About 125,000 salmon have died due to a disease outbreak at two fish farms on the Isle of Lewis, BBC Scotland has learned.
Marine Harvest confirmed that the sites in Loch Erisort have been hit by the bacterium Pasturella Skyensis.
The company has apologised to local people concerned about the smell of decay in the area and the sight of lorries carrying away dead fish.
The pathogen is believed to have taken hold at the farms at the end of August.
One theory behind the emergence of the disease is that climate change and rising ocean temperatures could be making Scottish fish farms more vulnerable to bacterial infections.
Infected salmon become very lethargic, stop eating and as the illness progresses it can prove fatal.
Marine Harvest’s business support manager Steve Bracken confirmed that the outbreak was “quite serious” and had taken its toll.
“The mortality is in the region of about 500 tonnes,” he said. “The fish are around about 4 kilos so it is about 125,000 fish we have lost during this period.
“It’s extremely unfortunate but we do see from time to time that there are these outbreaks of infections that are hard to explain. In this case we have seen it [Pasturella Skyensis] before.”
He said the pathogen was first identified at one of the company’s farms on Skye in 2002. It was also seen there in the late 1990s but at the time the company was not sure what it was.
“In its worst case it kills fish but it does respond to treatment and that is what we are carrying out at Loch Erisort at the moment,” Mr Bracken said. “We use antibiotics to control the infection and it has taken time but we are seeing that the antibiotics are working.
“I think we are through the worst of it, but we are still concerned of course and are not out of the woods yet. We are monitoring the situation very closely to make sure the disease is dying away.”
Stopping the spread
Lorries carrying thousands of dead fish have been a regular feature in the area over the last few weeks.
Specialist contractors are being used to transport the carcasses to the central belt where they are put into an anaerobic digester and turned into liquid fertiliser. Marine Harvest said this was done in line with strict bio security measures.
In the longer term, it is hoped a vaccine can be developed for the disease but for now the top priority for the company is to stop it getting into other sites.
The concentration of fish in salmon farm cages is high and one infected fish could quickly spread the illness to thousands of others.
Mr Bracken said that fortunately the company has managed to stop it from reaching another of their fish farms in Loch Erisort, just one mile away at Tavay.
“In a situation like this it is important to have good bio security in place to make sure we are not spreading the disease,” he said.
“What we do is we make sure we are not spreading equipment between sites and we are mindful to disinfect. From our past experience of the disease in Skye it didn’t spread from the farm that we had it on.
“And i think the fact that we have such good bio security measures in place we are very hopeful that the infection won’t spread.”
The aquaculture sector in Scotland has been affected by serious problems with sea lice and amoebic gill disease and it’s hoped that Pasturella Skyensis doesn’t become a third major issue.
Strains of the bacteria are present in wild fish which could be the origin of the infection, however, it’s feared that climate change could be making Scottish fish farms more vulnerable to bacterial diseases.
Not a lot is known about the infection but Mr Bracken said the company would learn a great deal more from this particular incident.
“We don’t know why it has come at this time,” he said. “It is very hard to say, but we are seeing changes in the environment, there is no doubt about it.
“We have got warmer sea water temperatures as a result of climate change. It’s possible it is linked to that. Once we are through this we’ll certainly look at this in a lot of detail to hopefully work out where it has come from.”
He added: “The marine environment is changing and this may bring new challenges for salmon farmers in the future.
“So it is up to us to make sure we are watching our fish very very closely that we are doing our research and that we are monitoring the environment just to check there is nothing in there that will come along and affect us in the future.”
Mr Bracken said that despite the deaths, Marine Harvest was having an excellent year all round.
“It’s just very unfortunate this has come along,” he said. “We have 46 sea farms on the west coast so we do have a good supply of fish so in the whole scheme of things it is not serious for the business but it is concerning from the point of view that we have got this infection that has come along and given us quite a side swipe.
“We have got to learn from this. We have to understand where it came from, best methods of treating and really about observation in the future what do we need to look out for are there any tell tale signs that it is coming back.
“Salmon farming is still a relatively new industry, with the first being being produced in 1971. So we have learned a lot but I think it is fair to say we still don’t have all the answers but we really do need to think about this kind of situation and focus and really just try to come up with solutions for the future.
“We want to apologise for any inconvenience caused to our neighbours and others nearby in the community.
“We don’t want to cause any upset at all and so we regret any disturbance that has gone on. Hopefully we are going to get through this in the very near future and we’ll get back on track, back to normal.”