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A National Treatment Strategy for the Control of Sea Lice on Scottish Salmon Farms

A Code of Practice - Published by Scottish Salmon - At the SSGA Technical Seminar in November 1997, Industry scientists reported on significant work regarding a strategic approach to control of sea lice. They have demonstrated that the reproductive capacity of female lice is compromised in the spring, resulting in reduced survival in their offspring.

A National Treatment Strategy for the Control of Sea Lice on Scottish Salmon Farms - A Code of Practice - Published by Scottish Salmon - At the SSGA Technical Seminar in November 1997, Industry scientists reported on significant work regarding a strategic approach to control of sea lice. They have demonstrated that the reproductive capacity of female lice is compromised in the spring, resulting in reduced survival in their offspring.

A National Treatment Strategy for the Control of Sea Lice on Scottish Salmon Farms

A late winter treatment offensive aimed at lowering the number of adult females to the lowest possible levels can have significant benefits. When data from the 1993 year class was compared with that of the strategically treated 1995 year class, the results showed that they could reduce fish mortalities by up to 83%, reduce fish being downgraded at harvest by 80%, have a 62% reduced mobile louse infestation level and reduce treatments by as much as 46%.


It is suggested in this document that control of sea lice at a national or regional level will be improved if salmon farmers adopt a collaborative policy of coordinated treatments much like those that were recently proposed for the eradication of sheep scab by the Crofters of Skye (see attached papers for information).

Progress with this initiative depends on the maximum cooperation from all Scottish salmon farmers. In order to initiate improved control of lice farmers are invited to try out the strategy on a regional basis in the first instance. This will involve working with neighbouring salmon farmers to form cooperative groups and all treating together. The way to go about it is outlined below and expanded in the text.

A Code of Practice for Salmon Farmers

  1. Define the Area to be Managed.
  2. Identify all the salmon farming operatives in the Area.
  3. Elicit a commitment from them towards the National Strategy.
  4. Form a local area Management Group - including veterinarians.
  5. Agree monitoring protocol and frequency.
  6. Agree timing and threshold of strategic treatments.
  7. Carry out strategic treatments.
  8. Continue monitoring.
  9. Continue consultation with stakeholders.

1. Define the Area to be Managed.

You must first identify the area to be managed and to help in this an attempt has been made to divide Scotland into manageable hydrographic areas . These are shown on the maps attached to the document.

2&3. Identify all the salmon farming operatives in the area and elicit a commitment from them towards the National Strategy.

You will know who your neighbours are. If they are not SSGA members, it may fall upon you or the Chairman of the Management Group to identify them and make an approach in order to find out what their views are after seeing the evidence presented in this document.

4. Form a local area Management Group, including Veterinarians.

A positive response will mean that a local area Management Group can be formed and of course this must include the veterinarians who have responsibility for the fish.

The Management Group has two main tasks to agree and oversee. All lice populations will have to be monitored at agreed intervals to a standard protocol and regional treatments will have to be carried out according to agreements drawn up between neighbouring farms.

5. Agree the Monitoring Protocol and Frequency.

Lice numbers on all the farms in one area will have to be regularly monitored and a suggested protocol is as follows: Starting in February, two pens on each site will be monitored weekly. Ten fish should be sampled from each pen and the number of chalimus, mobiles and females with eggs recorded. A useful article on this subject, titled Optimal timing for lice treatments was recently published in Fish Farmer (see references).

Results should be audited by neighbouring farmers or whoever the Management Group decides.

6. Agree Timing and Threshold of Strategic Treatments.

It is expected that monitoring will show the population of lice is low in March. The Management Group will decide when to treat. The main aim of the treatments is to prevent the large settlement of chalimus which has been shown to occur between weeks 20-25 and the subsequent build up in mobile lice. The idea is, therefore, to treat all the lice in the management area at the agreed time, probably in March, in an attempt to remove all the gravid females. A follow up treatment should then be given as required to kill any lice which had grown to pre-adult. Thereafter lice will be treated in order to keep females with eggs at the lowest possible level to stop recruitment. It is important to synchronise the early treatments.

Some farmers/veterinarians may feel that the level of lice is too low to warrant treatment, however, in order for the strategy to be successful, lice must be treated at the crucial time before numbers build up. Suggested thresholds for mandatory treatment during the crucial period are as follows:

  1. For any age of fish - one adult female louse with eggs in the ten fish sample.

  2. For fish in the sea for less than six months - as in 1, or an average of 1 mobile louse per fish.

  3. For growers - as in 1, or an average of two mobile lice per fish.

The Management Group may wish to coordinate treatments throughout the year but, to be successful, must revert to this code before the Spring louse increase.

7. Carry out the Strategic Treatments.

If the Management Group is organised in time to try to initiate the strategy in 1998, the only available medicines are likely to be Aquagard and Hydrogen Peroxide. Though less than perfect, these medicines were effective on the trial sites and farmers are encouraged to repeat the work using them (provided the farmer is confident of achieving a safe to the fish treatment).

The success of the Strategy will be improved when new, fully authorised medicines become available. These new bath and in-feed treatments should be used wherever possible. In-feed treatments will be particularly useful for small fish but are unlikely to become available for strategic use until 1999.

It is envisaged that, by the winter of 1999-2000, the Strategy will be well established and it will become increasingly effective as these new, safe and effective medicines become available. The result will be an overall reduction in the numbers of lice around the Scottish coast and farm louse numbers will be at their lowest when wild salmon and sea trout smolts are migrating.

8. Continue Monitoring.

It is important to continue monitoring in order to keep control of the lice but also to report to the Management Group so that the success of the strategy can be recorded and used to make plans for the following year.

9. Continue Consultation with Stakeholders.

In preparing this Code of Practice SSGA has taken account of constructive comments from salmon farmers, shellfish farmers, Regulatory Authorities, Fisheries Trusts, anglers and fishermen. Management Groups should continue this process in order to take advantage of their collective experience and to take account of their concerns. For example, farmers should be mindful that, due to the presence of juvenile stages of commercial species in the plankton, fishermen would like strategic treatments to be complete before week 13 each year.

References:

Simon Wadsworth,1997. Strategic Management of Sea Lice. MHM Internal Report.
West Highland Free Press, 3rd October 1997. Bid to make Skye scab-free.
Treasurer and Grant,1997. Optimal timing for lice treatments. Fish Farmer Nov/Dec.

The following were consulted in the preparation of this Code of Practice. Their input is gratefully acknowledged, however, mention here does not imply that they have endorsed the strategy.

Association of Scottish Shellfish Growers
Association of West Coast Fisheries Trusts
Atlantic Salmon Trust
Crown Estate Commissioners
Mallaig and North West Fishermens Association
Scottish Anglers National Association
Scottish Environment Protection Agency
Scottish Natural Heritage
Shetland Salmon Farmers Association
The Scottish Office - Fisheries Research Service

  • Division K, Aquaculture
  • Environmental Protection Unit
  • Freshwater Fisheries Laboratory

Source: Scottish Salmon - February 1998

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