Field trials on the efficacy of the organophosphorus compound azamethiphos for the control of sea lice (Copepoda: Caligidae) infestations of farmed Atlantic salmon ( Salmo salar)

A study of the efficacy of azamethiphos (S-6-chloro-2,3-dihydro-2-oxo-1,3-oxazolo[4,5-b]pyridin-3-ylmethyl 0,0-dimethyl phosphorothioate) is described when used to treat Atlantic salmon ( Salmo salar) infected with sea lice ( Lepeophtheirus salmonis and Caligus elongatus) at several salmon farms on the west coast of Scotland. Prior to commencing the cage trials, relative lice sensitivity was evaluated through a series of in vitro exposures which demonstrated differences in the sensitivities of different lice populations to both azamethiphos and dichlorvos. Treatments were carried out by completely enclosing pens within tarpaulins, or by surrounding pens with a tarpaulin skirt, and were l h in duration. A range of treatment concentrations were studied which included 0.05, 0.1 and 0.2 mg −1 L azamethiphos, in addition to reference treatments with dichlorvos (0,0-dimethyl-2,2-dichlorovinyl phosphate) at a concentration of 1.0 mg −1 L. Lice response was measured before treatment, 1 h, 24 h, and 1 week after treatment. Azamethiphos was highly efficacious (85% or better) (Site 3) against adult and pre-adult lice, from a sensitive population, at a concentration of 0.05 mg −1 L where complete tarpaulins were used. Where resistant populations were tested, a concentration of 0.2 mg −1 L (complete tarpaulins) resulted in efficacies ranging from 7.1 to 82.1% (mean 57.1%) (Site 1) and 41 to 96.6% (mean 69.4%) (Site 2) against adult and pre-adult lice, and were generally more variable depending on local treatment conditions. Skirt treatments, tested only on resistant populations of lice, were found to be ineffective. A general trend was observed indicating that pre-adult I male lice were the most sensitive followed by, in order of sensitivity, pre-adult I females, pre-adult II females, pre-adult II males, adult males and adult females. Larval lice numbers, from both sensitive and resistant populations, appeared unaffected by treatment with azamethiphos following treatments with either complete tarpaulins or skirts. When used at the concentrations discussed above, fish tolerated treatments and did not display signs of acute mortality. However, hyperactivity was displayed by several of the treatment groups during treatments.