Concentration of 22,23-dihydroavermectin B1a detected in the sediments at an Atlantic salmon farm using orally administered ivermectin to control sea-lice infestation

An HPLC analytical method based on a methanol extraction, C clean-up, derivatization using 8 trifluoroacetic acid anhydride and reverse-phase HPLC was developed for the detection of 22,23-dihydroavermectin B in marine sediment. The limit of quantitation was determined as 1a Ž . 0.93 ng 22,23-dihydroavermectin B H B per g of sediment. This method was used to 1a 2 1a determine the concentrations of H B in sediments taken from under a fish farm where 2 1a ivermectin had been administered orally, at 0.05 mgrkg twice weekly, to control sea-lice infestation in 350,000 post-smolt Atlantic salmon. Prior to sampling, a total of 23 g of ivermectin had been administered during the current production cycle. The sediment was sampled using diver-collected cores at sample sites 25 m apart. The mean concentration of H B detected in the 2 1a six sediment cores taken directly under the cage block was 5.0 ngrg in the top 3 cm, 3.1 ngrg in the 3–6 cm fraction and 0.7 ngrg in the 6–9 cm fraction. Concentrations were under the limit of quantitation at depths below 9 cm in all cores taken from directly under the cage block. H B 2 1a was not detected in any samples taken further than 22.5 m from the cage block in the eastern, western and southern directions. To the north of the cage block, H B was detected in samples 2 1a taken 31 m from the cage block and the limit of distribution in this direction was not established. Despite the limitations of this study, it is difficult to reconcile the concentrations of H B 2 1a ) Corresponding author. Fax: q353-91-750514; E-mail: [email protected] 0044-8486r00r$ see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0044-8486 99 00259-8 ( ) A. CannaÕan et al.rAquaculture 182 2000 229–240 230 detected in this study with long term accumulation and persistence of ivermectin in sediments under fish farms. q 2000 Elsevier Science B.V. All rights reserved.