Sea lice were difficult to control last year due to “abnormally high water temperatures and insufficient cleaner fish capacity”, Marine Harvest said in its latest annual report.
Marine Harvest’s Scottish salmon harvest was down 10.2% to 45,046t in the year to December 31. The aquaculture business blamed the difficulty in controlling sea lice on its fall in yield, but said alternative treatments could improve harvest in the future.
Salmon stock suffered from reduced survival because of sea lice treatment. Common treatments included adding the chemical ‘Slice’ to salmon feed, or using hydrogen peroxide. But, increasing resistance to treatments was becoming a costly problem.
What are sea lice?
- Lethal parasites found on salmon’s body
- Eat skin, blood and mucus of salmon
- Kill or weaken salmon
- Common on adult salmon
- Can live off one salmon for up to three weeks
- Two species found on salmon
‘Decrease in harvest volume’
Referring to global harvest, the report said: “The harvest volume for the year ended December 31 was 235,962t gutted weight, a reduction of 18,789t from 2015. The decrease in harvest volume is due to biological challenges, in particular related to sea lice.
“Sea lice control remains the most challenging area within salmon farming globally. Our research and development efforts relating to this issue are broad and will continue to be so going forward.”
Almost seven times as many Scottish sites were above sea lice limits in 2016 compared with 2013. In total, the number of Scottish sites above the limit were more than four times the average for the group’s overall operations, featuring sites in Norway, Canada, Chile and the Republic of Ireland.
But, Marine Harvest said that sea lice numbers could be reduced by using non-medicinal treatments.
Marine Harvest ceo Alf-Helge Aarskog said: “It has become apparent that sea lice numbers can be brought under control through increased use of non-medicinal treatment methods. Although we still have a way to go, we increased our use of non-medicinal tools in 2016 and expect to reap the benefits of our efforts going forward.
‘Non-medicinal treatments’
“The latter included the more widespread application of preventative measures, use of cleaner fish and non-medicinal treatment systems.”
Non-medicinal treatments included four techniques: ‘flusher’, ‘thermal’, ‘freshwater’ and ‘cleaner fish’.
Flusher involved the physical removal of sea lice by using jets of seawater.
Thermal removal meant using warmer seawater, to which the lice were sensitive.
Freshwater involved using a freshwater bath to remove sea lice.
Cleaner fish was a biological control, which used some fish species, including wrasse and lumpsuckers, to remove the lice.
The report said: “Our research and development focus areas include prevention, as well as further development and optimisation of non-medicinal treatments, cleaner-fish farming and the optimisation of cleaner-fish use, as well as biotechnological solutions.”