Department of Fisheries and Marine Biology, University of Bergen, Norway
Being in close contact with salmon in netpens, feeding partly from their mucus layer or on lice filled with salmon blood, as well as on the carcasses of dead salmon, wrasse would be likely to contract pathogens present in the salmon population. Given the wrasses ability to escape from salmon pens, diseases could be spread to wild wrasse populations, or to new salmon farms with subsequent capture of escaped wrasse.
Likewise the possibility of introducing new pathogens to salmon farming, by stocking wild wrasse with an unknown history of disease, has been a concern of the salmon farming industry. In some areas this has hindered the commercial use of wrasse as delousers.
This article summarizes the results from a three-year project on wrasse health, including screening of wild wrasse populations, the role of wrasse as a vector for diseases, and the prevention and treatment of diseases in wrasse. The goldsinny (Ctenolabrus rupestris) was chosen as a model, as it is the species most frequently used as a delouser in salmon farms.
DISEASES OF WILD WRASSE
A goldsinny population from the western part of Norway was sampled every second month from February to November. Simultaneously fish from the same wild population that had been transferred to a salmon farm was sampled. For each population and sample date 30 fish were examined immediately after transfer to the laboratory, and another 30 were put into a latent carrier test. The goldsinny was screened for presence of bacterial diseases, IPN-virus and parasites.
Kidney samples from wild or transferred goldsinny gave no cytopatic effect on CHSE-cells. The CHSE-cells, however, will only sustain growth of a limited number of viruses, so the presence of "new" viruses in goldsinny can not be ruled out by this screening. There is no evidence from the literature that wild wrasse carry viruses, but experiments have shown that goldsinny are susceptible to IPNV (Gibson and Sommerville, 1996).
Examination of freshly caught goldsinny from the wild revealed no external or internal signs of bacterial diseases, and pathogenic bacteria could not be isolated from the kidney. After the Latent Carrier test the picture was completely altered. In one particular sample a maximum of 33 % of the goldsinny were found to have covert infections of an atypical Aeromonas salmonicida (June-96), while the common incidence was 6-10%.
Only parasites with a direct life cycle, which might be transferred from wrasse to salmon without intermediate hosts, would pose a potential problem in salmon farming. In this study the most common parasites were trichodinids on the gills of the goldsinny. The trichodinids did not cause any pathogenicity, even in the most heavily infected goldsinnys, and seemed to be species specific for wrasse. The screening of the parasite fauna was done as a M.Sc. project, and performed by Mrs Aina Solberg (Dept. of Fisheries and Marine Biology, University of Bergen).
WRASSE AS A POSSIBLE VECTOR OF DISEASES
Infectious Salmon Anemia (ISA)
This viral disease causes mortality and clinical signs of disease mainly in farmed Atlantic salmon (Salmo salar), but sea trout (Salmo trutta) (Nylund and Jacobsen, 1995), rainbow trout (Oncorhynchus mykiss) (Nylund et al., 1997) and herring (Clupea harengus) (Nylund, unpublished) have proved to be asymptomatic carriers with the ability to transfer ISA to healthy salmon. To either confirm or rule out the wrasse as a possible carrier of ISA a thorough transmission experiment was carried out.
Blood from salmon positive for ISA, from natural outbreaks and from earlier transmission trials, was used for infection. There were 6 experimental groups:
2 x 30 goldsinny injected with ISA;
2 x 30 goldsinny cohabiting with 5 ISA-injected salmon;
2 x 30 salmon cohabiting with 5 ISA-injected goldsinny;
30 goldsinny injected with HBSS (negative control);
30 untreated salmon kept with 5 untreated goldsinny (negative control);
30 salmon injected with ISA (positive control).
From all treatment groups, and from goldsinny negative control, five fish were taken out for examination every fifth day from day five until day forty after infection. Moribund fish were taken out for examination on a daily basis. All fish were examined externally and internally for clinical signs of disease. Blood samples were taken for determination of hematocrit. Tissue samples from several organs was fixed for examination in light and electron microscope, and frozen for later immunohistology. From the ISA-injected goldsinny blood and tissue was frozen for later re-infection experiments with salmon.
While mortality started on day 17 and ceased on day 28 (90% mortality) in the ISA injected salmon, there were no mortality in the goldsinny injected with ISA, in goldsinny cohabiting with injected salmon, or in salmon cohabiting with injected goldsinny. Neither were there any clinical signs of disease or drop in hematocrit in these groups. Samples of tissue processed for light microscopy or for immunohistology did not show any signs of tissue damage, or positive reaction for the presence of ISA Virus.
In the re-infection experiment blood and tissue taken from the ISA-injected goldsinny (day 5, 10, 15, 20, 25, 30, 35 and 40) was prepared and intraperitoneal injected into eight groups of healthy salmon. There were no mortality in any groups of salmon, neither clinical signs of disease or drop in hematocrit in salmon sampled from these groups. In a positive control group of salmon the mortality reached 100 %.
The conclusion from the experiments must be that it is highly unlikely that goldsinny will be able to transfer ISA to or between salmon groups under natural conditions. The virus does not seem to be able to establish an infection in goldsinny, and through the cohabitation trials the role of goldsinny as a possible passive vector seems to be ruled out.
At present all material from the experimental groups, frozen and stored in liquid nitrogen, are being screened by the use of PCR, to examine clearance rate and possible traces of ISA- virus in the samples.
Atypical and classical furunculosis
As wild wrasse may carry an atypical variant of Aeromonas salmonicida it is of great importance to find out if this bacteria may be transmitted to, and cause disease in, salmon. Earlier work have demonstrated that wrasse are susceptible to classical furunculosis from salmon, both in laboratory experiments (Bricknell et al., 1996) and in a farming situation (Hjeltnes et al., 1992).
In a laboratory experiment, groups of 2 x 30 goldsinny wrasse and Atlantic salmon were intraperitonally injected with "high" (107CFU/fish) and "low" (105CFU/fish) doses of Aeromonas salmonicida subsp. salmonicida from salmon or atypical Aeromonas salmonicida from goldsinny.
In salmon injected with a "low" dose of Aeromonas salmonicida subsp. salmonicida mortality started at day three and reached 100 % within day eight post-injection. In the salmon groups injected with the atypical variant there was no mortality, and sequential examination of fish showed no signs of disease. The experiment gave no indications that the atypical furunculosis from wrasse could be transmitted to salmon, which corresponds to the findings of Costello et al. (1994).
In the goldsinny groups injected with "high doses" the mortalities for both isolates reached about 80 % within 15 days. In thewhile in the "low dose" groups mortalities were a bit higher in the atypical group (45 %) than in the classical group (35 %). When given an intraperitoneal injection goldsinny seems almost equally susceptible to classical furunculosis from salmon as to atypical furunculosis from wrasse. This implies that under farming situations with classical furunculosis present in the salmon population the use of goldsinny as cleaner fish should be avoided, because of the risk of mortalities in the wrasse and spreading the disease through escaped wrasse.
Prevention and treatment of atypical furunculosis in wrasse
Preliminary trials with vaccination of freshly caught goldsinny in April-May demonstrated the problems with vaccinating a carrier population. The stress of handling was sufficient to cause an outbreak of atypical furunculosis in the goldsinny, and from 400 fish ip injected with an commercial oil-based triple-vaccine only 50% survived the five-week long immunization period. A following challenge with atypical and classical furunculosis proved to be rather inconclusive.
In a later experiment goldsinny were vaccinated in December with an oil-based trial vaccine made of formaldehyde-killed atypical Aeromonas salmonicida bacteria. As goldsinny are scarcely available in early spring, when the salmon smolts are put to sea, trials with winter-storage of goldsinny have been conducted. Simultaneous vaccination would give several advantages with the possibility of disease outbreaks being lower during the winter months, lower social interactions between the goldsinny, and a much longer immunization period (5-6 months) compensating for a slower immune reaction caused by low water temperatures.
The winter-vaccination experiment gave promising results as mortalities in vaccinated group receiving subsequent handling-stress was significantly lower than in unvaccinated groups. Challenge experiments with ip injection of atypical Aeromonas salmonicida however failed to demonstrate any effect of vaccination, but this method for challenge may be unsuitable.
Treatment with antibiotics
Given the high mortality caused by stress-induced outbreaks of atypical furunculosis and the difficulties in vaccinating the fish, a more applicable approach to reduce mortality could be treatment with antibiotics. An experiment using three different antibiotics to treat a natural outbreak of atypical furunculosis was therefore conducted in the laboratory. Three antibiotics were chosen on the basis of sensitivity studies of the bacteria isolate.
Treatment was commenced when mortality caused by atypical furunculosis started. For each treatment 2x55 fish was used. Fish in two groups were ip. injected with 0.5 ml Tribrissen ¨vet. (400 mg/ml Sulfadiazine 80 mg/ml Trimethoprim), and two with 0.5 ml Aquacycline ¨ vet. (50 mg/ml Oxytetracycline). Two groups were bath treated with Flumequine (0.5 g/l) for three hours, and two groups kept as control. Dead and moribund fish were registered daily.
At the termination of the experiment, 28 days after treatment, the cumulative mortality was 47.5 % both in control groups and Tribrissen injected groups. In the Flumequine treated groups mortality reached 34% while in the Aquacyclin-treated groups mortality was only 14%.
It will not be possible to administer chemotherapeutics to wild-caught goldsinny via the oral route, but even though ip. injection is quite labor-intensive it will reduce the amount of antibiotics used and thereby spare both costs and environment. A commercial fishfarm with 100,000 smolts using 5,000 goldsinny (1: 20) will need 250 ml Aquacycline ¨ vet., or 12.5 g Oxytetracycline. Further investigation will probably show that the dose could be further reduced.
The screening of a goldsinny population from the West Coast of Norway did not reveal any pathogens that should pose an threat to salmon farming, neither were the goldsinny able to transmit ISA or atypical furunculosis to salmon. The main problem seems to be the mortality in the goldsinny, caused by stress-induced outbreaks of atypical furunculosis, after transfer to netpens. This disease may be treated efficaciously by injection of Oxytetracycline. Vaccination of goldsinny is difficult when the population contains carriers with covert infections, but vaccination combined with winter-storage has given promising results.
Bricknell, I.R., Bruno, D.W. and Stone, J., 1996. Aeromonas salmonicida infectivity studies in goldsinny wrasse, Ctenolabrus rupestris. Journal of Fish Diseases, 19: 469-474.
Costello, M., Deady, S., Pike, A. and Fives, J., 1994. Parasites and diseases of wrasse (Labridae) being used as cleaner-fish on salmon farms in Ireland and Scotland. In Wrasse Biology and use in Aquaculture, Sayer, M.D.J., Treasurer, J.W. Costello, M.J. (Eds.), pp: 211 227.Oxford: Fishing News Books.
Gibson, D.R. and Sommerville, C., 1996. The potential for viral problems related to the use of wrasse (Labridae) in the farming of Atlantic salmon. In Wrasse Biology and use in Aquaculture (Sayer, M.D.J., Treasurer, J.W. and Costello, M.J., eds.), pp: 240 - 246. Oxford: Fishing News Books.
Hjeltnes, B., Bergh, O., Holm, J.C. and Wergeland, H., 1995. The possibility of transmission of furunculosis from farmed salmon to marine fish. Diseases of Aquatic Organisms, 23: 25-31.
Nylund, A. and Jacobsen, P., 1995. Sea trout as a carrier of infectious salmon anaemia virus. Journal of Fish Biology, 47: 174 - 176.
Nylund, A., Kvenseth, A.M., Kross¿y, B. and Hodneland, K., 1997. Replication of the infectios salmon anaemia virus (ISAV) in rainbow trout, Oncorhynchus mykiss (Walbaum). Journal of Fish Diseases, 20: 275-27
Summary of the Irish sea trout problem
R. Poole and K. Whelan
Salmon Research Agency, Co. Mayo, Ireland
This article was circulated on the sealice e-mail discussion group in August 1998
In reply to those requests for information on the sea trout collapse in the west of Ireland and whether sea lice were the cause of the collapse, it was clear from the uninformed statements and questions that have appeared recently on e-mail that it is necessary to restate the problem and where it has occurred. There is a substantial body of published reports and papers which are referenced at the end.
Because of the marginal existence of the species, there has been increasing evidence during the past two decades of a slow decline in some stocks and this was largely attributed to poaching with fine mesh monofilament and a range of environmental problems such as: field drainage, stream drainage and maintenance fertilization of the hillsides, afforestation and more recently hillside erosion, due to overgrazing by sheep (Whelan, 1992).
Whelan (1991, 1992, 1993) and Poole et al. (1996), described the appearance of a more serious decline which appeared in many fisheries along the western seaboard in 1986 which, by 1989, resulted in a population collapse in most mid-western sea trout fisheries. Subsequent research programmes (Anon 1990, 1992, 1993, 1994) confirmed that the declining rod catches reflected an actual spawning stock collapse, approximately 90-98% reduction in ova deposition rates in one catchment studied.
The major collapse occurred in 1989 when, tragically, there was little sea trout research taking place and only some anecdotal information is available on the sequence of events which took place during the May/June period of 1989. However, the following details are known:
In 1990 the Salmon Research Agency co-ordinated a broadly based research programme which, it was hoped, would identify the extent of the problem and define a possible cause or causes. The results of this 'STAG' programme, implicating sea lice as the major component of the collapse, are summarised in Whelan (1992). During the following two years the research programme concentrated on five principal areas:
In 1991 the Department of the Marine established a Sea Trout Working Group and the results of the sea trout research programme were examined by this group at the end of each year (Anon, 1991, 1992, 1993, 1994). The "sea trout problem" in the Irish context was defined as the following:
The following points from the research programmes are worth summarising:
The only consistent factors to emerge from the research carried out to date are the early return of both smolts and kelts to the estuaries in early to late May and the presence of intensive infestations of juvenile salmon lice (Anon. 1991, 1993a, 1993b, 1994; Tully & Whelan 1993, Tully et al. 1993a, Tully et al. 1993b). The sea trout problem, as defined above, has only been observed in areas adjacent to intensive salmon farming. The problem is therefore geographically distinct and any valid hypothesis must be able to account for this fact.
Environmental factors have often been promoted as having been involved but results to date, particularly the high survival rates of stocked parr, would indicate that it is a marine based mortality of trout and no common factor, or suite of factors, have been identified as having caused the sea trout stock collapses in the west of Ireland. The "building of dams, roads, gravel digging, silage etc" (Kvenseth, Caligus 20/7/98) do not have any connection with the current sea trout situation in the west of Ireland.
In the latter half of the 1980's, farm salmon production in the bays of the mid-west of Ireland expanded to an unprecedented extent. Sea lice levels rose during this period and as a consequence juvenile lice production soared. No observed data exist for juvenile lice production during this period but extrapolation from the lice production figures indicates the massive release of larvae which took place at this time. It has also been shown that increasing sea temperatures in the late 1980's were directly linked with faster generation times in the louse life cycle (Tully & Whelan, 1993).
Two vital pieces of evidence have been shown; the sea trout smolts, and adults where present, are consistently infested by juvenile lice and that in the mid-west of Ireland during 1991, 95% of the total nauplius larval production of Lepeophtheirus salmonis, the louse causing the problem, was of fish farm origin. It has also been shown that sea trout were infested with juvenile lice within two to three weeks of migrating to sea and that morphological and physiological impact of the lice on the trout was sufficient to cause mortality. In the past number of years, correlations have also been found between both abundance and intensity of lice on sea trout with distance from neighbouring fish farms. While these correlations are by their very nature relatively crude, they have provided the impetus for further detailed in-bay research into the mechanisms governing sea lice infestation from both salmon farms and wild sources.
In 1996 the Salmon Research Agency attended the ICES Working Group in Edinburgh and the main conclusions of this group were as follows:
The commisioned report examining the 1992 - 1996 sea trout sampling programme, undertaken by Dr Ian Cowx, identified a number of perceived and actual inaccuracies in the programme. These were fully addressed in the 1997 programme. The inaccuracies identified were found to have not significantly altered either the database or the conclusions drawn from it and the 1997 modified programme completely corroborated the 1992-1996 findings.
Hopefully this description of the current status of our knowledge regarding the sea trout population collapses in the west of Ireland will help to inform those looking for such information.
Anon., 1990. Declining sea trout stocks in the Galway / south Mayo region - A scientific appraisal. The Salmon Research Agency of Ireland Inc.. Internal Report. (unpubl.). 206pp.
Anon., 1991-94. Reports of the Sea Trout Working Group. Fisheries Research Centre, Department of the Marine, Dublin.
Mulloy, S., Holland, C. and Poole, R. 1993. Helminth parasites of brown and sea trout Salmo trutta L. from the west coast of Ireland. Biology and Environment: Proc.. Roy. Ir. Acad., Vol. 93b no. 3. 137-142.
Murphy, T., Drinnan, E.H., Poole, W.R. and Whelan, K.F. 1993. Histological and virological investigations into the collapse of sea trout populations in the west coast of Ireland. ICES. CM 1993: M57:7.
Poole, W.R., Whelan, K.F., Dillane, M.G., Cooke, D.J. and Matthews, M. 1996. The performance of sea trout, Salmo trutta l., stocks from the Burrishoole system western Ireland, 1970-1994. Fisheries Management & Ecology, 3 (1). 73-92.
Poole, W.R. & Whelan, K.F. (1996). The sea trout stock in the Burrishoole catchment and a review of the symptoms of the west of Ireland sea trout collapse. Paper presented to: ICES Workshop on the Interactions between Salmon Lice and Salmonids, ICES CM 1997/M:4.
Tully. O., 1991. Assessment of the impact of sea lice (Lepeophtheirus salmonis) infestation on sea trout smolts on the west coast of Ireland during 1990 and 1991. The Salmon Research Agency of Ireland Inc. Internal Report. (unpubl.) 37pp.
Tully, O., Poole, W.R. and Whelan, K.F. 1993a. Infestation parameters for Lepeophtheirus salmonis (Kroyer) (Copepoda:Caligidae) parasitic on sea trout (Salmo trutta L) post smolts on the west coast of Ireland during 1990 and 1991. Aquaculture and Fisheries Management, 24 (4).545-557.
Tully, O., Poole, W.R. and Whelan K.F. and Merigoux 1993b. Parameters and possible causes of epizootics of Lepeophtheirus salmonis (Kroyer) parasitic on sea trout (Salmo trutta L) on the west coast of Ireland. Proceedings of the First European Crustacea Conference. Paris, August 31 - September 5, 1992 Pathogens of wild and Farmed Fish, Sea Lice (ed. by G.A. Boxshall). Ellis Horwood Ltd, London. 202-212.
Tully, O. and Mulloy, S. 1993c. Infestation of sea trout (Salmo trutta L.) by the salmon louse (Lepeophtheirus salmonis (Kroyer) in Ireland during 1993. ICES CM 1993/M:14.
Tully, O., Gargan, P. and Whelan, K.F. 1993d. Infestation of sea trout (Salmo trutta L.) by sea lice (Lepeophtheirus salmonis (Kroyer)) in systems close to and distant from salmon farms in Ireland. ICES CM 1993/M:56.
Tully, O. and Whelan, K.F. 1993e. Production of nauplii of Lepeophtheirus salmonis Kroyer) (copepoda:caligdae) from farmed and wild Atlantic salmon (Salmo salar L) on the west coast of Ireland during 1991 and its relation to infestation levels on wild sea trout (Salmo trutta L). Fisheries Research 17. 187-200.
Whelan, K.F. 1991. Disappearing sea trout - decline or collapse? The Salmon Net No. 23. 24-31.
Whelan, K.F. 1992. Management of salmon and sea trout stocks. Environment and Development in Ireland. Proceedings of a Conference held at University College Dublin. The Environmental Institute, University college Dublin. 457-466.
Whelan, K.F. 1993. Historic overview of the sea trout collapse in the west of Ireland. In: Aquaculture in Ireland - towards sustainability. Ed. J. Meldon. Proceedings of a Conference held at Furbo, Co. Galway. 30th April - 1 May. 1993. 51-53. An Taisce, Dublin.
Whelan, K.F. 1993. Decline of sea trout in the west of Ireland: an indication of forthcoming marine problems for salmon? Proceedings of the Fourth International Atlantic Salmon Symposium, St. Andrews, N.B. Canada, June 1992. Salmon in the Sea and New Enhancement Strategies - (Ed. D. Mills) Fishing News Books, Chapter 9.
Sea lice management methods in Scotland
Marine Harvest McConnell, Lochailort, Inverness-shire PH38 4LZ, U.K.
This paper was presented at the workshop on sea lice control on fish farms in Trondheim, November 1997.
This paper examines:
(a) sea lice infestation patterns on farms in Scotland investigating whether the source of infection is external or is generated from within the farms;
(b) factors governing infestation cycles and the pattern of infection;
(c) methods to prevent and treat sea lice and
(d) the choice of medicine, the timing of the treatment and how frequently the fish should be treated.
Internal or external infection patterns?
Smolts (1992 year class) stocked on a production farm in west Scotland with one sea winter production fish already present (1991 year class) were rapidly infected with Lepeophtheirus salmonis and copepodids were found on the newly stocked fish within 3 days (Fig. 1). Up to 8 mobile L. salmonis were recorded 4 weeks after stocking and the fish were treated with Aquagard (dichlorvos) at regular intervals through the first summer. In the following year the farm was fallowed for several weeks and this subsequent year class remained relatively uninfested with lice through the first year and only one treatment with Aquagard was required during this period. In many farms where fallowing is applied, lice treatment is not required for up to 15 months following transfer to sea and this treatment can be then solely for Caligus elongatus rather than L. salmonis. This was also the conclusion from an intensive study of four Scottish salmon farms that established that recruitment of lice was initially slow following fallowing and emanated from wild fish (Bron et al., 1993). Thereafter the build up of lice was largely internally generated within the farm itself.
The pattern of infestation with Caligus follows a well established direct route of transfer from wild fish from late June onwards (Fig. 2) and therefore fallowing has little effect on controlling this species. Caligus numbers on salmon often decline naturally with the onset of colder weather in November or an influx of freshwater. The recruitment of Caligus varies greatly between sites and it is difficult to conclude whether Caligus are an increasing problem. Annual fluctuations in settlement of Caligus on salmon were evident on a farm in Loch Sunart over four year classes (Fig 2). Treatment with Aquagard-Novartis (dichlorvos 50% w/v) is normally very effective in removing Caligus, and there is no indication of reduced sensitivity to this compound.
In farms that have been fallowed the recruitment of lice in late spring in the second year is a critical period in sea lice control (Fig. 3). In this example there was an initial infestation with Caligus and this declined naturally without treatment. In week 16 of the second year numbers of chalimi of L. salmonis increased rapidly followed by a rise in weeks 18 to 20 of mobile L. salmonis to an average of 12 per fish. At this point fish were treated with Aquagard. One further treatment was required at the beginning of August. The increase in lice numbers on fish was also not gradual but logarithmic. Several factors may be involved in this rapid increase in sea lice infestation in weeks 16-20, including increasing day length, rising water temperatures and increasing salinity (allowing greater larval survival). In addition, copepodids may be less viable in winter due to smaller egg size and a reduced nutrient reserve, although larger numbers of smaller eggs are produced. In late spring fewer larger eggs are produced, and larger eggs may give more viable copepodids (Ritchie et al., 1993).
Estuarine plankton are adapted to prevent wash out including mechanisms for vertical movement in seawater. Nauplii and copepodids of sea lice are likely to behave in the same way as other zooplankton. Bron et al. (1993) indicated the average survival time of copepodids at 10°C as 7 days but it could be as much as 21 days in a farm situation in a confined bay area. Two tides each day, each with flow and ebb movements, provides four opportunities per day for copepodids to pass through a farm. Over a week this is 28 occasions, and over 3 weeks 84 opportunities. Together with high egg production this can explain the rapid increase in lice numbers from a relatively small number of gravid lice per fish.
Lice control methods in Scotland
On many farms integrated pest control management is being followed and this may involve management agreements between companies in certain sea loch areas.
1. Fallowing of farms
A management agreement between three companies operating in Loch Sunart included specifying a fallow period, on stocking a single year class, and on exchanging information on the health of the fish. Prior to this, in the 1989 year class, lice infested fish rapidly following stocking and remained high through the production cycle (Fig. 4). The first treatment with Aquagard was in September of the first year and a total of 22 treatments was required in this year class. In 1991 when a fallow period of 8 weeks was used, numbers of L. salmonis were low through the first year with only Caligus present in any numbers and not sufficiently high to require treatment. It was 15 months before the first treatment was required. Fallowing was therefore very effective with a 50% reduction in the number of treatments compared with the 1991 year class. The option to fallow may not be available to small companies who have a limited number of farms or only one farm and where fish of various sizes are required for harvest.
In 1994 a questionnaire was sent to fish farming companies in Scotland asking about their use of wrasse (Treasurer, 1996a). Wrasse were stocked widely with the smolt input, 150,000 wrasse in 1994 representing 33% of farms and 39% of fish put to sea that year. Wrasse have been effective in the first year of the production cycle where wrasse have been treated carefully and hides have been used to give protection in cages. On one farm used as an example (Fig. 5) smolts were stocked with larger fish, as this was a broodstock site. In 1994 smolts were treated with Aquagard on three occasions but no treatment was required in 1995 after wrasse were stocked on a group of 8 cages.
Wrasse have been found to be less effective in the second year of the production cycle. I have carried out a trials with larger corkwing wrasse on 2 cages but with little effect. An alternative would be to try ballan wrasse as good results using ballan have been reported from Norway. Ballan are relatively uncommon in Scotland (Treasurer, 1996b), about 1% of the catch, and culture of ballan would be required.
Part of the ineffectiveness of wrasse in the second year is the disappearance of wrasse over the first winter. Special hides have been developed by Martin Sayer of the Dunstaffnage Marine laboratory, Oban providing a buffer to sudden changes in salinity and temperature. These have been trialled on farms but with no conclusive results; although survival of wrasse improved, they continued to be lost from the cages. Wrasse were not found dead and therefore it was assumed that they have escaped through the net.
Another problem has been the occurrence of an atypical furunculosis, Aeromonas salmonicida, in wrasse following capture, transfer and stocking in cages. Wrasse should be vaccinated although the induction period is too short and there would have to be a simultaneous treatment with antibiotic.
3. Treatment with medicines
There are only three licensed medicines for treatment of salmon infested with sea lice in Scotland. Dichlorvos (Aquagard-Novartis) as 1 ppm active is applied as a bath treatment with the cage enclosed with a tarpaulin. The discharge of this medicine is regulated by the Scottish Environmental Protection Agency and the volume of dichlorvos that can be discharged has been reduced in many cases in line with the Paris Convention 1974 for the protection of the North Sea and North east Atlantic. This has a requirement to reduce the use of various toxic and persistent chemicals including dichlorvos. Therefore discharge consent may only be available to treat a farm once per annum. Also many lice populations are resistant to the use of this organophosphate, particularly at lower water temperatures, and treatments can be ineffective (Jones et al. 1992). Azamethiphos (Salmosan, Novartis) has recently been granted a marketing authorisation and discharge consents have been obtained for a few farms.
The third medicine is hydrogen peroxide, supplied as Paramove by Solvay Interox and Salartect by Brenntag. This is also applied as a bath treatment and can be very effective (Fig. 6). Although up to 80% of lice recovered from treated cages were active after an hour (Treasurer & Grant, 1997), there have been no reported cases of significant resettlement on salmon. The main problem with hydrogen peroxide is toxicity at water temperatures in excess of 14°C. In addition, chalimi are unaffected, effectiveness against mobile lice may not be complete, and egg bearing females are more difficult to remove.
These medicines are both applied as a bath treatment and there can be variations in enclosed volume compared with target volume. When the tarpaulin has not been filled adequately a higher more toxic concentration is achieved and with a large fill the target concentration is not attained, giving an ineffective treatment. Bath applications are labour intensive, may stress fish, and licensed medicines do not kill the larval stages. Furthermore, the appetite of fish is frequently affected by treatment and fish are starved on the day before and during treatment.
Within the terms of the Medicines Act 1968, any substance intended to be used as a veterinary medicine has to have a marketing authorisation in the U.K. Applications for a marketing authorisation are submitted to the Veterinary Medicines Directorate (VMD) and are assessed on the quality, efficacy and safety of the product. The VMD is advised by the independent Veterinary Products Committee. Obtaining a marketing authorisation can be very time consuming and expensive process, e.g. azamethiphos took 6 years from the beginning of the Animal Test Certificate (ATC) trials.
Un-licensed medicines can be prescribed by a veterinarian where treatment is necessary on welfare grounds and where all licensed alternatives are considered as being ineffective. The veterinarian makes a series of decisions termed the 'cascade principle'.
In addition, the medicine has to be granted a discharge consent by the Scottish Environmental Protection Agency (SEPA). Under section 23 of the Water Act 1989 fish farms are classified as trade premises and any wastes are classified as trade effluent and require a discharge consent. An application for a consent has to made for each individual farm and depending on the size of the farm extensive hydrographic data are required to predict the scale of discharge that can be made without breaching short term Environmental Quality Standards. An application for a discharge consent is advertised in the local press.
Following extensive laboratory and field trials and environmental impact studies, SEPA gave a limited issue of restrictive consents for ivermectin for a trial period. Efficacy has been good but there have been political issues in marketing these fish.
Cypermethrin (Excis, Grampian Pharmaceuticals) has been trialled under an Animal Test Certificate and this medicine will shortly receive a marketing authorisation. The product is very effective against mobile and to a lesser extent larval stages. Discharge consents will also need to be obtained.
No in-feed treatment is licensed (but see ivermectin), although three possibilities have been used in limited trials under ATC. Onions and garlic have been tried in bags and in feed but were ineffective. Neither have light lures demonstrated efficacy.
The development of a successful vaccine is not imminent. While immune stimulants have given good laboratory results (Simon Wadsworth, pers. comm.), they have not been found to be effective in farm conditions (pers. obs.).
Immediate assistance would be the provision of other more effective medicines but at present the most effective use of current medicines must be considered, particularly hydrogen peroxide. This involves strategic or winter treatments; treating fish when sea lice numbers are low and stable at the beginning of the year. Ritchie et al. (1993) showed that many small eggs are produced at this time and survival of copepodids is poor because of a low nutrient reserve. In an example from Loch Sunart fish were treated with hydrogen peroxide on 3 occasions from March at approximately 6 weekly intervals, weeks 10, 17 and 23 (Fig. 7). This had the effect of reducing the spring increase in sea lice numbers. A comparison is made of the 1993 with the 1995 year class in the second year of the production cycle. Chalimus numbers in 1996 were significantly less (ANOVA on log transformed data, P<0.05), 5.5 on average compared with 21.8 in 1994 (Fig. 7). Numbers of mobile L. salmonis were also less, 13.4 weekly average for the second year compared with 31.1 in 1994 (Fig 8). The number of treatments in the second year was reduced by up to 46%, the length of time between treatments was extended, damage to fish was reduced, there were fewer mortalities and improved fish quality (Wadsworth et al., 1998).
This policy and procedure should be allied to coordinating treatments between farms in a prescribed area and followed up with subsequent treatments based on a lice surveillance system. The availability of more effective medicines will enhance the success of this initiative. Recently a national strategy was launched by the Scottish Salmon Growers Association to coordinate planned sealice treatments throughout designated areas in spring to coincide with the time of year when copepodid survival has been shown to be lowest (Wadsworth et al., 1998). The results of the strategy have not been published but have been reported to be favourable and further improvement will be possible if more effective medicines such as cypermethrin are more widely available.
Bron, J. E., Sommerville, C., Wootten, R. & Rae, G. (1993). Fallowing of marine Atlantic salmon, Salmo salar L. farms as a method for the control of sea lice, Lepeophtheirus salmonis (Kroyer, 1837). Journal of Fish Diseases 16, 487-493.
Jones, M.W., Sommerville, C. & Wootten, R. (1992). Reduced sensitivity of the salmon louse, Lepeophtheirus salmonis to the organophosphate dichlorvos. Journal of Fish Diseases 15, 197-202.
Ritchie, G., Mordue, A.J., Pike, A.W. & Rae, G.H. (1993). The reproductive output of Lepeophtheirus salmonis adult females in relation to seasonal variability of temperature and photoperiod. In : Pathogens of wild and farmed fish: sea lice. Boxshall, G.A. & Defaye, D. Ellis Horwood, Chichester, 153-165.
Treasurer, J.W. (1996a). Wrasse (Labridae) as cleaner-fish of sea lice on farmed Atlantic salmon in west Scotland. In: Wrasse: biology and use in aquaculture. Sayer, M.D.J., Treasurer, J.W. & Costello, M.J. Fishing News Books, Oxford, 185-195.
Treasurer, J.W. (1996b). Capture techniques for wrasse in inshore waters of west Scotland. In: Wrasse: biology and use in aquaculture. Sayer, M.D.J., Treasurer, J.W. & Costello, M.J. Fishing News Books, Oxford, 74-90.
Treasurer, J.W. & Grant, A. (1997). The efficacy of hydrogen peroxide for the treatment of farmed Atlantic salmon, Salmo salar L. infested with sea lice (Copepoda: Caligidae). Aquaculture 148, 265-275.
Wadsworth, S., Grant, A. & Treasurer, J.W. (1998). A strategic approach to lice control. Fish Farmer 21 (2), 8-9.
Vaccine against salmon lice
Dr Rob Raynard
FRS, Marine Laboratory, Victoria Road, PO Box 101, Aberdeen AB11 9DB, Scotland, U.K.
Summary of a project funded under the EC FAIR Research programme: AIR2-CT93-1079
The immune system of fish, as in human, reacts against invading foreign substances. Faced with a disease, the immune system will try to eliminate the infectious agent or substance, for example infectious bacteria, as soon as it has been identified. However, the infection may be too advanced by the time the immune system is fully reacting. Vaccines can accelerate this process. During vaccination, specific portions of the infectious agent (antigens) are injected to stimulate a specific reaction of the immune system (build up of antibodies) to prevent recurrence of the disease. The risk of infection is limited, as only antigens and not the disease agent are injected. During future infections, the immune system will react quicker to the already known disease and consequently improve the chances of survival of the animal.
This common method of vaccination can not be developed against lice as only the exoskeleton and mouth parts of the lice are in contact with the fish. The immune system does not recognise any foreign substances in the blood stream and the active defence mechanisms are not being triggered. However, lice feed on skin and mucus of salmon and thereby take blood meals. As the antibody, which recognise the target, and the white cells, which destroy it, are present in the blood, it was suggested that fish might fight against lice externally through the action of their bloods defence mechanisms ingested by the lice. Indeed, if the immune system is prepared to react against gut cells of lice, it will be triggered within the lice gut and against it. Therefore, lice could be eradicated by the action of the fish immune system following a blood meal. Such vaccination techniques have been used to fight bloodsucker parasites of cows and sheep.
The partners of this project managed to design several vaccines aimed to be used as described above. However, tests on salmon challenged with lice showed only a limited efficacy of the vaccination. Several reasons may explain this poor success. Firstly, lice, unlike bloodsuckers, only ingest a limited amount of blood. Secondly, there is still room for some improvements of the vaccine. Nevertheless, this new approach for treatment of parasites in fish is of great interest and research efforts are being continued to improve the present vaccine. Further development may be expected in a near future.
Semiochemicals for sea lice control
Greg Devine and Jenny Mordue
Zoology Department, Aberdeen University, Tillydrone Avenue, Aberdeen AB24 2TZ.
Semiochemicals are naturally occurring chemicals which communicate information to living organisms. Some of these compounds have been exploited in order to regulate insect behaviour. For example, sex pheromones are used to lure, trap and disrupt the behaviour of some moth pests1 and the compounds that attract some veterinary pests to their mammalian hosts have also been used in traps2. Such approaches now form important options for the design of effective pest management strategies.
The theory, methodologies and technologies that have given rise to such strategies are now being applied to the problem of Lepeophtheirus salmonis. We are in the process of identifying the semiochemicals which attract salmon lice to their hosts and male lice to their mates*. It has long been suggested that parasitic copepods use fish-derived odours, at least in part, to identify their hosts3,4 and preliminary work has shown that lice do have a behavioural response to these stimuli5. The successful identification of such compounds will allow us to investigate the possibilities of designing lice traps or of creating disruptive techniques for use in fish farms.
Currently, our research focuses on host-finding behaviour by adult lice. Significant numbers of lice transfer between hosts in sea cages, and host-finding may be important in the reattachment of lice dislodged from their hosts, and for the redistribution of males searching for unmated females6. It may also prove possible to encourage lice to leave their host given sufficiently strong alternative attractants.
In this initial phase, the project centres on the design and use of simple flow chambers allowing different stimuli, present in hostconditioned seawater, to be proffered to lice. The copepods are monitored for the presence or absence of behavioural responses. Using a digital tracking system, the relative strength of these responses can be assessed by measuring the speed and direction of movement. Other assays, which allow the lice to exhibit preference for one stimulus over another, are used to gauge whether a stimulus is an attractant. By making a choice of odours available to the lice, we can discern which are the more effective.
As a result of these studies, we now have unequivocal evidence that male and female salmon lice exhibit behavioural responses to a variety of fish-derived stimuli, and that the molecules involved are quite stable in seawater. These responses are directional and kinetic, and provide the first evidence that L. salmonis exhibits a positive rheotaxis to fish-conditioned water.
Once we have established which semiochemical sources elicit the greatest attractant response, we will extract the organic components of these from the water and use electrophysiological techniques to record the sensory responses of lice to these compounds. The aim of this is to show a clear relationship between the chemical cue, the stimulation of the chemoreceptors and the triggering of host-finding behaviour. At this point we can embark on detailed studies of the particular molecules eliciting this chain of events. It is these we hope to investigate for their potential in trapping and disruption techniques.
*Link Aquaculture (SAL 11). Aberdeen University - AJ Mordue, AW Pike, W Mordue; Nottingham University - I Duce; IACR-Rothamsted - JA Pickett, L Wadhams. Funded by Natural Environment Research Council, the Scottish Salmon Growers Association and the Shetland Salmon Farmers Association. In collaboration with Marine-Harvest Mc Connell and Landcatch UK.
(1) Howse P, Stevens I, Jones O (1996) Insect Pheromones and Their Use in Pest Management. Chapman and Hall, London 256pp.
(2) Torr SJ, Mangwiro TC (1996) Upwind flight of tsetse (Glossina spp) in response to natural and synthetic host odour in the field. Physiological Entomology 21 (2) 143-150.
(3) Fasten N (1913) The behaviour of a parasitic copepod Lernaepoda edwardsii Olsson. J Anim Behav 3 36-60.
(4) Boxshall GA (1976) The host specificity of Lepeophtheirus pectoralis (Muller 1776)(Copepoda:Caligidae). J Nat. Hist 8 681-700.
(5) Hull MQ (1997) The Role of Semiochemicals in the Behaviour and Biology of Lepeophtheirus salmonis (Kroyer 1837): Potential for Control? PhD Thesis. University of Aberdeen.
(6) Hull MQ, AJ Mordue, AW Pike, G Rae (1998) Patterns of pair formation and mating in an ectoparasitic caligid copepod Lepeophtheirus salmonis (Kroyer 1837); implications for its sensory and mating biology. Phil.Trans.R.Soc.Lond.B. 353 53-764.
Report on the 1998 sealice conference in Amsterdam
About 70 persons attended the sealice sessions at the Fourth International Crustacean Congress, July 20-24, Amsterdam. Delegates were from industry (43%) universities (41%), government (14%) and non-governmental (2%) organisations. Many of these papers have been submitted for publication and are currently undergoing peer review. The proceedings will be published in the journal Contributions to Zoology (formerly Bijdragen tot de Dierkunde). The following papers were presented at the sealice workshop sessions. Note that the titles and authorships of papers submitted for publication in the proceedings do not necessarily match those presented at the conference.
Banks, B.A., A.P. Shinn, J.E. Bron & C. Sommerville. The use of RAPDs to establish the interspecific relationships of the ectoparasitic caligid, Lepeophtheirus salmonis (Krøyer, 1837) in Scotland.
Bashirullah, A.K. Non-interactive coexistence of two parasitic copepods of Caranx hippos in eastern Venezuela.
Bell, S., J.E. Bron, & C. Sommerville. The distribution of exocrine glands in Lepeophtheirus salmonis (Krøyer, 1837) and Caligus elongatus Nordmann, 1832.
Boxaspen, K. & T. Næss. Development of eggs and planktonic early life stages of salmon lice (Lepeophtheirus salmonis) at low temperatures.
Braidwood, J.C. The use of Crangon Crangon to investigate the potential environmental impact of Excis sea lice treatment.
Bron, J.E., A.P. Shinn & C. Sommerville. Ultrastructure of the cuticle of the chalimus larva of the salmon louse Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda: Caligidae).
Bron, J.E., A.P. Shinn & C. Sommerville. A description of moulting in the chalimus larva of the salmon louse Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda: Caligidae).
Bron, J.E., G. Wainwright, R.P. Smullen & C. Sommerville. The cuticle and ecdysis in larval stages of Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda: Caligidae).
Costello, M.J. & A.W. Pike. Towards a quantification of salmon lice population dynamics and infestation potential.
Dawson, L.J., A.W. Pike, D.F. Houlihan & A.H. McVicar. Effects of Sea Lice, Lepeophtheirus salmonis, on Sea Trout, Salmo trutta, at different times after seawater transfer.
El-Rashidy, H. & G.A. Boxshall. Coevolution of the parasitic copepods of the family Ergasilidae (Poecilostomatoida) and host fishes of the family Mugilidae.
Firth, K.J., S.C. Johnson & N.W. Ross. Investigation on the role of skin mucus proteases of Atlantic salmon during Lepeophtheirus salmonis infestation.
Grimnes, A., B. Finstad & P.J. Jacobsen. Salmon lice: comparative study on two hosts.
Haji Hamin, H.L., J.E. Bron, A.P. Shinn & C. Sommerville. The occurrence of blood feeding in Lepeophtheirus salmonis (Krøyer, 1837).
Hull, M.Q., A.W. Pike, A.J. Mordue & G.H. Rae. Should I stay or should I go? New on- and off- host parasite data having implications for inter-host transfer of Lepeophtheirus salmonis.
Ibrahim, A., B.M. MacKinnon & M.D.B. Burt. The influence of sub-lethal levels of zinc on smoltifying Atlantic salmon Salmo salar L. and on their subsequent susceptibility to infection with Lepeophtheirus salmonis (Krøyer, 1837).
Jackson, D., S. Deady, D. Hassett & Y. Leahy. Population dynamics of sea lice on wild sea trout post smoults.
Jackson, D., S. Deady, D. Hassett & Y. Leahy. Caligus elongatus Nordmann as parasites of farmed salmon in Ireland.
Jackson, D., D. Hassett, S. Deady & Y. Leahy. Lepeophtheirus salmonis (Krøyer) (Copepoda: Caligidae) on farmed salmon.
McAndrew, K., R. Wootten & C. Sommerville. Survival and egg production of Lepeophtheirus salmonis in experimental infections of Atlantic salmon (Salmo salar).
Nordhagen, J.R., P.A. Heuch & T.A. Schram. Size as indicator of origin of salmon lice Lepeophtheirus salmonis (Copepoda: Caligidae).
Roth, M. The availability and use of chemotherapeutic sea lice control products.
Schram, T.A. The egg string attachment mechanism in salmon lice Lepeophtheirus salmonis (Copepoda: Caligidae).
Shinn, A.P., B.A. Banks, N. Tange, J.E. Bron, C. Sommerville, T. Aoki & R. Wootten. Comparison of 18S and 1TS sequences obtained from Lepeophtheirus salmonis parasitising Atlantic salmon (Salmo salar) in Scotland.
Shinn, A.P., Bron, J.E., Gray, D.J. & C. Sommerville. Elemental analysis of Scottish populations of the ectoparasitic copepod Lepeophtheirus salmonis (Krøyer, 1837).
Treasurer, J.W., A. Grant & P.J. Davies. Physical constraints of bath treatments of Atlantic salmon (Salmo salar) infested with sea lice (Copepoda: Caligidae).
Tully, O., W.R. Poole, K.F. Whelan. Temporal variability in physiological conditions of sea Trout in the marine environment: Implications for the impact of Sea Lice on host survival.
Tully, O., P. Gargan, W.R. Poole, K.F. Whelan. Spatial and temporal variation in Sea Lice infestation of Sea Trout in Ireland (1990-1997).
Vikeså, V. & K. Boxaspen. The effects of salinity and temperature on early life stages of salmon lice, Lepeophtheirus salmonis.
Recent publications on sealice
Collier, L. M. and Pinn, E. H. 1998. An assessment of the acute impact of the sea lice treatment ivermectin on a benthic community. Journal of Experimental Marine Biology and Ecology, 230(1), 131-147.
Costelloe, M., Costelloe, J., Coghlan, N., O'Donohoe, G. and O'Connor, B. 1998. Distribution of the larval stages of Lepeophtheirus salmonis in three bays on the west coast of Ireland. ICES Journal of Marine Science 55(2) 181-187.
Davies, I. M, Gillibrand, P. A., McHenery, J. G. and Rae, G. H. 1998. Environmental risk of ivermectin to sediment-dwelling organisms. Aquaculture, 163, 29-46.
Dawson, L. H. J. 1998. The physiological effects of salmon lice (Lepeophtheirus salmonis) infections on returning post-smolt sea trout (Salmo trutta L.) in western Ireland. ICES Journal of Marine Science 55(2) 193-200.
Grant, A. and Briggs, A. D. 1998. Toxicity of ivermectin to estuarine and marine invertebrates. Marine Pollution Bulletin, 36, 540-541.
Grant, A. and Briggs, A. D. 1998. Use of ivermectin in marine fish farms: some concerns. Marine Pollution Bulletin, 36, 566-568.
Hull, M. Q., Pike, A. W., Mordue, A. J. and Rae, G. H. 1998. Patterns of pair formation and mating in an ectoparasitic caligid copepod Lepeophtheirus salmonis (Kroyer 1837): implications for its sensory and mating biology. Philosophical Transactions of the Royal Society of London B 353, 753-764.
MacKenzie, K., Longshaw, M., Begg, G. S. and McVicar, A. H. 1998. Sea lice (Copepoda: Caligidae) on wild sea trout (Salmo trutta L.) in Scotland. ICES Journal of Marine Science 55(2) 151-162.
MacKinnon, B. M. 1998. Host factors important in sea lice infections. ICES Journal of Marine Science 55(2) 188-192.
Mo, T. A. and Heuch, P. A. 1998. Occurrence of Lepeophtheirus salmonis (Copepoda: Caligidae) on sea trout (Salmo trutta) in the inner Oslo Fjord, south-eastern Norway. ICES Journal of Marine Science 55(2) 176-180.
O'Donoghue, G., Costelloe, M. and Costelloe J. 1998. Development of a management strategy for the reduction/elimination of sea lice larvae, Lepeophtheirus salmonis, parasites of farmed salmon and trout. Marine Resources Series No. 6, 51 pp.
Rolland, J. B. and Nylund, A. 1998. Infectiousness of organic materials originating in ISA-infected fish and transmission via salmon lice (Lepeophtheirus salmonis). Bulletin of the European Association of Fish Pathologists, 18(5), 173-180.
Schram, T. A., Knutsen, J. A., Heuch, P. A. and Mo, T. A. 1998. Seasonal occurrence of Lepeophtheirus salmonis and Caligus elongatus (Copepoda: Caligidae) on sea trout (Salmo trutta), off southern Norway. ICES Journal of Marine Science 55(2) 163-175.
Davies, I. M., McHenery, J. G. and Rae, G. H. 1997. Environmental risk of dissolved ivermectin to marine organisms. Aquaculture 158, 263-275.
McHenery, J. G., Linley-Adams, G. E., Moore, D. C., Rodger, G. K. and Davies, I. M. 1997. Experimental and field studies of effects of dichlorvos exposure on acetylcholinesterase activity in the gills of the mussels, Mytilus edulis L. Aquatic Toxicology 38, 125-143.
McVicar, A. H. 1997. Disease and parasite implications of the coexistence of wild and cultured Atlantic salmon populations. ICES Journal of Marine Science, 54:1093-1103.
Murison, D. J., Moore, D. C., McHenery, J. G., Robertson, N. A. and Davies, I. M. 1997. Epiphytic invertebrate assemblages and dichlorvos usage at salmon farms. Aquaculture 159, 53-66.
Thain, J E, Davies I M and G H Rae, 1997. Acute toxicity of ivermectin to the lugworm, Arenicola marina. Aquaculture, 159, 47-52.
Best current strategies for the control of lice on salmon farms
This document arose from the discussions at the Trondheim workshop on sea lice control on fish farms, as part of the EU Concerted Action on "Lice control in Fish Farms", under the FAIR programme, and was drafted by Kjell Maroni, KPMG Management Consulting, Flatanger, Norway
The following report is made as a pointed list, and not as an in depth advice. This is done as a consequence of the fast development of new methods for lice treatment, and also is the most correct picture from the discussions in Trondheim.
Prevention of infestation
These precautionary methods will also reduce the transfer of other diseases within and between farms.
The preferred method will always be the biological method, using cleaner-fish (wrasse). The advantage is that they clean the fish continuously, and pick off the lice with egg strings first. The disadvantage is their low activity at low temperatures (< 6 ° C), and the method is only partly developed for big salmon (> 2 kg). However, Ballan wrasse seem very promising on such big fish (see Kvenseth article, Caligus, Issue 2). It is necessary to avoid heavily fouled nets, because the wrasse will feed on the fouling rather than the lice. It is also possible to capture lice released when moving or grading fish.
Chemical treatment can be used if preventive methods or cleaner-fish do not do the job. The application of chemotherapeutants by spray or dip when grading is effective and uses and releases less chemicals. It was agreed that in-feed methods seem very promising, but documentation and practical results are still scarce. For example the time between dose and effect on lice and relative impacts on chalimus, mobile and egg production is unclear.
Chemical methods should be chosen so that the life cycle of the lice is broken. Bath methods effective against juveniles should be chosen when treating early in the life of the salmon, while methods killing the adult lice only can be used when it is near slaughtering time.
It is important to have cooperation between the fish farmers and the authorities. The authorities should go for a preventive strategy, and help the salmon farming industry to quantify benefits from lice control (economic, less lice pressure on wild fish, market image).
It is felt that some countries (apart from Norway) have an over-protective strategy when it comes to legislation of new treatment methods. This can result in resistance problems, and also leaves the farmers fewer methods to use under different conditions. The authorities must use their "law-power" to force fish farmers who do not follow the agreed (co-ordinated) treatments in a region. Both "carrot" and "stick" are necessary!
Most chemotherapeutants used against lice have been developed for other animals so considerable information already exists on their risk to staff and the consumer. However there is inadequate co-ordination between different regulations for chemotherapeutants both within and between countries, including the EU and EEA countries. Products may be licensed for use but not permitted to be released into the sea or may not have maximum residue limits established for salmon meat going to market.
Farmers need to have the methods and best advice on how to treat lice now. The priority for action here is to improve management and co-ordination of regulations. A longer term view identified novel areas of research which may produce more effective treatments. This includes research into how lice find their host, if it is possible to upset the digestive system of lice (e.g. through a vaccine), and to improve techniques for the use of different species of cleaner-fish to control lice. Another priority is for more sophisticated and quantitative models of lice population dynamics including their host location behaviour and hydrographic conditions.