Sea Lice


Sea Lice Monitoring

The Marine Institute operates the National Sea Lice Monitoring Programme on behalf of the State since it was first instigated in 1991. Since then, the monitoring programme has been refined and formalised through the government’s Monitoring Protocol No. 3 in 2000 and the publication of the Strategy for improved pest control in 2008. All active marine finfish farms are inspected 14 times per year, on a fortnightly basis from March to May and monthly from June to February (with December and January combined as one period). Inspection results are reported on a monthly basis and all the data is published in an annual report, available through the Marine Institute Open Access Repository.


What are sea lice?

Sea lice are a group of invertebrate ectoparasitic copepods, of the family Caligidae, found on fish world-wide. There are two species of sea lice commonly found around the coast of Ireland, Caligus elongatus, which infests over eighty different species of marine fish, and Lepeophtheirus salmonis (the salmon louse), which primarily infests salmonid species. In Ireland, L. salmonis is considered the more serious parasite on Atlantic salmon, both in terms of its prevalence and effects and is a common ectoparasite of both wild and farmed Atlantic salmon at sea.

Picture of Sea lice.

Caligid parasites generally have a direct lifecycle requiring only one host. Development to the adult stages involves hatching out of egg strings in the water column, after which they enter the planktonic stage of development. This stage consists of two free-living nauplii and one copepodid stage. It is during the copepodid stage that the louse must find and infect a host. Once a host is found, the chalimus stage begins, consisting of two stages during which the louse is attached to the host by a frontal filament and is not mobile. This is followed by two pre-adult and one adult stage, where the parasite is mobile and can move over the surface of the fish and swim in the water column.

Picture of Sea lice life cycle.
Life cycle of Lepeophtheirus salmonis.


The duration of each developmental stage is directly dependent on water temperature. Temperature also plays an important role in determining infestation success rates and reproductive capacity. Understanding the relationship between temperature and louse development can play an important role in determining the frequency of when control measures should be applied on farms.

In addition to the influence of temperature, salinity and light play an important role in the ability of the free-living planktonic stages to find a host. In general, salmon louse nauplii and copepodids avoid low salinities with nauplii in particular avoiding salinities below 30 ppt. Nauplii are phototactic and migrate upwards towards light, however it has been shown that the copepodid stage is less phototactic than previously believed.

The parasite is extremely well adapted to target its host species and is ubiquitous to all the coastal waters around Ireland, and indeed, throughout the migratory range of Atlantic salmon. Returning wild salmon have been found to carry an average of 10 or more adult egg bearing females on their return to the Irish coastline from their feeding grounds in the Atlantic.


Impact of Sea Lice

The effects of sea lice infestations on the host are directly related to the physical damage caused by the parasite through its attachment and feeding activities. The initial damage is caused by the attachment of the copepodid stage and subsequent feeding activity of the chalimus stages. As these stages are non-motile, the damage to the host is localised. It is the mobile, pre-adult and adult stages of sea lice which have the greatest effects on farmed and wild Atlantic salmon. These mobile stages attach to the host by suction, generated by the cephalothorax allowing them to move freely over the host body feeding on the skin, mucus and blood. This feeding behaviour can damage the integrity of the fish’s epithelium, disrupting the osmoregulatory ability of the host and may leave the fish open to secondary infections. The net effect of infestation, especially if it is left unchecked, is a reduced growth rate, poor welfare and an increased morbidity.

Atlantic Salmon
Factors influencing marine survival of Atlantic salmon at sea (adapted from the Technical Expert Group on Salmon (TEGOS) – annual report on the status of Irish salmon stocks).


There are many factors which can affect the marine survival of Atlantic salmon at sea. According to the International Council for the Exploration of the Sea (ICES), climatic factors modifying ecosystem conditions are considered to be the main contributory factors to lower productivity, which is expressed almost entirely in terms of lower marine survival.

Over the last number of decades’ concerns have been raised that sea lice, more specifically the salmon louse Lepeophtheirus salmonis, have been negatively impacting wild salmonid populations and this has been the subject of intense research and scientific debate.

Marine Institute studies, performed between 2001 and 2009 at eight locations along the west coast of Ireland showed this to be the case in some instances, but overall “the combined data suggest that while sea lice-induced mortality on outwardly migrating smolts can be significant, it is a minor and irregular component”. Taken together with other studies, performed in Ireland and Norway, there appears to be a marked variability in overall survival within individual studies with much of the variation explained by the release location, time period and overall baseline (background) survival. The main findings of these studies were summarised in an extensive report published by ICES.


Farm locations

Marine Atlantic salmon farming in Ireland is carried out in three distinct regions (Figure 2), namely the north-west (Co. Donegal), west (Co. Mayo & Co. Galway) and the south-west (Co. Kerry and Co. Cork). These regions are geographically separate from each other with distances between regions of around 160 km from north-west to west and 200 km from west to south-west.

Farm Locations


The Aquaculture Information Management System (AQUAMIS) contains mapping and details of licensed aquaculture sites including datasets from the National Parks and Wildlife Service and the Marine Institute.

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




20032002200120001995 to 2000.

Yearly trend of sea lice levels for one-sea-winter salmon for each year since monitoring began during the month of May, which is the key month for salmon smolt migration.

Ovigerous L. salmonis.


Total Salmonis.