Sea lice numbers on salmon farms double in a single year

Total sea lice numbers on salmon farms double in a single year.

Industry and official SEPA data underline how the rush to expand salmon farm production is massively increasing the risks to wild salmon and sea trout.

“A moratorium on salmon farm expansion is now more essential than ever”

Andrew Graham-Stewart, Director of Salmon and Trout Conservation Scotland (S&TCS).

Data recently published by the Scottish Salmon Producers Organisation (SSPO) shows that the average adult female sea lice count per fish on salmon farms increased from 0.25 in April 2018 to 0.49 in April 2019 – an increase of 96%.

Over the same period the total amount of salmon in Scotland’s farms rose by almost 25%, from 97,000 tonnes to 122,000 tonnes, according to data published by the Scottish Environmental Protection Agency (SEPA). The 122,000 tonnes figure is the highest on record.

By combining the above data sets and with the fair assumption that the average size of the fish across all farms is unchanged year-on-year, it is evident that total adult female sea lice numbers in April were more than double the total for a year earlier.

Multiplying (total biomass in 2019 / total biomass in 2018) by (average lice per farmed fish in 2019 /average lice per farmed fish in 2018) shows that, in April 2019, the production of juvenile sea lice by fish farms is likely to have been between two and three times higher than that in April 2018.

Andrew Graham-Stewart, Director of Salmon and Trout Conservation Scotland (S&TCS), said:

“The absolute number of adult female sea lice on farms is of far greater relevance as regards the impact on wild salmon and sea trout than the average number of lice per farmed fish.It is these adult female lice on the farms that produce the lice larvae that then infest wild fish in the sea lochs.There can be no doubt that there has been a dramatic year-on-year increase in total adult female sea lice numbers on Scotland’s salmon farms. The available data indicates there was a huge escalation in the production of sea lice larvae, at the most critical time in the spring when juvenile salmon migrate from their rivers to sea and are most vulnerable to fatal sea lice infestations.”

Guy Linley-Adams, Solicitor to S&TCS, added:

“The massive increase in sea lice numbers is a major concern. The situation is now far worse than it was during last year’s Scottish Parliamentary Inquiry. In the circumstances, a moratorium on salmon farm expansion is now more essential than ever, if further damage to wild fish survival is to be avoided. Scottish Ministers need to act now and stop kicking the necessary decisions into the long grass”.

Permissions for new salmon farms or salmon farm expansions are continuing on a regular basis. For example, in Argyll and Bute alone there have been ten planning decisions in favour of salmon farming expansion since the publication of the ECCLR Committee report in March 2018. Another three applications are awaiting decisions.

Following a warmer than average winter, salmon farming industry sources are predicting severe problems with sea lice on fish farms in Scotland this autumn. They report that sea lice levels have been highly challenging this summer with no end in sight.

They also report that some companies are choosing not to treat their fish for sea lice because of high levels of other disease and gill health problems, which weaken farmed salmon and reduce their ability to survive the chemical and physical treatments for sea lice.  This will almost certainly inflict even more damage on wild salmon and sea trout.



Issued by Corin Smith. (T: 07463 576892).

Notes for editors

1) Salmon & Trout Conservation UK (S&TC UK) was established as the Salmon & Trout Association (S&TA) in 1903 to address the damage done to our rivers by the polluting effects of the Industrial Revolution. Since then, S&TC UK has worked to protect fisheries, fish stocks and the wider aquatic environment for the public benefit. S&TC UK has charitable status in both England and Scotland (as S&TCS) and its charitable objectives empower it to address all issues affecting fish and the aquatic environment, supported by robust evidence from its scientific network, and to take the widest possible remit in protecting salmonid fish stocks and the aquatic environment upon which they depend.

2) Just what is the problem with sea lice?

Adult wild salmon are well adapted to coping with a few sea lice. Background levels of these parasites occur naturally in the sea. However, the advent of salmon farming, particularly in fjordic sea lochs, has led to a fundamental change in the density and occurrence of sea lice in parts of the coastal waters of the west Highlands and Islands. Even one or two mature female sea lice per fish within a set of cages housing hundreds of thousands of farmed salmon amounts to a rampant breeding reservoir pumping huge numbers of mobile juvenile sea lice out into the local marine environment. The consequences when wild salmon and sea trout smolts, the metamorphosing fragile skin of which is not adapted to cope with more than the odd louse, migrate from local rivers into this “sea lice soup” can be devastating.

Carrying an unnaturally high burden of sea lice is known to compromise severely the survival of juvenile wild migratory salmonids; research has shown that an infestation of ten to 12 lice 10 is likely to have fatal consequences. Lice feed by grazing on the surface of the fish and eating the mucous and skin. Large numbers of lice soon cause the loss of fins, severe scarring, secondary infections and, in time, death. Quite literally, the fish are eaten alive. Badly infested salmon smolts disappear out to sea, never to be seen again. In contrast afflicted sea trout smolts remain within the locality and they, together with the impact of the deadly burdens they carry, are more easily monitored through sweep net operations.

3) Research indicates that a 1,000 tonnes salmon farm with an indicative adult female sea lice count of 5.0 per fish could be producing as many as three billion sea lice larvae per month. Open cage salmon farming allows unhindered free flow of sea lice into the wider marine environment.