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Melanie Andrews is a researcher at the Veterinary College and Salmon Lice Centre in Bergen. Photo: Pål Mugaas Jensen / Kyst.no.
Melanie Andrews is a researcher at the Veterinary College and Salmon Lice Centre in Bergen. Photo: Pål Mugaas Jensen / Kyst.no.

The majority of lice removed from salmon by warm water treatment survive the ordeal, researcher Melanie Andrews told the Sea Lice 2018 conference in Punta Arenas, Chile.

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“It is therefore important that all lice be collected and taken care of,” Andrews said.

The scientist and her colleagues at the Veterinary College and Salmon Lice Centre in Bergen found several interesting results during attempts to develop bioassays for warm water and freshwater treatments. One was that nearly all lice survive warm water treatments.

Immobilised or dead?

In a research project, Andrews studied sensitivity and survival of lice in different stages and from different regions. The goal is to make a bioassay that can easily be used at the cage by fish health professionals before starting a given treatment.

“One of the questions we asked was what really happens with the lice after treatment. Is the louse only immobilised or is it dead? Besides that, we wanted to see if there were differences in lice from north to south in Norway,” she said.

They also wanted to say something about the potential for developing resistance.

The results of the experiment have only begun to be analysed, so the results she showed at the conference are preliminary, she stressed.

Still active

The scientist reported that the normal temperature of the water used for warm water treatment is between 30 and 34 degrees Celsius. And usually the exposure time is 30 seconds. Andrews exposed the lice for more than two minutes.

“At 29-30 degrees there was still a large proportion of lice that were still active. However, there were far fewer male lice than female lice who were active.”

Even up to 35 degrees there was a dog that had not been knocked out. (See picture 1 at the end of the article)

And after 24 hours there was a striking number of lice that had woken up after being transferred to 12 degrees (See picture 2 at the end of the article). Even after treatment at 40 degrees there was a louse that had survived. And researchers also saw that there were differences in different lice populations.

Water filter

“The fact that so many survive means that having a good water filter is very important,” stressed Andrews.

One of the tasks in the future will also be to create a single bioassay that fish health professionals can apply when considering the treatment to be done.

“As we see that there are differences between populations for sensitivity, it’s important to use the right treatment so you do not treat anything that does not work. This is also important in preventing resistance development,” the researcher pointed out.

Figure 1. Percentage of pre-adult salmon lice active after two minutes of exposure at different temperatures (bar chart) and active after 24 hours of recovery (line). Yellow is female lice, green is male. Source. Melanie Andrews.
Figure 1. Percentage of pre-adult salmon lice active after two minutes of exposure at different temperatures (bar chart) and active after 24 hours of recovery (line). Yellow is female lice, green is male. Source. Melanie Andrews.
Picture 2. Survival of copepodid-stage lice 24 hours after two minutes' exposure to different temperatures. Different colours are different populations. Source: Melanie Andrews
Picture 2. Survival of copepodid-stage lice 24 hours after two minutes' exposure to different temperatures. Different colours are different populations. Source: Melanie Andrews
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