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What is a sea louse? Dr Sally Campbell

Sea liceWhy is it that sea lice have become such a focus of attention, even to the extent that the collapse of wild salmon stocks in west of Scotland rivers has been linked to sea lice endemic in salmon fish farms on the west coast?

 

COAST's Dr Sally Campbell provides answers to why and how the sea lice that spread from fish farms cause such damage to the wild salmon population...

Diagram courtesy of Thomas Schram (who retains copyright)

The media has carried articles about sea lice causing the death of young wild salmon, called smolt, as they make their way to sea. In any coastal environment one would expect to find parasitic organisms preying on their hosts and sea lice take their place in natural ecosystems.   Indeed, it is not a problem confined to Scotland; Norway, Ireland, Chile and British Columbia are plagued with sea lice infestations in the overcrowded conditions of fish cages.

 

 

 

Like all highly specialised concentrated monocultures the species reared in such conditions is always vulnerable to the spread of disease and infestation and fish in confined farm cages in the sea are no exception. So sea lice can easily multiply and rapidly infect the imprisoned fish population. This artificially enhanced population of parasites is not confined to the fish cages but migrates into the wider environment, vastly increasing the incidence of parasitism.

 

COAST has been asked what they are, how do they get to the salmon farms and why are they so damaging to the smolt stages of wild salmon? This month we get to grips with the fish louse and its life cycle. Next month we will tackle the vexed question of control of lice in fish farms and why the salmon farming industry spends millions of pounds a year combating the lice in a battle for economic survival against nature which some say is being lost.

Sea lice is the common term used for one group of parasitic copepods which occur naturally on fish world-wide. Copepods are crustaceans - minute marine or freshwater crustaceans usually having six pairs of limbs on the thorax; some abundant in plankton and others parasitic on fish. The order they belong to are the Branchiura - copepods with suctorial mouthparts which are parasitic on fish. They “moult” to grow or in other words replace their protective shell, the chitinous exoskeleton, more widely known to be associated with crabs, prawns and shrimps. Copepods are found in both fresh and seawater. However, sea lice die once they enter fresh water. Sea lice are marine ectoparasites (external parasites) that feed on the mucus, epidermal tissue and blood of marine fish. Species from two groups- Lepeophtheirus and Caligus have been recorded on farmed fish, and Lepeophtheirus salmonis is the major problem in farmed Atlantic salmon, wild salmon, sea trout and arctic char. When full grown they are about 1cm long and can easily be spotted clinging on to the body of the fish. A sheep tick is very small by comparison.

Sea lice have both free swimming, planktonic and parasitic life stages. See www.upei.ca. In the adult, the body consists of 4 regions: cephalothorax, fourth leg-bearing segment, genital complex, and abdomen. The cephalothorax forms a broad shield that includes all of the body segments up to the third-leg bearing segment. It acts like a suction cup in allowing the louse to cling on to the fish. All species have mouth parts shaped as a siphon or oral cone (characteristic of the order of Copepods, the Siphonostomatoida). The second antenna and oral appendages are modified to assist in holding the parasite on the fish. The second antenna is also used by males to grasp the female during copulation.

 

The sea louse, which resembles a tiny horseshoe crab, is well adapted to life as a marine ectoparasite. Its flattened head is covered by a shield, and its legs are specialized to allow it to grasp on to the host and feed on fish. Female sea lice have what looks like two tails trailing behind that are actually strings of eggs. The adult females are always much larger than males and develop a very large genital complex which in many louse species makes up the majority of the body mass. Two egg strings of 500 to 1000 eggs (L. salmonis) that darken with maturation are approximately the same length as the female’s body. One female can produce 6-11 pairs of egg strings over a short lifetime of approximately 7 months. The female sea lice carry the eggs in long trails and lays in favourable conditions. The  life cycle consists of 10 different stages. The entire life cycle from egg to adult for L. salmonis is 7-8 weeks at about 10° C. All stages are separated by moults.

The first three life stages of sea lice are motile and non-parasitic. The egg hatches into so-called Nauplius 1 that gives rise to Nauplius 11 after moulting. Both naupliar stages are non-feeding, depending on yolk reserves for energy, and adapted for free swimming.  They possess distinctive appendages and are actively motile. The third motile stage of sea lice is the Copepodid stage. The louse at this development stage is more rice grain shaped with less noticeable appendages. This is the infectious stage and at this point it on the search for an appropriate host. How planktonic stages of sea lice disperse and find new hosts remains somewhat of a mystery. Temperature, light and currents are major factors and survival depends on salinity above 25%. Sea water approaches 33% of dissolved salts. Several field and modelling studies have examined copepodid populations in intertidal zones and have shown that planktonic stages can be transported tens of kilometres from their source. The Copepodid undergoes moulting, which develops to the first parasitic larval stage with a frontal filament, the Chamilus. This structure allows the louse to attach to the fish. At this point it gets more complicated still!

The Chamilus moults through four successive stages after finding its host and is found commonly on the fins and tails of marine fish. The Chamilus larvae then develops into preadults 1 and II. These are not usually attached to the fish by a frontal filament, but often burrow under the scales to keep very close to the host fish. Finally, the preadult II moults and develops into the adult sea lice that are visible to the naked eye. The adult female is larger than male sea lice and possesses long trails. The preadult and adult stages of sea lice cause maximum mortality by weakening the life support system of the fish.  The lifespan of adult sea lice may range from 6-8 weeks depending upon the species. The preadult stage tends to be the most damaging, particularly as these concentrate on the head region of the fish where there are no protective scales and is therefore more susceptible to attack.

Sea lice cause physical and enzymatic damage at their sites of attachment which results in abrasion-like lesions. The infection can cause a generalized chronic stress response in fish. Heavy infection can cause deep lesions particularly in the head region of farmed Atlantic salmon. Adult wild salmon may survive otherwise critical numbers of sea lice, but small, thin skinned juvenile wild salmon migrating to sea are highly vulnerable, and sea trout populations in recent years have seriously declined and higher than normal populations of sea lice generated in the proximity of fish farms have been implicated in this decline. Recently published reports on work in western Canada showed up to 28 lice on a single juvenile wild sockeye salmon (see www.farmedanddangerous.org/uploads. The River Trusts in Western Scotland are concerned by high levels of lice infestation in salmon and trout.

In summary the sea louse displays a complex life cycle in the complexity of the marine environment. Much is still to be understood about the way it is dispersed, effect of currents, temperature, and its response to density of fish or host biomass concentration. But like many infectious agents- for example sheep ticks- the more sheep there are in an area, the more ticks are able to find hosts and therefore complete their life cycle. So it is with the sea lice.

References:

Morton, A. and Routledge, R.D. (2005). Mortality rates for Juvenile Pink Oncorhynchus gorbushca and Chum O. keta salmon infested with Sea Lice Lepeophtheirus salmonis in the Broughton Archipelago. The Alaska Fisheries Research Bulletin. 11(2): 146-152
Schram, T.A. (2004). Practical identification of pelagic sea lice larvae.
Journal of Marine Biological Association of the United Kingdom, 84: 103-110.
Watershed Watch Salmon Society (2004) Sea Lice and Salmon: Elevating the Dialogue.

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