‘guard cages’ and counting how many lice have infested the fish after two weeks in the ocean.

The lice trap utilises natural current conditions in the ocean and requires no electricity or pumps to filter seawater.

Lice larvae captured in open sea

Researchers at NINA have collaborated with NTNU to develop a new method for counting salmon lice larvae in the open ocean. This is ground-breaking work that can be scaled up and contribute to cost-effective monitoring of lice larvae along the entire Norwegian coast.

The plankton samples are analysed at NINA's genetics lab.

The lice trap filters seawater and collects plankton. The plankton samples are analysed at NINA's genetics lab, where the researchers have developed DNA-based methods for calculating how many salmon lice larvae are present per litre of seawater. Thus, the method can be used to calculate the density of lice larvae in the ocean.

The net filters the water and collects lice larvae and other plankton.

Analyse spread of salmon lice larvae in Hardangerfjord

A total of 12 lice traps have been put out in Etne and Rosendal municipalities in the Hardangerfjord this summer. The Norwegian Institute of Marine Research already has ongoing salmon lice monitoring with guard cages, so the researchers can compare their data with the monitoring results from the Institute of Marine Research. Development of the new method will also help to calibrate the models used in the traffic light system and make the dispersal models even more accurate.

PhD candidate Nathan Mertz and master's students Ingvild Tryggestad and Amanda Andersson at NTNU Sealab have collected large amounts of plankton from various localities in order to analyse the spread of salmon lice in the fjord system.

“These data will be very important and will complement the various national dispersal models for salmon lice developed by the Institute of Marine Research, SINTEF and the Norwegian Veterinary Institute. The method could become an important part of the traffic light system,” says Bengt Finstad, a professor at NTNU Sealab.

Salmon lice (Lepeophtheirus salmonis)

Salmon lice are a parasitic crustacean having salmonids as hosts. Their life cycle consists of eight stages.

Mature female lice hatch an average of 300 eggs each week. These develop in three free-swimming larval stages that disperse with the ocean currents.

In the third stage, the larva is called a copepodite and is dependent on attaching itself to a host fish; otherwise it dies of starvation. In the later attached stages and moving stages, the parasite feeds on mucus, skin and blood and can inflict major wounds on the host fish if there are a lot of them.

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