Sea lice are tiny parasites that live on the surface of wild marine fish.  Sea lice are close relatives of other, better known, crustaceans such as shrimps and lobsters, and several hundred different species are found on fish throughout the world. In the UK, and in particular where Atlantic salmon live, two species of sea lice are commonly found: Lepeoptheirus salmonis (often referred to as “Leps”) and Caligus elongatus (often called “caligus”). Lepeoptheirus salmonis is generally found only on Atlantic salmon and sea trout, whereas Caligus elongatus is found on a much wider range of fish, including mackerel, herring as well as Atlantic salmon and sea trout.

The life cycle of sea lice is very complex and includes a number of distinct stages, which can be grouped into three phases.

At the beginning of the life cycle, minute sea lice nauplii (nauplii 1) hatch from the eggs released into the sea by mature female lice. These nauplii, swim freely in the sea amongst the zooplankton. As they grow, they develop through a second nauplii (nauplii 2) stage and eventually turn into copepodids. It is the copepodid stage that marks the end of the sea louse’s planktonic existence, and the young sea lice move on to seek out and attach to a fish.

Once the copepodids find a fish, they attach themselves on to its surface by a slender stalk. Once they have settled on a fish they grow and develop through four further juvenile chalimus stages (i.e. chalimus I, chalimus II, chalimus III and chalimus IV). As chalimus, they remain attached to the fish in the same spot. As development continues, the chalimus turn into pre-adults, passing through two pre-adult stages (pre-adult I and pre-adult II) before becoming adult sea lice. Pre-adult and adult sea lice are able to move freely around on the surface of the fish. Only when they become adults are they able to reproduce and produce eggs.

The time it takes for sea lice to develop from egg to adult is strongly affected by temperature – at warmer temperatures development takes place more quickly.

"Leps" are so-called ‘obligate’ parasites of salmon and sea trout (i.e. they live exclusively on these species) and have evolved along with these fish over tens of thousands of years. They occur naturally on wild salmon and, when the farming of salmon began in Scotland about forty years ago, "Leps" from wild salmon began to appear on the farmed salmon too. 

Lice feed by grazing on the surface of the fish and eating the mucous and skin. When they are properly managed and controlled by the farmers and their vets, they represent a minimal risk to the welfare of farmed salmon. For this reason, it is important to keep their numbers as low as possible and work to prevent them reaching the stage of development where they can reproduce.    

Being responsible for the care of sentient animals, salmon farmers and their veterinary surgeons have ethical and legal responsibilities to maintain the highest standards of health and welfare in the fish in their charge. Keeping fish healthy also makes good business sense, because well cared for fish grow and perform well and yield good returns of a high quality product at the end of the production cycle.  Underpinning these practical considerations is a broad range of EU, UK and Scottish legislation, including the Animal Health and Welfare (Scotland) Act (2006) and the Aquaculture and Fisheries (Scotland) Act (2007), put in place to ensure that certain minimum legal standards covering the health and welfare needs of all farmed fish are met. Above and beyond this, high standards of good practice on health and welfare and, specifically, the way in which sea lice should be managed and controlled, are set out in the Code of Practice for Scottish Finfish Aquaculture.  Thus, Scottish salmon farmers are obliged to meet a wide range of legal and voluntary standards in relation to their day to day activities.

Salmon farmers continuously monitor their fish for sea lice as this provides them with the best information to effectively manage lice. It is a legal requirement for farmers to monitor the presence of sea lice weekly and compliance with this legislation is regulated by the Scottish Government’s Fish Health Inspection Services. Furthermore, compliance with the CoGP requires regular and defined monitoring of sea lice.

Scotland is proud to have pioneered the development and application of integrated sea lice management strategies.  These strategies take a holistic approach to lice management and control, based on the use of licensed and approved sea lice treatment medicines, single year class production, area management, synchronisation of production, and fallowing at the end of the production cycle. More recently, biological control, involving the use of cleaner fish including wrasse, has emerged and this has been added to the list of options available to the farmers.