farmed salmon leaping

Transparent lice add to salmon farm fears

  12th December 2018

Fears are growing that lice numbers on Atlantic salmon farms may quickly become even more difficult to control – due to ‘see-through’ lice.

An inspection by the Norwegian Food Safety Authority (NFSA) this autumn (2018) revealed several salmon farms contained only populations of clear or transparent female lice and reports suggest this type of louse is spreading rapidly throughout Norway. Lice are typically a huge issue on at-sea salmon farms, not only in terms of producing salmon for the global food market and fish welfare (see here) but their abnormally high numbers are known to have massive impacts on any wild salmon or sea trout populations nearby – something strongly refuted by salmon producers despite a raft of scientific evidence. Both sea and salmon lice are rightly identified as one of the most serious threats to wild salmonids.

Like it or not, so-called ‘cleaner fish’, such as ballan wrasse (Labrus bergylta), which eat lice, are routinely taken from the wild (unsustainable, to say the least…) and used on salmon farms to reduce lice numbers. If the lice are lacking their normal skin pigments and are transparent then cleaner fish, which feed largely by sight, simply cannot see them and so do not eat them.

transparent sea lice
The difference is clear. A normal louse on the right, a transparent one on the left. Underwater, the transparent lice are even harder to see. Credit: NFSA

“The dark moving lice are eaten by cleaner fish and the transparent ones become sexually mature and spread further without being discovered”

Speaking about the surprising and rapid developments being seen in lice populations, in Norway at least, senior NFSA advisor Mette Kristin Moen warns “Lice with the trait of little or no pigmentation have now had a great advantage over the well-pigmented brown lice for several years and therefore the transparent lice constitute the bulk of all moving lice. The dark moving lice are eaten by cleaner fish and the transparent ones become sexually mature and spread further without being discovered”.

Despite mounting environmental and public health concerns, trade in farmed Atlantic salmon has increased at an average of 10% per year in value terms since 1976, and since 2013 it is the largest single fish commodity by value. In fact, global production has increased by 1000% since 1990. Norway, the world’s largest salmon producer, produced 1.2m tonnes in 2017 alone, worth an estimated €7b while Marine Harvest, the world’s largest salmon producing company, made a pre-tax profit of €792m.

Given the popularity of salmon farming globally, it seems likely this form of adaptive evolution in lice – where a species responds to environmental pressures and changes in order to survive – will soon be noticed elsewhere around the world, possibly including Ireland where almost 50 salmon farms exist in inland bays along our southern, western, and northern coasts.

Moen points out that it is important that farmers now focus on the problem and more people have to become involved in working on it. “In a couple of years, all moving lice are transparent and the effect of cleaner fish has ended – what then?” Farmers are investigating the use of lasers to control lice populations and some are considering farming salmon alongside mussels and scallops, which may eat larval sea lice.

However, to many this latest development just adds to the growing list of reasons that salmon-at-sea farming must be quickly phased out in favour of more controllable, sustainable and less environmentally-damaging land-based systems, such as those highlighted here.