A REVIEW OF THE USE OF IVERMECTIN AS A TREATMENT FOR SEA LICE (LEPEOPHTHEIRUS SALMONIS KRØYER AND CALIGUS ELONGATUS NORDMANN) INFESTATION IN FARMED ATLANTIC SALMON (SALMO SALAR )

I.M. Davies & G.K. Rodger

Ectoparasitic sea lice (Lepeophtheirus salmonis Krøyer and Caligus elongatus Nordmann) browse on the skin of farmed Atlantic salmon (Salmo salar). The resulting lesions cause stress and increase susceptibility of the fish to secondary infections. In extreme infestations, fish can suffer from osmoregulatory failure and death. The most immediate treatment for the relief of sea lice infestations at fish farms is the use of chemotherapeutants, either by bath or oral administration. One compound that has been investigated for use as a chemotherapeutant is ivermectin.

Ivermectin is a member of the avermectin group of compounds, which are macrocyclic lactones. The avermectins are neurotoxins, which have been used successfully in the treatment of helminthic parasitic infections in a number of terrestrial farm animals and also in the treatment of river blindness in humans. Due to the low solubility of the compound, ivermectin has been investigated as an oral treatment administered with the feed and has been found to be effective for the treatment of sea lice on Atlantic salmon. Studies have found that ivermectin is poorly absorbed by fish with a high percentage of the administered dose being excreted in the faeces. The highest concentrations of the absorbed ivermectin were found in lipid-rich organs. The ivermectin remained in the tissues of the treated fish for a prolonged period of time and was excreted mainly in the unchanged form.

Ivermectin can reach the marine environment via excretion from the bile, unabsorbed via the fish faeces and by uneaten food pellets and has a strong affinity to lipid, soil and organic matter. The fate of orally administered ivermectin is discussed in relation to the available information on its toxicity to non-target species.

A summary of the licensing procedure for the use of chemicals in the treatment of farmed fish in the United Kingdom is also given. This review paper ends with details of the development and the status of the use of ivermectin in the treatment of sea lice infestations at Scottish fish farms.

REDUCED SENSITIVITY OF SALMON LICE, LEPEOPHTHEIRUS SALMONIS, ON FARMED ATLANTIC SALMON TO HYDROGEN PEROXIDE

Jim Treasurer¹, Simon Wadsworth¹ & Andrew Grant²

¹ Marine Harvest McConnell, Lochailort, Inverness-shire PH38 4LZ, Scotland.

² Marine Harvest McConnell, Farms Office, Blar Mhor, Fort William PH33 7PT, Scotland.

Hydrogen peroxide has been the main medicine used to control sea lice on many salmon farms in Scotland since 1992 and anecdotal evidence has recently suggested reduced treatment efficacy. The present study tests the sensitivity of sea lice to exposure to hydrogen peroxide under farm conditions and also in vivo by comparing lice from a farm with suspected resistance and lice from a farm that had not been pre-exposed to hydrogen peroxide. In bin treatments on a farm that had used hydrogen peroxide on 41 occasions numbers of ovigerous lice declined from 2.7 and 3.8 per fish in two replicates to 2.4 and 2.1 respectively (by 11% and 45%), and from 22.9 and 17.8 other mobile stages to 14.7 and 12.1 (36% and 32%). The overall removal of all mobile stages was 33% and 34%. In fish from a farm where hydrogen peroxide had not been used before ovigerous females declined from 7.7 to 0.8 and 1 per fish (90% and 87%) and other mobile lice from 14.8 to 0.4 and 0.1 (97% and 99%), and overall efficacy was 95% and 96% respectively in replicates. These trial results indicate the development of resistance in lice to hydrogen peroxide and this may be greater in ovigerous females. The mechanisms involved in the development of resistance through detoxifying enzymes such as catalase and the implications for lice control strategies relying on hydrogen peroxide are discussed.

MEDICAL TREATMENT THAT PROTECTS AGAINST SEA LICE INFESTATION IN FARMED ATLANTIC SALMON (SALMO SALAR) AND SEA TROUT (SALMO TRUTTA )

Bernt Martinsen & Svein Alexandersen

ALPHARMA, Aquatic Animal Health Division, Harbitzalleen 3, PO Box 158 Skoyen, N-0212 OSLO, Norway.

Alpharma has tested a new medical treatment, currently under development, that protects Atlantic salmon and sea trout from sea lice (Lepeophtheirus salmonts) infestation for a significant period of time after treatment. The medical treatment may be administered as an in-feed treatment or as a bath formulation. The treatment may be performed in fresh water just before the salmon smolts are transferred to sea water, or the treatment can be performed in sea water after the fish have been infected. The protection period is dependent on the drug dose and sea water temperature. At low sea water temperatures a protection period up to five months has been obtained. No adverse drug reactions have been associated with the treatment.

The tested treatment will be most suitable for smolts prior to sea transfer, as a long withdrawal period would be expected. By combining the protective property of the treatment and use of wrasse, it should be possible to have sea lice free fish the first summer in sea without additional chemical treatment.

Data on sea lice protection, fish growth and drug depletion will be presented.

DELOUSING OF FARMED SALMON DURING GRADING BY SPRAYING WITH PY-SAL 25, A NATURAL PYRETHROID COMPOUND. A COMBINATION OF MECHANICAL AND CHEMICAL DELOUSING

Per Gunnar Kvenseth¹ & Anne-Mette Kvenseth²

¹ KPMG Management Consulting as - Centre for Aquaculture and Fisheries, Sandviksboder 5, N-5035 Bergen, Norway.

² University of Bergen, Department of Fisheries and Marine Biology, High Technology Centre, Pb 7800, N-5020 Bergen, Norway.

A co-operation project between Melbu Verft AS, producing stainless steel equipment for aquaculture and fisheries purposes, and the salmon producer AS Mowi, have developed a live fish grader that combines size grading and delousing.

A Silkstream live fish pump was used to bring the salmon from the pen to the grading machine. The pump alone removed 40-70% of the sealice, which were subsequently collected in a sieve. When concentrated Py-Sal 25 was sprayed on the salmon during the grading process, 95% of the salmon lice were removed. Py-Sal 25 diluted in seawater (5%) gave variable delousing effect (37-83%).

Mechanical separation of Py-Sal 25 and seawater for recycling of the pyrethroid solution was found to be difficult, mainly because it blended with fish mucous. The method proved to be effective but gave ten times higher chemical costs compared with traditional delousing methods.

DOSE DETERMINATION OF EMAMECTIN BENZOATE FOR THE TREATMENT OF SEA LICE, LEPEOPHTHEIRUS SALMONIS KRØYER, INFESTATIONS OF ATLANTIC SALMON, SALMO SALAR L.

J. Stone¹, I.H. Sutherland², C. Sommerville¹, R.H. Richards¹ & K.J. Varma³

¹ Institute of Aquaculture, University of Stirling, Scotland.

² 31/10 Hermitage Drive, Edinburgh, Scotland.

³ Schering-Plough Animal Health, Union, New Jersey, USA.

The determination of an efficacious dose of emamectin benzoate, as a treatment for sea lice, Lepeophtheirus salmonis, infestations of Atlantic salmon, was evaluated in three tank trials. Fish were repeatedly challenged with larval and motile stages of sea lice in four replicate tanks and then randomly re-distributed to each of two or three replicate control or treatment tanks. Doses of 25, 50 and 100m g kg^-1 emamectin benzoate were evaluated and comparisons made with control fish given the same diet without emamectin benzoate. Treatment was administered daily as a coating on feed pellets for 7 consecutive days. At 7, 14 and 21 days from the start of treatment, all fish were anaesthetised and the number of each developmental stage of louse recorded.

In-feed treatment with emamectin benzoate was effective against all chalimus and motile stages of L. salmonis. Lice numbers were reduced at all treatment doses but the optimum therapeutic dose was established as 50 m g kg^-1 day^-1. This dose was well tolerated by Atlantic salmon and no adverse effects were attributed to treatment. As early as day 7, lice numbers were significantly (P<0.001) reduced relative to the control groups, in all three trials. By the end of the trial at day 21, many treated fish were completely free of sea lice and the prevalence of lice damage was much lower than on control fish. Fish mortalities attributed to sea lice damage were reduced and, in one trial survival of treated fish was 48% higher than survival of control fish.

FIELD TRIALS WITH EMAMECTIN BENZOATE 0.2% AQUACULTURE PREMIX (SLICEä , SCHERING-PLOUGH ANIMAL HEALTH) FOR THE ORAL TREATMENT OF SEA LICE INFESTATIONS IN FARMED ATLANTIC SALMON, SALMO SALAR L. IN NORWAY – 1997 GENERATION

A. Ramstad¹, R. Nordmo¹, I. Sutherland² & R. Simmons³

¹ Veso Vikan AkvaVet, Namsos, Norway.

² 31/10 Hermitage Drive, Edinburgh, Scotland.

³ Schering-Plough Animal Health, Union, NJ, USA.

In Norway during the summer/early autumn of 1997, four farms were recruited to a programme of field trials to evaluate the efficacy of emamectin benzoate (SLICEä , Schering-Plough Animal Health) against natural infestations of sea lice on Atlantic salmon. A total of 1,170,543 first year class fish were involved, with 561,266 receiving emamectin benzoate as a premix coated onto the feed with oil to supply a dose rate of 50µg/kg bodyweight daily for seven consecutive days (Study Day 0 – 6). Test groups (pens) were compared with parallel positive control groups totalling 610,411 first year class fish that received a commercially available oral sea lice treatment, teflubenzuron (Skretting A/S, Ektobann), fed at a dose rate of 10mg/kg bodyweight daily for seven consecutive days (Study Day 0 – 6). All groups (both test and control) received medicated feed at 0.5% bodyweight/day. Lice were counted on 20 fish in each group on Study Days –2, 1, 7, 14 and 21. In addition, lice were counted on Study Day 36 at one site and on Study Day 51 at another site.

Both emamectin benzoate and teflubenzuron reduced the mean number of sea lice/fish within 7 days from the start of treatment from approximately 5 lice (all stages)/fish to <1. Sea louse infestation rates remained low during the 21-day observation period. However, statistical analysis of the data showed that at Study Day 21, sea lice numbers on fish that received emamectin benzoate were significantly lower (p<0.0001) than on fish treated with teflubenzuron i.e. 0.32 vs. 1.18 lice/fish, respectively. On Study Day 51, a mean of 13.9 lice (all stages)/fish were observed on teflubenzuron-treated fish vs. a mean of 0.23 lice (all stages)/fish on fish that received emamectin benzoate. Also, no pre-adult or adult sea lice were found on emamectin benzoate-treated fish by Study Day 51. Feed consumption in all groups was satisfactory throughout the study although at two study sites slight appetite depression was observed that may in part have been attributable to a different basal ration and/or size of pellets for the medicated feed. No fish mortality or adverse effects attributable to treatment with emamectin benzoate were noted.

EFFICACY OF SLICEÔ (EMAMECTIN BENZOATE 0.2% AQUACULTURE PREMIX) AGAINST SEA LICE, CALIGUS FLEXISPINA AND CALIGUS TERES, ON ATLANTIC SALMON, SALMO SALAR, IN CHILE.

M. Romero¹, I.H. Sutherland³, T. Katz² & R.G. Endris²

¹ Schering-Plough Animal Health Cia. Ltda., Chile.

² Schering-Plough Animal Health Corp. Union, NJ, USA.

³ 31/10 Hermitage Drive Edinburgh, Scotland.

Sea lice infestations are the primary health problem of farm-reared salmonids in Chile. Caligus flexispina is the dominant sea louse species but C. teres is present as well. Six sea cages of Atlantic salmon (mean weight, 1.35 kg) naturally infested with sea lice of both species were included in the study. Three cages were untreated and 3 cages received SLICEÔ (emamectin benzoate 0.2% aquaculture premix) in medicated feed at a dose rate of 50 m g/kg biomass/day for 7 consecutive days (Study Day 0 – 6). Ten fish from each cage were anaesthetised and the number of chalimus (I-IV), preadult/adult and gravid female sea lice/fish were counted on Study Days 1, 26, 46 and 102. Prior to treatment the mean numbers of chalimus (I-IV), preadult/adult and gravid female Caligus spp./fish were 15.9, 26.3 and 12.7 respectively. On Study Day 26 (20 days after treatment), the efficacy against chalimus (I-IV), preadult/adult and gravid female was 79.3%, 84.3%, 88.2%, respectively. On Study Day 46, the efficacy against chalimus (I-IV), preadult/adult and gravid female was 92.1%, 90.3%, 96.3%, respectively. The efficacy against gravid females was 83.7% (3.8/SLICE-treated fish vs. 23.5/untreated fish) on Study Day 102. Salmon were under constant reinfestation pressure throughout the study. The mean total number of sea lice/fish in the untreated cages on Study Day 1 was 91.5 and by Study Day 46 the mean total number/fish had increased > 87% to 171.4. SLICEÔ (emamectin benzoate 0.2% aquaculture premix) fed to sea lice infested Atlantic salmon provided effective control of non-motile and motile sea lice for at least 40 days after treatment and it effectively controlled reproductive female sea lice for up to 96 days after treatment. Results of additional field trials with similar results will be discussed

A FIELD EVALUATION OF THE EFFICACY OF EMAMECTIN BENZOATE 0.2% AQUACULTURE PREMIX (SLICE, SCHERING-PLOUGH ANIMAL HEALTH) AGAINST SEA LICE, LEPEOPHTHEIRUS SALMONIS, INFESTING ATLANTIC SALMON, SALMO SALAR, IN NEW BRUNSWICK, CANADA

Robert Armstrong¹, Daniel MacPhee², Terry Katz³ & Richard Endris³

¹ 6545 First Street Forestville, CA, USA 95436.

² P.O. Box 1120 St. Andrews, New Brunswick Canada E0G2X0.

³ Terry Katz3 Schering-Plough Animal Health Box 3182 Union, New Jersey USA 07083.

The efficacy of emamectin benzoate 0.2% aquaculture premix was evaluated against Lepeophtheirus salmonis on Atlantic salmon at two sites in New Brunswick, Canada. Salmon pens were randomly allocated to either a test group that received emamectin benzoate in-feed at 50m g per kg body weight per day for 7 consecutive days or a control group that received the same diet with no added medication. Each pen could also receive, at the discretion of the site veterinarian, a bath treatment with azamethiphos (Salmosan, Novartis). Three bath treatments were applied to each control pen during the trial while the treatment pens received no bath treatment. The mean number of lice per fish was statistically significantly lower (p<0.05) in the experimental group when measured 1, 3, 4 and 6 weeks after the start of medication. Treatment efficacy was 70, 88, 95 and 61% respectively, although these figures were reduced by bath treatment of the controls. The number of sea lice/fish was also determined at one of the sites 8, 10, 13 and 16 weeks after the start of medication and the efficacy was 96, 91, 79 and 63% respectively. No gravid female lice were observed on any fish in the treatment group between 1 and 10 weeks after the start of medication while a low number of gravid females were observed on fish in the control group. Ten weeks after the start of medication, fish in the test group at one trial site were statistically significantly heavier than fish in the control group. Orally administered emamectin benzoate proved to be highly effective for control of sea lice on farmed salmon under commercial production conditions in Eastern Canada.

SLICEÔ (EMAMECTIN BENZOATE 0.2% AQUACULTURE PREMIX): A SUMMARY

R.G. Endris¹, I.H. Sutherland², R.D. Simmons¹ & K.J. Varma¹

¹ Schering-Plough Animal Health Corp; Union, NJ, USA.

² 31/10 Hermitage Drive, Edinburgh, Scotland.

SLICEÔ is a feed premix containing 0.2% emamectin benzoate developed for use in aquaculture to control sea lice infestations in salmonid species. The active ingredient, emamectin benzoate (4"’-deoxy-4"-epi-methylamino avermectin B₁), is a 16-membered lactone in the avermectin class of antiparasitic compounds. The premix is based on starch and maltodextrin formulated for use as an oil-bound coating on pelleted feed. The premix is stable for at least 2 years under ambient conditions and SLICE-medicated feed is stable for at least 3 months under usual field conditions. SLICE is administered to salmonids in feed at a dose rate of 50 m g/kg biomass/day for 7 consecutive days. The concentration in feed is 10 ppm (10 mg/kg) when a feeding rate of 0.5% biomass/day is employed. Emamectin benzoate is well tolerated by both Atlantic salmon (Salmo salar) and Rainbow trout (Oncorhynchus mykiss) with signs of toxicity observed at dose rates in excess of 3.5X and 4.4X than recommended, respectively. SLICE has been evaluated for efficacy against sea lice, Lepeophtheirus salmonis, Caligus elongatus and Caligus flexispina, in Scotland, Norway, Canada and Chile. It kills all parasitic stages of sea lice infesting both salmon and trout at sea temperatures ranging from 5° – 15° C. Typical efficacy patterns are: 80 - 90% efficacy within 14 days of treatment, > 90% efficacy within 21-28 days of treatment and sustained efficacy > 90% for up to 10 weeks. Under practical conditions of use, emamectin benzoate did not adversely effect pelagic or benthic organisms, vertebrate or invertebrate.

EFFICACY OF CALICIDEÒ FOR THE TREATMENT OF SEA LICE (LEPEOPHTHEIRUS SALMONIS) INFESTATIONS OF FARMED ATLANTIC SALMON (SALMO SALAR )

Edward Branson¹, Stig Sverre Rønsberg² & Gordon Ritchie²

¹ Trouw Aquaculture Ltd., Wincham, Northwich, Cheshire CW9 6DF, UK.

² Nutreco Aquaculture Research Centre, Sjøhagen 3, 4016 Stavanger, Norway.

The efficacy of CALICIDEÒ (chemical name teflubenzuron), for the treatment of farmed Atlantic salmon (Salmo salar) infested with sea lice (Lepeophtheirus salmonis), was investigated on 2 commercial salmon farms in Norway. CALICIDEÒ , coated onto commercial feed pellets, was administered orally at 10mg/kg/day for 7 consecutive days. Fish were randomly sampled and lice numbers recorded from both treated and control groups on 4 sampling occasions post-medication. Statistically significant reductions in the number of L. salmonis / fish were recorded. Maximum efficacy (>90%) was observed toward chalimus and preadult stages of L. salmonis, and was achieved 7-10 days post-medication. No apparent effect was observed on adult lice. No adverse drug reactions or palatability problems were associated with the treatments.

EFFECT OF CALICIDEÒ ON THE CUTICLE STRUCTURE OF LEPEOPHTHEIRUS SALMONIS

Ove Martin Grøntvedt & Gordon Ritchie

Nutreco Aquaculture Research Centre, Sjøhagen 3, 4016 Stavanger, Norway.

CALICIDEÒ (teflubenzuron), is proposed for the control of sea lice on farmed Atlantic salmon (Salmo salar), is highly efficacious toward the developing stages of Lepeophtheirus salmonis. Teflubenzuron acts by inhibiting the biochemical synthesis of chitin in insects, thus disrupting the moult. SEM, TEM and LM techniques were used to examine the effect of teflubenzuron on the cuticular structure and ultrastructure of larval and adult L. salmonis exposed to teflubenzuron. Deformations were observed on the cuticle surface of exposed larvae. The procuticle was thinner, lacking any definite structure, and the basal membrane was absent in exposed larvae. No differences were observed between the cuticle of exposed and control adult L. salmonis. Chitin synthesis was inhibited and no new cuticle was synthesised or deposited by moulting stages of L. salmonis exposed to teflubenzuron.

FOOD SAFETY ASPECTS BY THE USE OF LEPSIDON® (DIFLUBENZURON) MEDICATED PELLETS IN FARMED ATLANTIC SALMON (SALMO SALAR L.)

J.I. Erdal¹, M.I. Toneby² & A.C. Dykstra³

¹ EWOS as, P.O.Box 424, N-1471 Skaarer, Norway.

² MIT miljolab AB, Dal, S-153 94, HÖLÖ, Sweden.

³ Uniroyal Chemical BV. P.O.Box 58217, 1040 HE Amsterdam, The Netherlands.

Pharmacokinetic properties of orally administrated diflubenzuron in Atlantic salmon are presented. Absorption: Bioavailability after a single oral administration of diflubenzuron equal to 3 mg kg^-1 body weight is 31%. Maximum serum concentration (C_max) is found after 24 hours, and is 0.14 µg ml^-1. Metabolism: Chromatographic extracts of tissue showed that the major component is diflubenzuron. This was further confirmed by mass spectral analysis. Four minor components (4-chlorophenyl urea and three unidentified compounds) have been detected, which all disappeared by 7 days after dosing. Excretion: Major route of excretion is via bile and faeces as diflubenzuron. Excretion from tissues is rapid. Depletion of residues have been studied at two different water temperatures, 6 °C and 15 °C. Mean residues were found to be < 0.05 mg kg^-1 14 and 21 days after end of medication, respectively. CVMP has recommended a provisional MRL for diflubenzuron in Atlantic salmon fillet of 1 mg kg^-1 valid until 01.07.2000.

ENVIRONMENTAL SAFETY ASPECTS OF THE USE OF LEPSIDON^® (DIFLUBENZURON) MEDICATED PELLETS IN FARMED ATLANTIC SALMON (SALMO SALAR L.)

Mark Toneby¹, Per Johannessen², Helge Botnen², Staffan Hjohlman², Gisle Vassenden², Christian Wallace³ & Jon Inge Erdal⁴

¹ MIT miljölab AB, Dal, S-153 94 Hölö, Sweden.

² UNIFOB, Section of Applied Environmental Research, Bergen High Technology Centre, N-5020 Bergen, Norway.

³ Ewos Research Station Lønningdal, N-5200 Os, Norway.

⁴ Ewos as, P.O. Box 424, N-1471 Skårer, Norway.

Studies were conducted to investigate potential effects on the marine fauna in the vicinity of farms with Atlantic salmon treated orally with diflubenzuron at 3 mg/kg body weight daily for 14 days for the control of sea lice. Sediment and water were sampled up to 6 months after treatment and chemically analysed for diflubenzuron. Total fauna > 1mm in the sediment samples was characterised. The variation of fauna within and between farms and the relationship to the concentration of diflubenzuron in sediment was studied by multivariate statistical methods. In a separate study plates were deployed in the water column close to farms for exposure to natural settling of cirripedia larvae and other organisms. Settling and growth of fauna was observed over 6 months. In both types of studies, untreated farms or farms not using diflubenzuron for the control of salmon lice served as control farms.

Diflubenzuron was found in sediments beneath the treated farms. The concentration decreased with distance from the farm and time post medication. No statistically significant adverse effects on fauna elated to treatment were found in sediment or on plates in the water. It is concluded that diflubenzuron used according to the recommendations for oral treatment against sea lice, under the field conditions of the investigations, did not appear to cause detectable adverse effects on the marine fauna.

ALPHA MAX^TM VET., AN EFFECTIVE TREATMENT AGAINST SEA LICE (LEPEOPHTHEIRUS SALMONIS (KROYER)) INFECTING FARMED ATLANTIC SALMON (SALMO SALAR) AND RAINBOW TROUT (ONCHORHYNCHUS MYKISS )

Svein Alexandersen & Bernt Martinsen

ALPHARMA, Aquatic Animal Health Division, Harbitzalleen 3, PO Box 158 Skoyen, N-0212, OSLO, Norway.

Alpharma AS has developed a new delousing compound, ALPHA MAX^TM VET, which contains the pyrethroid deltamethrin as active ingredient. ALPHA MAX^TM VET is administered as a bath treatment by use of a closed treatment tarpaulin, a well boat or a treatment skirt surrounding the net pen. Due to a combination of adequate effect against sea lice and a short treatment period (30 minutes), ALPHA MAX^TM VET has within one year become the most popular delousing compound among fish farmers in Norway.

Field data shows that deltamethrin, formulated as ALPHA MAX^TM VET, is effective against sea lice, especially against preadults and adult males, where the effect is close to 100%. It seems, however, that the immediate effect is influenced by the sea water temperature. Bath treatments conducted at cold sea water temperatures (<6° C) show that the measured efficacy of the treatment improves up to 17 days after bath treatment. This indicates that the judgement of the effect should be determined 10 days or later after treatment at low temperatures. Laboratory studies have confirmed that the compound is more effective against sea lice at high temperatures compared with cold temperatures, and that adult males are more vulnerable to the compound, compared with the adult female sea lice. LC50-values from laboratory studies will be presented and discussed together with effect data from field trials conducted under different sea water temperatures.

THE EFFICACY OF CYPERMETHRIN (EXIS®) UPON THE SURVIVAL AND RATE OF DEVELOPMENT OF CHALIMUS STAGES OF LEPEOPHTHEIRUS SALMONIS (KRØYER, 1837) (COPEPODA: CALIGIDAE)

Simon Wadsworth¹, Julian Braidwood² & Nicola Fraser²

¹ Marine Harvest McConnell, Lochailort, Inverness-shire, Scotland. PH38 4LZ.

² Vericore Ltd, Talkin, Brampton, Cumbria. CAB 1LE.

A major problem currently facing the Scottish salmon farming industry is a lack of an available medicine that is effective against the chalimus of Lepeophtheirus salmonis. Use of compounds that are only effective against post-larval stages can result in stress, damage, lost growth and mortalities in the affected fish populations. The efficacy of the synthetic pyrethroid, cypermethrin (EXIS®) was assessed against L. salmonis chalimus. Exposure of the chalimus to 5 µg l^-1 cypermethrin for 1 h resulted in a significant reduction (83%, p<0.001) in the mean number of chalimus 12 d post-treatment. The developmental stages observed in the untreated group were predominantly pre-adult II males. The majority of the remaining lice observed in the treated group were chalimus III indicating a profound effect of cypermethrin on the rate of development of L. salmonis. At 1 µg l^-1 cypermethrin for 1 h there was no significant reduction in the mean number of lice compared to controls (p=0.161) although there was a significant delay in the rate of development. There was no significant reduction (p=0.719) in the efficacy of cypermethrin at 5 µg l^-1 by reducing the treatment duration from 1 h to 0.5 h.