Sea lice are a naturally occurring marine parasite of fish. There are over 500 species and they are found on most fish species worldwide. Two main species affect salmonids in Ireland, Lepeophtheirus salmonis (the salmon louse) and Caligus elongatus. They are small copepods ranging in size from 0.5-2cm and graze on the mucus, skin and blood of the 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. Current Marine Institute research on the the west coast of Ireland has found average levels of sea lice on returning wild salmon to be10.9 L. salmonis per fish.
Sea lice only effect fish and cause no harm to humans. The itching effect sometimes observed by swimmers, is not caused by sea lice but by the planktonic larval stages of certain jellyfish, sea anemones and other cnidarians.
Sea lice are a concern in salmon aquaculture where the health and quality of the fish can be affected. Effective control depends on all sites in a bay having a co-ordinated 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 Co-ordinated Local Aquaculture Management Systems (CLAMS).
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 Norway (Skilbrei et al, 2013) with both studies generating similar results. 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)’. Related research papers:- Jackson et al. 2011a Jackson et al. 2011b.
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 inspection of sea lice levels on all fish farms in Ireland in accordance with the Department of Agriculture, Food and 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 interested parties 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 sea board namely; Bantry Bay and Kenmare Bay in the South; Connemara bays and in Clew Bay in the West; and Donegal Bay, Mulroy Bay and Lough Swilly in the Northwest.
Sea Lice Levels On Farmed Salmonids
Yearly trends of sea lice levels on farmed fish shows the mean L. salmonis levels for the month of May for each year since monitoring began. 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 treat to reduce sea lice levels.
Marine Institute Annual Sea Lice Reports containing results for all the sea lice inspections carried out can be found here: 2012, 2011, 2010, 2009, 2008, 2007, 2006, 2005, 2004, 2003, 2002, 2001, 2000, 1995 to 2000.