The future of therapeutic agents in aquaculture.

The implementation and monitoring of fish health regulations vary extensively in aquaculture throughout the world. In the main salmon-producing countries, there is strict regulatory oversight of the use of pharmaceutical drugs. Such controls have supported the sustainable growth of the aquaculture industry and, in Norway, aquaculture has been able to reduce its total consumption of antibiotics by more than 99% between 1995 and today, yet there has been a 20-fold rise in production volume. Other countries on other continents may have less control, with no mandatory prescription regulations and variable quality of the pharmaceutical products available. A good regulatory framework, with control and monitoring systems, should be established in all countries where aquaculture is practised and veterinary medicinal products should only be available under veterinary prescription and supervision. Many drug resistance genes have been identified, and molecular methods should be applied to control drug resistance in the microbial and parasitic populations of all major aquaculture-producing regions.

Les niveaux de mise en place et de suivi des réglementations applicables à la santé des poissons en aquaculture sont extrêmement variables d'une région à l'autre. Les pays où la salmoniculture prédomine pratiquent un contrôle réglementaire strict de l'utilisation de produits médicamenteux. Ces contrôles ont permis une croissance durable du secteur ; ainsi, en Norvège l'aquaculture a pu réduire sa consommation totale d'antibiotiques de plus de 99 % depuis 1995 tout en multipliant par vingt le volume de production. Le niveau des contrôles est parfois moindre dans d'autres pays d'autres continents, où aucune réglementation n'impose l'obligation d'une prescription vétérinaire et où les produits pharmaceutiques disponibles sont de qualité variable. Tous les pays ayant une production aquacole devraient mettre en place un cadre réglementaire solide doté de systèmes de contrôle et de suivi ; par ailleurs, les produits pharmaceutiques vétérinaires ne devraient pouvoir être obtenus que sous prescription et supervision vétérinaires. Plusieurs gènes responsables de résistances aux agents médicamenteux ont été identifiés ; il conviendrait que toutes les régions dédiées à l'aquaculture recourent à des méthodes moléculaires pour contrôler le phénomène de la résistance aux agents médicamenteux dans les populations de microbes et de parasites.

La aplicación de reglamentos de salud piscícola y las actividades de control reglamentario en la acuicultura varían sobremanera según el lugar del mundo de que se trate. En los principales países productores de salmón, el uso de productos farmacéuticos está sujeto a una estricta supervisión reglamentaria. Estos controles han favorecido el crecimiento sostenible del sector de la acuicultura, sector que en Noruega ha logrado reducir su consumo total de antibióticos en más de un 99% entre 1995 y la actualidad, pese a haber multiplicado por veinte su producción. En otros países de otros continentes hay a veces un menor control, debido a la ausencia de reglamentos de carácter prescriptivo y a la irregular calidad de los productos farmacéuticos disponibles. Sería menester que todos los países donde se practica la acuicultura contaran con un buen ordenamiento reglamentario, dotado de sistemas de control y seguimiento, y que en ellos solo se pudieran obtener productos de uso veterinario con receta y bajo supervisión veterinarias. Sabiendo que se han encontrado muchos genes que determinan resistencia a los medicamentos, convendría aplicar métodos moleculares para controlar la aparición de farmacorresistencias en las poblaciones microbianas y parasitarias de todas las regiones que albergan un importante sector acuícola.

A screening of multiple classes of pharmaceutical compounds for effect on preadult salmon lice Lepeophtheirus salmonis.

The salmon louse, Lepeophtheirus salmonis Krøyer, is the major obstacle facing a sustainable future for farmers of salmonids in the North Atlantic Ocean. Medicinal compounds have been the most utilized tool to prevent salmon lice infestation; however, the active compounds have become less effective or considered environmentally unfriendly in the past years. Novel medicinal compounds are thus highly desired. In two experiment series, 26 medicinal compounds were screened for their efficacy against salmon lice, in a 30-min exposure and 24-h exposure, respectively. Pyriprole, imidacloprid, cartap and spinetoram were effective at 50 mg L(-1) in the short-time exposure. In the 24-h exposure, pyriprole, propoxur, cartap, imidacloprid, fenoxycarb, pyriproxyfen, nitenpyram, spinetoram, spiromesifen and diflubenzuron induced a high level of immobilization at 5 mg L(-1) . The EC50 values of the effective compounds were calculated in further titration studies for both exposure periods. Several physiological and biochemical pathways were discovered as possible targets for medicinal intervention against the salmon louse.

A screening of medicinal compounds for their effect on egg strings and nauplii of the salmon louse Lepeophtheirus salmonis (Krøyer).

Egg strings and nauplii of the salmon louse Lepeophtheirus salmonis were exposed to a variety of medicinal compounds at 50 mg L(-1) for 30 min in two experiment series. This medicine concentration was selected as a starting point for a screening series. Hatching of egg strings and development to copepodid larvae were monitored in one experiment, and the survival and development of nauplii were monitored in the other. Two compounds, emamectin benzoate and cypermethrin, inhibited hatching effectively. Several compounds affected nauplii, either directly or through inhibiting development to the infective stage. A total of 50 mg L(-1) of azamethiphos, cypermethrin, emamectin benzoate and propoxur was lethal to >70% of the larvae. Diflubenzuron, fenoxycarb, pymetrozine, pyriprole and tebufenozide diminished the ability of nauplii developing to copepodids.

Deltamethrin resistance in the sea louse Caligus rogercresseyi (Boxhall and Bravo) in Chile: bioassay results and usage data for antiparasitic agents with references to Norwegian conditions.

The sea louse Caligus rogercresseyi is a major threat to Chilean salmonid farming. Pyrethroids have been used for anticaligus treatments since 2007, but have shown reduced effect, most likely due to resistance development. Pyrethroid resistance is also a known problem in Lepeophtheirus salmonis in the Northern Hemisphere. This study describes the development of deltamethrin resistance in C. rogercresseyi based on bioassays and usage data for pyrethroids in Chilean aquaculture. These results were compared to bioassays from L. salmonis from Norway and to Norwegian usage data. Available deltamethrin bioassay results from 2007 and 2008, as well as bioassays from Norway, were collected and remodelled. Bioassays were performed on field-collected sea lice in region X in Chile in 2012 and 2013. Bioassays from 2007 were performed prior to the introduction of pyrethroids to the Chilean market. Both the results from 2008 and 2012 showed an increased resistance. Increased pyrethroid resistance was also indicated by the increased use of pyrethroids in Chilean aquaculture compared with the production of salmonids. A similar trend was seen in the Norwegian usage data. The bioassay results from Chile from 2012 and 2013 also indicated a difference in the susceptibility to deltamethrin between male and female caligus.

Single-dose field bioassay for sensitivity testing in sea lice, Lepeophtheirus salmonis: development of a rapid diagnostic tool.

Sea lice on farmed salmonids are often treated with chemicals. Sensitivity testing of sea lice can reduce the number of treatments by identifying substances the sea lice are susceptible to. This study describes a simpler protocol for field sensitivity testing than today's six-dose bioassay. The protocol, which uses a single dose of the delousing agents deltamethrin, azamethiphos and emamectin benzoate, was developed on four different strains of sea lice and their subsequent generations. A sensitive strain and a strain showing reduced sensitivity were identified for each chemical after performing traditional bioassays and small-scale treatments. The single doses for each chemical were established by modelling dose-response curves from 24-h bioassays on strains with differences in sensitivity. The largest difference between the lower 80% prediction interval for the sensitive strain and the upper 80% prediction interval for the strain showing reduced sensitivity was identified for each delousing agent. The concentration of the chemical and the % mortality corresponding to each of the 80% prediction intervals were subsequently established. To validate the protocol for field use, further studies on both sensitive and resistant strains of sea lice under field conditions are required.

Salmon lice--impact on wild salmonids and salmon aquaculture.

Salmon lice, Lepeophtheirus salmonis, are naturally occurring parasites of salmon in sea water. Intensive salmon farming provides better conditions for parasite growth and transmission compared with natural conditions, creating problems for both the salmon farming industry and, under certain conditions, wild salmonids. Salmon lice originating from farms negatively impact wild stocks of salmonids, although the extent of the impact is a matter of debate. Estimates from Ireland and Norway indicate an odds ratio of 1.1:1-1.2:1 for sea lice treated Atlantic salmon smolt to survive sea migration compared to untreated smolts. This is considered to have a moderate population regulatory effect. The development of resistance against drugs most commonly used to treat salmon lice is a serious concern for both wild and farmed fish. Several large initiatives have been taken to encourage the development of new strategies, such as vaccines and novel drugs, for the treatment or removal of salmon lice from farmed fish. The newly sequenced salmon louse genome will be an important tool in this work. The use of cleaner fish has emerged as a robust method for controlling salmon lice, and aquaculture production of wrasse is important towards this aim. Salmon lice have large economic consequences for the salmon industry, both as direct costs for the prevention and treatment, but also indirectly through negative public opinion.

Pharmacokinetics and pharmacodynamic effects of meloxicam in piglets subjected to a kaolin inflammation model.

The pharmacokinetics and the analgesic, anti-inflammatory and antipyretic effects of meloxicam were investigated in a placebo controlled study in 2-week-old piglets. Inflammation was induced by a subcutaneous injection of kaolin in the left metacarpus, and 16 h later, meloxicam (0.6 mg/kg) or saline was administered intramuscularly. The absorption half-life was relatively short (0.19 h) and the elimination half-life was 2.6 h. Mechanical nociceptive threshold testing was used to evaluate the analgesic effect, but no significant effect of the meloxicam treatment was found. The skin temperature of the inflamed area increased after the kaolin injection, but no significant decrease in temperature was found after administration of meloxicam. Only limited pyresis was observed after the kaolin injection, and no significant antipyretic effect of meloxicam was found. The results indicated that this dose of meloxicam had very limited anti-inflammatory and analgesic effects in piglets.

Enantioselective pharmacokinetics of ketoprofen in piglets: the significance of neonatal age.

Following intravenous dose of 6mg/kg racemic ketoprofen, the chiral pharmacokinetics of ketoprofen was investigated in eight piglets aged 6 and 21days old. S-ketoprofen predominated over R-ketoprofen in plasma of the piglets in both age groups. The volumes of distribution of S-ketoprofen for the 6- and 21-day-old piglets were 241.7 (211.3-276.5) mL/kg and 155.0 (138.7-173.1) mL/kg, respectively, while the corresponding parameters for R-ketoprofen were 289.2 (250.3-334.2) mL/kg and 193.0 (168.7-220.8) mL/kg. The clearances of R-ketoprofen [948.4 (768.0-1171.2) mL/h/kg and 425 (319.1-566.0) mL/h/kg for the 6- and 21-day-old piglets, respectively] were significantly higher compared to the clearances of S-ketoprofen [57.3 (46.6-70.4) mL/h/kg and 33.8 (27.0-42.2) mL/h/kg for 6- and 21-day-old piglets, respectively]. The elimination half-life of S-ketoprofen was 3.4h for both age groups, while the elimination half-life of R-ketoprofen was 0.2h for the 6-day-old and 0.4h for the 21-day-old piglets. The clearances of both R- and S-ketoprofen were significantly higher in the 6-day-old piglets compared to when they were 21 days old. Furthermore, the volumes of distribution were larger in the youngest age group.

Ketoprofen in piglets: enantioselective pharmacokinetics, pharmacodynamics and PK/PD modelling.

The chiral pharmacokinetics and pharmacodynamics of ketoprofen were investigated in a placebo-controlled study in piglets after intramuscular administration of 6 mg/kg racemic ketoprofen. The absorption half-lives of both enantiomers were short, and S-ketoprofen predominated over R-ketoprofen in plasma. A kaolin-induced inflammation model was used to evaluate the anti-inflammatory, antipyretic and analgesic effects of ketoprofen. Skin temperatures increased after the kaolin injection, but the effect of ketoprofen was small. No significant antipyretic effects could be detected, but body temperatures tended to be lower in the ketoprofen-treated piglets. Mechanical nociceptive threshold testing was used to evaluate the analgesic effects. The piglets in the ketoprofen-treated group had significantly higher mechanical nociceptive thresholds compared to the piglets in the placebo group for 12-24 h following the treatment. Pharmacokinetic/pharmacodynamic modelling of the results from the mechanical nociceptive threshold testing gave a median IC(50) for S-ketoprofen of 26.7 µg/mL and an IC(50) for R-ketoprofen of 1.6 µg/mL. This indicates that R-ketoprofen is a more potent analgesic than S-ketoprofen in piglets. Estimated ED(50) for racemic ketoprofen was 2.5 mg/kg.

Pharmacokinetics and pharmacodynamics of meloxicam in piglets.

The pharmacokinetics and pharmacodynamics of meloxicam in piglets (16-23 days old) were studied using a stratified parallel group design. One group (n = 13) received 0.4 mg/kg meloxicam intravenously, while the other group (n = 12) received physiological saline solution. A carrageenan-sponge model of acute inflammation was used to evaluate the effects of meloxicam. The plasma clearance was low (0.061 L/kg/h), the volume of distribution was low (0.19 L/kg) and the elimination half-life was short (2.7 h). At most time points, the mean concentration of meloxicam in plasma exceeded the concentrations in exudate indicating a limited accumulation of the drug at the site of the inflammation. There were significant differences between the groups in the exudate prostaglandin E2 (PGE2) concentration, but the inhibition of PGE2 in the meloxicam group was limited. The inhibition of thromboxane B(2) (TXB2) production in serum in the meloxicam group was extensive, but of shorter duration than the PGE2 inhibition in exudate.

Tolerability and pharmacokinetic profile of a novel benzene-poly-carboxylic acids complex with cis-diammineplatinum (II) dichloride in dogs with malignant mammary tumours.

The pharmacokinetic profile, tolerability and efficacy of benzene-poly-carboxylic acids complex with cis-diammineplatinum (II) dichloride (BP-C1) were studied in dogs with mammary cancer. A three-level response surface pathway designed trial was performed on seven dogs. At each level BP-C1 was administered subcutaneously daily for 7 days followed by a 7-day rest period in a dose escalating manner. Adverse events according to VCOG-CTCAE, performance status and tumour progression were recorded. The pharmacokinetic profile followed a two-compartment model with rapid absorption, short distribution, and a slow elimination phase. The overall elimination half-life was 125 h. The maximum tolerated dose of BP-C1 was estimated to be above 0.46 mg kg-1 . A significant reduction in VCOG-CTCAE toxicity which correlated negatively with increasing dose was found. The dogs' general performance status remained unchanged. No decrease in total tumour burden was found, although temporary tumour reduction was seen in some target tumours.

Disposition of 14C-flumequine in eel Anguilla anguilla, turbot Scophthalmus maximus and halibut Hippoglossus hippoglossus after oral and intravenous administration.

The absorption, distribution and elimination of 14C-labelled flumequine were studied using whole body autoradiography and liquid scintillation counting. Flumequine was administered to eel Anguilla anguilla, turbot Scophthalmus maximus and halibut Hippoglossus hippoglossus intravenously and orally as a single dose of 5 mg kg(-1), corresponding to 0.1 mCi kg(-1). The turbot and halibut studies were performed in salt water (salinity of 32%) at temperatures of 16 +/- 1 degrees C (turbot) and 9.5 +/- 0.5 degrees C (halibut). The eel study was conducted in fresh water at 23 +/- 1 degrees C. In the intravenously administered groups flumequine was rapidly distributed to all major tissues and organs. After oral administration flumequine also appeared to have rapid and extensive absorption and distribution in all 3 species. After the distribution phase, the level of flumequine was higher in most organs and tissues than in the blood, except in muscle and brain. The most noticeable difference between the species was the slow elimination of flumequine from eel compared to turbot and halibut. In orally administered eels, substantial amounts of flumequine remained in all major organs/tissues for 7 d. At 28 d significant levels of flumequine were present in liver, kidney and skin (with traces in muscle), and at the last sampling point (56 d) in eye, bone, bile and posterior intestine. In orally administered turbot significant levels of flumequine were observed over 96 h in bile, urine, bone, skin, intestine and eye, and traces were detected over 28 d in bone and eye in addition to a significant level in bile. In orally administered halibut, significant levels of flumequine were observed in bile, skin, intestine and eye over 96 h. Traces were present in skin and eye over 7 d. The maximal flumequine concentrations in blood were calculated to be 2.5 mg equivalents l(-1) (eel at 12 h), 0.8 mg l(-1) (turbot at 6 h) and 0.6 mg l(-1) (halibut at 6 h) after oral administration.

Consumption of drugs for sea lice infestations in Norwegian fish farms: methods for assessment of treatment patterns and treatment rate.

Sea lice are a major problem in Norwegian fish farms; however, data on drug treatment patterns or treatment rates of sea lice infestations are not available. Such data are important for analysing resistance patterns against drugs used for such infestations. The main objective of the present study was to develop a method to estimate the treatment patterns and treatment rates for drugs used in the treatment against sea lice (Lepeophtheirus salmonis and Caligus elongatus) in farm salmonids by means of national sales statistics. Annual sales figures, as weight of active substances, were obtained from the drug wholesalers and the feed mills. The weight of active drug substances is not useful as a unit of measurement of drug use in an epidemiological context because it does not correct for dosage differences and number of repeat treatments. To correct for these factors, we introduced approved daily dose (ADD(farm fish)) and treatment course-doses(farm fish) kg(-1) live-weight fish. To express the drug treatment patterns, the biomass (in weight) of farm salmonids treated with 1 course of a drug were estimated. When measured as kg active substance, the quantities of drugs for the treatment of sea lice infestations declined by 98% during the study period (1989 to 2002) but this figure increased 5-fold when it was corrected for differences in dosage. To correct for amounts of farm salmonids liable to require treatment we estimated the annual treatment rate, defined as the number of treatments for sea lice infestations per biomass slaughtered Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss. The annual treatment rate increased gradually during the study period; however, it varied considerably (range 0.45 to 1.34, mean 0.90). Before 1995, organophosphates were the most frequently used drugs against sea lice; since then pyrethroids have become the dominating drug group.

Tissue distribution of 14C-diflubenzuron in Atlantic salmon (Salmo salar).

Diflubenzuron is a potent inhibitor of chitin synthesis, with potential use against salmon lice infestations. The absorption, distribution and elimination of the substance in Atlantic salmon was examined after a single, oral dose of 75 mg/kg bodyweight. The kinetic properties were studied by whole-body autoradiography, liquid scintillation counting and thin layer chromatography, using a 14C-labelled isotope of the substance. The drug was poorly absorbed from the intestine, but reached a concentration of more than 4 micrograms/g in the mucus layer of the skin 2 days after administration. If maintained for several days, this concentration is probably sufficient to control all moulting stages of sea lice in Atlantic salmon. The main route of excretion was via the bile.

Organophosphate poisoning of Atlantic salmon in connection with treatment against salmon lice.

Three incidents with high mortality in Atlantic salmon after trichlorfon treatment against salmon lice are described. All 3 incidents occurred at water temperatures of 12 degrees C or higher. The mean brain acetylcholinesterase (AChE) activity of dead fish was less than 20% of normal activity, while survivors showed mean activities of 22-61% of normal levels. Dichlorvos residues in muscular and liver tissues ranged from nondetectable levels to 0.2 micrograms/g tissue. The strongest inhibition of brain AChE was found in association with the highest dichlorvos residues. Substantial AChE-inhibition was, however, also found in samples in which dichlorvos residues could not be detected. AChE-determination was found to be more reliable than residue analysis for the diagnosis of organophosphate poisoning in salmon.

Prudent use of antibacterial drugs in Norwegian aquaculture? Surveillance by the use of prescription data.

Antibacterial drug treatment in aquaculture during 1991-1996 was investigated using prescription data provided by the Norwegian Government Fish Inspection and Quality Control Service (NFCS). The majority of prescriptions (n = 5401) were for Atlantic salmon and rainbow trout (salmonids), while 383 prescriptions were for other species. Of the 13 different single substances or combinations prescribed during the study period, only 5 were approved for or had been subjected to clinical trials in salmonids. Of the prescriptions for the salmonids, 99% were for approved drugs or drugs subjected to clinical trials. The major proportion of the antibacterial drugs prescribed for other fish species were drugs which were approved for or which had been subjected to clinical trials in salmonids. In all fish species, the prescribing of antibacterial drugs which were neither approved for nor had been subjected to clinical trials was mainly for fish far below slaughter weight. The prescription data were validated against the drug statistics from the wholesalers and feed mills. It was concluded that the data indeed represented antibacterial drug prescribing in Norwegian aquaculture. The prescribing of antibacterial drugs proved to be almost completely reported to NFCS, which is responsible for the control of drug residues in farmed fish in Norway.

Distribution of emamectin benzoate in Atlantic salmon (Salmo salar L.).

The aims of this study were to investigate the content of emamectin in blood, mucus and muscle following field administration of the recommended dose, and correlation with sea lice infection on the same fish (elimination study). The tissue distribution of tritiated emamectin benzoate after a single oral dose in Atlantic salmon was also investigated by means of whole-body autoradiography and scintillation counting (distribution study). In the elimination study, concentrations of emamectin benzoate reached maximum levels of 128, 105 and 68 ng/g (p.p.b.) for blood, mucus and muscle respectively, on day 7, the last day of administration. From day 7, the concentration in the blood declined until concentration was less than the limit of detection on day 77. The concentration was higher in mucus compared with plasma (P < 0.05) except on days 7 and 21. The concentration of emamectin benzoate decreased gradually from the end of treatment (day 7) to day 70 with half-lives of 9.2, 10.0 and 11.3 days in muscle, plasma and mucus respectively. The distribution study demonstrated a high quantity of radioactivity in mucous membranes (gastrointestinal tract, gills) throughout the observation period (56 days). Activity was high in the epiphysis, hypophysis and olfactory rosette throughout the study. The highest activity was observed in the bile, indicating this to be an important route for excretion. The distribution study confirmed the results from the elimination study with respect to concentrations in blood, skin mucous and muscle.

Single-dose pharmacokinetics of flumequine in cod (Gadus morhua) and goldsinny wrasse (Ctenolabrus rupestris).

Knowledge of the pharmacokinetic properties of drugs to combat bacterial infections in cod (Gadus morhua) and wrasse (Ctenolabrus rupestris) is limited. One antimicrobial agent likely to be effective is flumequine. The aim of this study was to investigate the pharmacokinetic properties of flumequine in these two species. Flumequine was administered intravenously to cod (G. morhua) at a dose of 5 mg/kg bodyweight and wrasse (C. rupestris) at a dose of 10 mg/kg. Flumequine was also administered orally to both species at a dose of 10 mg/kg body weight, and as a bath treatment at a dose of 10 mg/L water for 2 h. Identical experimental designs were used otherwise. The study was performed in seawater with a salinity of 3.2% and a temperature of 8.0 +/- 0.2 degrees C (cod) and 14.5 +/- 0.4 degrees C (wrasse). Pharmacokinetic modelling of the data showed that flumequine had quite different pharmacokinetic properties in cod and wrasse. Following intravenous administration, the volumes of distribution at steady-state (Vss) were 2.41 L/kg (cod) and 2.15 L/kg (wrasse). Total body clearances (Cl) were 0.024 L/hxkg (cod) and 0.14 L/hxkg (wrasse) and the elimination half-lives (t1/2lambda z) were calculated to be 75 h (cod) and 31 h (wrasse). Mean residence times (MRT) were 99 h (cod) and 16 h (wrasse). Following oral administration, the t1/2 lambda z were 74 h (cod) and 41 h (wrasse). Maximal plasma concentrations (tmax) were 3.5 mg/L (cod) and 1.7 mg/L (wrasse), and were observed 24 h post-administration in cod and 1 h post-administration in wrasse. The oral bioavailabilities (F) were calculated to be 65% (cod) and 41% (wrasse). Following bath administration, maximal plasma concentrations were 0.13 mg/L (cod) and 0.09 mg/L (wrasse), and were observed immediately after the end of the bath.

Single-dose pharmacokinetics of flumequine in the eel (Anguilla anguilla) after intravascular, oral and bath administration.

Knowledge of the pharmacokinetic properties of drugs to combat bacterial infections in the European eel (Anguilla anguilla) is limited. One antimicrobial agent likely to be effective is flumequine. The aim of this study was to investigate the pharmacokinetic properties of flumequine in European eels in fresh water. Flumequine was administered to eels (Anguilla anguilla) intravenously (i.v.) and orally (p.o.) at a dose of 10 mg/kg body weight, and as a bath treatment at a dose of 10 mg/L water for 2 h. The study was performed in fresh water with a temperature of 23 + 0.3 degrees C, pH 7.15. Identical experimental designs were used. Two additional bath treatments were also performed, one in which the pH in the water was lowered by approximately 1 unit to 6.07 (dose: 10 mg/L) and one at a dose of 40 mg/L for 2 h in a full-scale treatment. Following i.v. administration, the volume of distribution at steady state was 3.4 L/kg. Total body clearance was 0.012 L/h per kg and the elimination half-life (t1/2lambda z) was calculated to be 314 h. Mean residence time was 283 h. Following oral administration, the t1/2lambda z was 208 h. Maximal plasma concentration (Cmax) was 9.3 mg/L, at 7 h after administration (Cmax). The oral bioavailability (F) was calculated to be 85%. Following bath administration in 10 mg/L for 2 h, maximal plasma concentration was 2.1 mg/L, observed immediately after the end of the bath. The 'bioavailability' in eel following a 2-h bath treatment was 19.8%. Reducing the pH in the bath to 6.07 produced a maximal plasma concentration of 5.5 mg/L, observed immediately after the end of the bath. The 'bioavailability' was increased to 41% by the lowering of the pH. A similar effect was observed in a full-scale treatment (1 kg eels/L water). The CO2 produced by the eel lowered the pH and increased 'bioavailability' to 35%.