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Argulus (fish louse)

The fish louse Argulus is a highly damaging parasite and one of the greatest disease threats to stillwater trout fisheries. National Fisheries Laboratory have been trialling a new approach to help control infections of this parasite in stillwaters.

The aim is to provide fishery owners with a cheap, simple and effective way to break the life cycle of the parasite and reduce future disease problems.

Argulus biology

Argulus reproduce by laying eggs in long strings on any hard substrate. Female parasites start laying their eggs in spring when water temperatures rise above 10ºC and will lay through the year until water temperatures drop below 10ºC. Eggs hatch in around 16 days at 20ºC, longer in colder temperatures. During winter many adult parasites will die off and it is mainly egg strings that overwinter, with these egg strings dictating infections the following spring. Argulus is a common parasite of freshwater fish, found in fisheries throughout the UK. Adult lice are flat, round, jelly-like parasites that feed on skin and mucus. Heavy infections can quickly cause irritation, condition loss and death.

Managing Argulus infections

In 2005, the Environment Agency part-funded a PhD project in partnership with the trout industry and Stirling University to improve understanding of Argulus infections in stillwater trout fisheries. This focused on identifying the risk factors for disease problems and how these could be avoided through fishery management approaches. This work confirmed that a number of fishery management measures, in particular stocking practices, can help reduce the impact of the parasite and, in combination, prevent the build-up of infections.

You can find current approaches for managing Argulus infections in the Environment Agency's guidance for fisheries (Argulus: Management in Stillwater Trout Fisheries factsheet). You can request a copy of this guidance using the details at the bottom of the page or by contacting your local Environment Agency fishery officer.

Desperate times bring desperate measures

Despite these approaches, Argulus continue to cause serious problems in stillwater fisheries every year. These measures are not always feasible and in some cases can be more expensive. Similarly, they are primarily focused on trout fisheries and offer limited value for coarse fisheries, where Argulus can also be problematic.

The scale of this problem has led to some drastic measures in an attempt to control the parasite. These include closing fisheries, culling stock during the summer months, illegal use of chemicals and draining and liming fisheries, sometimes year upon year.

Argulus in trout and other fish
Argulus parasites

An additional difficulty with managing Argulus in fisheries is being able to monitor parasite levels before they become a problem. Counting lice on the fish caught by anglers is important, but can lead to an underestimate of a problem as only the healthier fish tend to get caught. This alone can hinder awareness of how the parasite population is developing through the year.

These challenges have highlighted the need for a simple, cheap and effective solution to break the life cycle of the parasite and help fishery owners to both monitor and reduce lice burdens in stillwater fisheries.

Studies on a new approach

From early 2017, Environment Agency staff at National Fisheries Laboratory (NFL), Brampton have been trialling a new approach to break the life cycle of Argulus by harvesting the egg strings of the parasite. These are based on the use of plastic pipes, placed vertically in the margins of fisheries where Argulus can lay their eggs and then be removed easily from the water. Trials suggest that this approach, in combination with other fishery management tools, could provide an effective way to monitor egg laying activity and also kill large numbers of parasites in the process.

Egg strings Argulus
Testing fisheries

These trials have shown that through the use of plastic pipes, in combination with other management practices, large numbers of egg strings can be successfully harvested and removed from fisheries with Argulus problems. It also provides a cheap and simple approach that can be applied to any water, allowing easy monitoring of egg string numbers and removal of eggs before they hatch. In some fisheries, over half a million eggs have been removed on each pipe, with a reduction in lice burdens on the resident fish. It is too early to say whether this provides a long-term solution for Argulus control, and results are not guaranteed, but we are keen to raise awareness of the work and are supporting trials at more fishery sites.

If you are interested in exploring this approach at your fishery, please contact the Environment Agency so that they can offer advice on the best way to proceed. The success of this approach is not guaranteed and will rely on many different factors, including the size and characteristics of the fishery, existing egg laying sites and other management practices, so these need to be considered as part of a wider management plan. You may be able to obtain some funding for this initiative through national Fisheries Improvement Funds. For more information on this, please contact the Environment Agency using the details below or through your local fisheries officer.

Contact National Fisheries Laboratory

NFL Brampton can help with any fish health related query or suspected disease problem and encourage you to get in touch if you would like to discuss this article further. They're interested in hearing your experiences of this initiative or any ideas of how you think they could improve it for other fisheries.

National Fisheries Laboratory, – Fish Health, Ageing and Species, Environment Agency, Bromholme Lane, Brampton, Huntingdon, PE28 4NE

Call: 02084 745244 or 07768 022726

Email: [email protected]


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Last Edited: 26 July 2023

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