Sea Lice

Picture of Sea lice.

Sea lice are a naturally occurring parasite found on marine fish. They are small ecto-parasitic copepod crustaceans and there are approximately 559 species, made up of 37 genera found worldwide on most fish types. Two main species affect salmonids in Ireland, Lepeophtheirus salmonis (the salmon louse) and Caligus elongatus. They range in size from 0.5-2 cm and graze on the mucus, skin and blood of fish. They have a complex life cycle with numerous moults from egg to adult stage. Their presence would traditionally have been considered a sign of a returning fish, fresh from the sea. Marine Institute research on the the west coast of Ireland has found average levels of sea lice on returning wild salmon to be 10.9 L. salmonis per fish.

Sea lice only affect marine fish and cause no harm to humans. The itching effect sometimes observed by swimmers, is not caused by this type of sea lice but most likely by the planktonic larval stages of certain jellyfish, sea anemones and other cnidarians.

Cage View

Sea lice are a concern in salmon aquaculture where the health and quality of the fish can be affected. Effective control depends on all aquaculture sites in a bay having a coordinated sea lice management approach. This approach is facilitated by the Single Bay Management process. An extension of the Single Bay Management approach to all aquaculture species within a bay has lead to the development of a Coordinated Local Aquaculture Management Systems (CLAMS).

Potential effects of sea lice on wild salmon stocks

There has been a long standing debate about the potential effects of sea lice on wild salmonid stocks. Sea lice infestations as a source of marine mortality of outwardly migrating ranched Atlantic salmon has been investigated in long term studies in Ireland (Jackson et al, 2013) and in Norway (Skilbrei et al, 2013) with both studies generating similar results. The Norwegian study not only used the same statistical methodology to analyse its results, it also reported similar findings (Jackson et al, 2014). This article also validated the analytical methodology and recognized that over time the level of sea lice-induced mortality has remained relatively constant at approximately 1%, or 10 fish in 1000. Other mortality factors, such as by-catch, climate change, freshwater habitat, predation, etc., have increased substantially leading to a drop in the survival of Atlantic salmon over the study period from the region of 20% to <5%.

The Marine Institute study comprising 28 releases of 352,142 salmon smolts at 8 locations along Ireland's coast from Donegal to Cork, over a 9 year period showed that sea lice were "a minor and irregular component of marine mortality in the stocks studied and is unlikely to be a significant factor influencing conservation status of salmon stocks. The level of sea lice-induced mortality is small as a proportion of the overall marine mortality rate, which is in the region of 90%, and in absolute terms represents 1% (10 fish in a thousand)". Part of the original data used in this study was published in Jackson et al. 2011a and Jackson et al. 2011 b. All the raw data is published in Irish Fisheries Bulletin No. 43.

Graph of Correlation of water quality with river basin district stock status.A Marine Institute paper into Impacts of Aquaculture and Freshwater Habitat on the Status of Atlantic Salmon Stocks found aquaculture had no impact on the status of stocks at river basin district level and the performance of adjacent wild salmon stocks. Rivers in the River Basin Districts with adjacent salmon farms have performed best in terms of meeting their conservation limits nationally, and also in terms of their ability to support a commercial catch by way of a commercial draft net and rod fisheries. Critically, freshwater pollution was found to have a highly significant impact on stock status. These finding supports previous research on farm escapees and sea lice which found little influence of escaped farmed salmon in Ireland on spawning stocks, and that sea lice were a minor and irregular component in marine mortality.

The Marine Institute Irish Fisheries Bulletin No. 43 on Sea lice Epidemiology and Management in Ireland sets out the evidence against a legal complaint to the EU Commission (EU Pilot Case 764/09/ENV1) that Ireland was failing to comply with the EU Habitats Directive and the Environmental Impact Assessment Directive in relation to three named rivers; the Bundorragha, the Newport and the Ballinahinch. The complainants also cited a failure to protect the freshwater pearl mussel (Margaritifera margaritifera) in these rivers. This report concludes that there was no evidence to support any suggestion that the three named rivers were being adversely affected by levels of sea lice infestation, whether from farmed or other sources.

The Marine Institute has also carried out research on sea lice infestation patterns, to determine sources of infestation pressure and the dynamics of lateral and vertical transmission, using sentinel cages and 3D modelling of particle distribution to analyse sea lice dispersal - Jackson et al 2012.

Sea lice Monitoring

The Marine Institute carries out regular inspections of sea lice levels on all fish farms in Ireland in accordance with the Department of Agriculture, Food and the Marine's sea lice Monitoring Protocol (2000) and Strategy (2008). All stocks of fish are inspected by Marine Institute Inspectors on 14 occasions throughout the year. Results from the programme are reported each month to stakeholders and all the data is published on an annual basis. This monitoring programme has been in operation since 1991 and is widely regarded as international best practice.

"In Ireland the control protocols in respect of sea lice are operated by the Marine Institute on behalf of the State and are more advanced than those operated in other jurisdictions for the following reasons: the inspection regime is totally independent of the industry; data obtained as a result of inspections is published; and treatment trigger levels are set at a low level. These controls are widely accepted as representing best practice internationally" Minister for Agriculture, Food and the Marine Simon Coveney, Dail debate, 27 Nov 2012, written answer no. 551

Location of Fish Farm Sites

Fish farms are located in bays along the western seaboard namely; Bantry Bay and Kenmare Bay in the South; the connemara bays and Clew Bay in the West; and Donegal Bay, Mulroy Bay and Lough Swilly in the Northwest. View location of Salmonid sites.

Sea Lice Levels On Farmed Salmonids

Yearly trend of sea lice levels for one-sea-winter salmon for each year since monitoring began shows sea lice levels have been steady for the past five years. The month of May is the key month for salmon smolt migration.

Sea lice levels are determined from the monitoring process carried out by independent Marine Institute Inspectors and measured against treatment trigger levels set out in the protocol. Where levels are higher than treatment trigger levels (0.5 egg bearing females L. salmonis per fish in the spring) farms are instructed to take action to reduce sea lice levels.

The Marine Institute Annual Sea Lice Reports containing results for all the sea lice inspections carried out can be found here:




20032002200120001995 to 2000.