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.

An evaluation of the duration of efficacy of emamectin benzoate in the control of Caligus curtus Müller infestations in Atlantic cod, Gadus morhua L.

The duration of efficacy of emamectin benzoate in the control of Caligus curtus infestations in Atlantic cod, Gadus morhua L., was studied following an administration of 50 µg kg⁻¹ for seven consecutive days. No lice were found on medicated fish when challenged 1 week (challenge 1) or 5 weeks (challenge 2) following termination of medication, whereas the mean abundance of lice among the unmedicated fish was 17.9 and 19.3 lice per fish in challenge 1 and 2, respectively. Muscle concentrations of 19.5 ± 8.2 ng g⁻¹ and 3.4 ± 0.9 ng g⁻¹, respectively, and skin concentrations of 23.1 ± 10.8 and 4.2 ± 1.0 ng g⁻¹, respectively, were found 27 and 55 days following the termination of medication. Tissue concentrations and the duration of efficacy indicate a dosing regime for emamectin in cod, similar to the regime used for Atlantic salmon, Salmo salar L.