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Sea Lice
Species of parasitic sea lice currently affecting farmed fish
| Species |
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Known range |
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Hosts |
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|
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| Lepeophtheirus salmonis |
|
North Atlantic and north |
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Specific to salmonid species e.g., Pacific affecting farms in Atlantic salmon |
| Salmo salar |
|
Canada, USA, Japan & Europe |
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Pacific salmon, Onchorhynchus sp, Arctic charr Salvelinus |
| Caligus elongatus |
|
Atlantic coast of Canada |
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Not host specific - found on > 80 species of fish including salmonids |
| Caligus curtus |
|
Atlantic coast of Canada |
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Not host specific - occasionally found on salmonids |
| Caligus clemensi |
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Pacific coast of Canada, USA |
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Not host specific - occasionally found on salmonids |
| Caligus rogercresseyi |
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Pacific coast - Chile |
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Not host specific - found mostly on salmonids in Chile (previously C. flexispina) |
| Caligus teres |
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Pacific coast - Chile |
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Occasionally found on salmonids in Chile |
Introduction
Sea lice is the common term used for one group of parasitic caligid copepods which occur naturally on fish world-wide
Copepods are crustaceans found in both marine and freshwater environments. Most are planktonic, while others are found living in the sediments. Some species are specialised to live as parasites, on or in host organisms at some stage in the lifecycle, although one or more stages are free-living as plankton in the water, usually during the early stages of development. Under intensive salmon farming conditions where fish are kept in close proximity, sea lice numbers can increase rapidly and cause serious problems as a result of the opportunistic response of lice to the high density of available hosts. While low numbers of sea lice cause only minimal damage to the host, high numbers can result in severe effects and even death of the host fish.
Epidemiology
There are approximately 200 known species of Caligus and 90 species of Lepeophtheirus, most of which are parasitic on marine fish Although large numbers of sea lice are usually associated with commercial fish farming, sea lice originated from wild fish and, as early as 1940, long before the development of intensive fish farming, there were reports of high numbers of lice causing severe damage/mortality in wild fish
More recently, research in Norway and Japan has shown that in some areas sea lice are common on wild salmonids in the open
While Lepeophtheirus salmonis and Caligus elongatus are the most important species affecting farmed fish in the northern hemisphere, other species of parasitic copepods are becoming a problem as fish farming expands world-wide and more species of fish are being cultured
Life-cycle of Sea Lice
This life-cycle consists of three phases:
- Free-swimming larval stages - Nauplius I, nauplius II & copepodid.
- Immature attached stages - Copepodid, chalimus I, II, III and
- Motile stages - (Lepeophtheirus) Pre-adult I, Pre-adult
- (Caligus) Mature adults
Diagnosis
Visible effects of sea lice on host fish may vary by location and severity. Damage to the host fish is caused by the feeding activity of sea lice. Chalimus stages cause only mild, localized damage although even this may have adverse effects on fish health.
The most damaging stage of L. salmonis tends to be the pre-adults, particularly as these concentrate on the head region which has no protective scales and is therefore more susceptible to damage Although adult female L. salmonis are the largest stage and can cause more damage, once they become gravid the majority tend to move to a position behind the dorsal and anal fins where damage is less severe and not so detrimental to the fish
Disease Prevention And Management
While sea lice have caused serious economic and welfare problems to the aquaculture industry in the past, there are now very effective treatments available which will allow effective control of sea lice populations. The recent development of treatments which are effective against both the immature chalimus and mature motile stages of lice has led to improved control and thereby reduces the need for frequent repeat treatments With a range of different treatments now available, the potential for resistance can be reduced while good management practices and rotation of different treatments will further help to ensure continued success in the control of sea lice in farmed fish.
Treatment
References
J. Stone, University of Stirling, Scotland
© 2002 Schering-Plough Animal Health Corporation
All Rights Reserved.
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