Farmed salmon lice problems exaggerated 'millionfold'

March 9, 2010 -- A new three-year study completed last year and awaiting publication says salmon farms are not the source of sea lice for wild salmon, refuting some earlier studies that claimed this is the case.

The study of the Loch Shielddaig site in Scotland showed the lice came from the wild, and somehow congregated at fish farms after hitching rides on other species, or being carried there by currents.

'They come from the wild, and that infection pressure seems to be over-magnified within the farms, so the amount of parasites released from the farms is overestimated in those models and is not as pronounced as was published,' said Ian Bricknell, a professor of aquaculture biology and director of the Aquaculture Research Institute at University of Maine.

'There as an overestimation of about a millionfold.'

The conclusion was a lot of the pioneering lice populations came from the wild salmonid populations, or a transfer hosts that have yet to be identified, he said.

Researchers put 150 salmon into three cages, 50 salmon a cage, and left them in the loch for one-week. The fish were killed, and taken to a lab, where the number of lice on them was counted, and further broken down into the various stages of their lifecycle. The three-year test period equaled 1.5 production cycles for the salmon farmers in the area.

What the researchers found was a strong correlation between the flow rate of the water and the number of lice that were found, as faster flows mean less lice landed on the fish.

'It’s like trying to land on a jumbo jet with a parachute as it flies by, so if water is going fast, it’s much harder for these animals to land on the fish,' Bricknell said at the World Aquaculture Society (WAS) conference last week in San Diego, Calif.

While some research by Simon Jones in British Columbia identified a carrier species – the stickleback – as being able to take lice from the wild and transport them past farms, Bricknell said his team could not identify the species that was acting as a lifeboat for the lice, though they did rule out sticklebacks.

'It would appear in the Pacific other species like sticklebacks are important, but in Europe we haven’t identified what those species are yet,' he told IntraFish. “It’s quite clear other species seem to have this role as a lifeboat or peripatetic host, where they can go some way of the lifecycle. The lice then become mobile at the pre-adult size and have a second phase when they start seeking out salmonids.

'We didn’t realize that was so important. Simon Jones and my group found almost the same thing. He found on the sticklebacks they would go up to pre-adult and disappear, and we couldn’t find any on sticklebacks but we found the pre-adults and adults appearing on our sentinel fish, which were parasite-free when they went in, so we knew they were coming from somewhere.'

Bricknell and his team assumed they were being carried by sticklebacks (pictured), but a survey of sticklebacks in the loch didn’t find any sea lice on them. They then took some sticklebacks, and tried to infect them with sea lice, and then gave the lice the choice of attaching themselves to sticklebacks or salmon. 

'They always went on to the salmon,' said Bricknell. 'We did some work on the actual smell of the two species, and while we found sea lice from Scotland were attracted to sticklebacks, they were much more attracted to Atlantic salmon. So there’s a difference in either the sea lice or the sticklebacks we have in Europe compared to the Pacific. '

 The next step is to identify how the lice are being transported, as that will be crucial to understanding how to put together a strategy for this parasite, he said.