Fatal attraction: Sea lice roundup
The reason parasites are able to sniff out potential hosts, hard words from MSPs and questions over Norway’s ‘traffic light’ system all feature in our sea lice round-up.
Salmon give off a smell that sea lice find irresistible – and fish with lower resistance to the parasites are even more attractive.
New research has enhanced our understanding of why sea lice are predominantly attracted to salmon.
The international study, involving scientists from a leading team of scientists from Norway, the UK, USA, Canada, Sweden and Australia, is investigating the chemicals released by Atlantic salmon that serve as attractants to sea lice.
The encounter between a salmon louse and its host begins when the louse is in its free-swimming larval stage, known as a copepodite. But being tiny organisms in a vast ocean, the scientists wanted to know how they locate salmon as their host.
“Chemical signalling is believed to play a key role in host-parasite communication, and scientists have confirmed this,” says Nicholas Robinson from Norwegian research institute Nofima.
Robinson is coordinating the CrispResist project, a collaboration between 12 partners from science and industry. In particular, Rothamsted Research (UK), University of Gothenburg (Sweden), Bigelow Laboratory of Ocean Science (USA) and Nofima (Norway) have contributed to this part of the project. Their goal is to uncover the mechanisms behind cross-species variation in host resistance to sea lice and apply this knowledge to enhance Atlantic salmon resistance in aquaculture.
A crucial objective is to identify and document genes and mechanisms responsible for the difference in sea lice resistance between salmonid species. It is well-established that certain Pacific salmon species are resistant to sea lice and can kill them in the early stages of parasitism, whereas Atlantic salmon are highly susceptible.
Holyrood inquiry calls for tougher action on sea lice
The move to mandatory weekly reporting of sea lice numbers is “a step towards” the aim of ensuring that maximum permitted levels are “challenging and of the highest international standards”, but more needs to be done.
That is one of the conclusions of the Scottish Parliament’s Rural Affairs and Islands (RAI) Committee in its report on the salmon industry. The committee was following up on a 2018 report by its predecessor, the Rural Economy and Connectivity (REC) Committee.
In its report, published last month, the RAI Committee welcomed the introduction of mandatory weekly reporting of sea lice counts. The committee expressed some concern at the number of “no counts” permitted for a variety of reasons including bad weather and veterinary treatments.
The lobby group WildFish had told the committee that as many as 20% of data entries from the industry were “no counts”.
The report concludes: “Whilst members recognise there will be weeks when it is not possible for fish farms to undertake a sea lice count, the reporting mechanism needs to be robust enough to be comprehensive and accurate in order to inform the Fish Health Inspectorate’s oversight.
“The committee recommends the Scottish Government introduce stricter conditions on the accepted reasons for no counts with regards to stock that is subject to treatments and being held for harvest, and updates relevant guidance and enforcement approach accordingly.”
The RAI Committee report also argues there are gaps in the data on the long-term effects of using emamectin benzoate (“Slice”) a pesticide used to control sea lice numbers. Slice is deemed to be harmless to fish and mammals, but kills crustaceans.
The MSPs also said the four years given to farms to cut their use of Slice is too long. The report says: “The committee is concerned by the proposed four-year implementation period for the introduction of a revised environmental quality standard for emamectin benzoate, given the environmental risks from the chemical. It recommends the Scottish Government considers whether an expedited timetable may be appropriate.”
Traffic light system questioned
Norway’s “traffic light” system of regulation for salmon farming divides the country’s coastal waters into 13 production zones, in three categories: “Red”, “Yellow” and “Green”.
The colour of the zone determines whether there is scope to increase farming production (Green) or whether it must stay within existing limits (Yellow) or be reduced from current levels (Red).
The status of the zones is based on the perceived threat to wild fish from sea lice populations, which can be larger in areas where there is a significant farmed fish presence.
This basis has now come into question, however. A new study, carried out by scientists at the Department of Breeding and Genetics at the Norwegian research institution Nofima, suggests that the link between sea lice density at sea and the number of sea lice observed on wild fish is much more tenuous than had been previously thought.
The Nofima team (Bjarne Gjerde and Muhammad Luqman Aslam) revisited a study carried out by Myksvoll et al in 2018 (Evaluation of a national operational salmon lice monitoring system - from physics to fish).
The Myksvoll study analysed observed mean lice burden (OMLB) per gram body weight of wild sea trout caught using traps and nets, and the predicted mean lice particle density (PMLD) in the sea at the same locations.
Myksvoll et al concluded that PMLD can provide reliable information about the spatial distribution of salmon lice infestation pressure all along the coast, and therefore could help inform decisions made by regulators about spatial planning for aquaculture.
The Nofima study, however, set out to cross-validate the Myksvoll data, and found that none of the three methods they applied – linear regression models, bivariate logistic regression models, and a threshold model with three assumed mortality classes – confirm the Myksvoll conclusions.
The problem is not the quality of the mathematical models used, the Nofima team say, but the lack of useful data. Many of the locations and wild trout samples tested found no sea lice present, and the fish that were found with parasites showed a very wide degree of variation in terms of the lice count.
The Nofima study says: “The very low predictive power of all models shows that PMLD is such a poor predictor of OMLB that assigning a Green, Yellow or Red colour to a PZ with a sufficiently high statistical power is only possible with PMLD values from many sea areas in sum larger than the area of the majority of the 13 PZs.”
In other words, the classification of the “traffic light” zones is based on models for which not only is there not enough data; the zones are not large enough to allow that much data to be gathered.
“Prediction of the salmon lice density on wild sea trout from the mean predicted lice density in the sea: A cross-validation of the data in Myksvoll et -al.” (2018) Aquaculture Volume 599, 15 April 2025, Bjarne Gjerde, Muhammad Luqman Aslam
doi.org/10.1016/j.aquaculture.2025.742149
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