Abstract

The loss of effectiveness of some used chemotherapeutants to control sea lice in Chile (Caligus rogercresseyi) has led to the need in some salmon-producing companies to evaluate parasite sensitivity over time to the chemotherapeutants used. The aim of this study was to monitor the seasonal sensitivity of C. rogercresseyi to antiparasitics based on deltamethrin, cypermethrin and azamethiphos in the Los Lagos Region of Chile (macrozones 1, 2, 3 and 5). Parasites were collected from anaesthetized fish at the farms and transported alive to the “Caligus Unit” of the Central Laboratory (Marine Harvest, Chile), to perform an accredited toxicological bioassay (NChISO-17025 INN). A total of 229 toxicological bioassays were conducted between January 2014 and March 2016 (approximately 48.000 parasites tested). Exposure time of parasites was 30 min for cypermethrin and azamethiphos, and 40 min for deltamethrin, to concentration gradients which included the manufacturer’s recommended concentrations. The responses were evaluated 24 h after treatment exposure and results are presented as EC50 and affected parasites (%) at the recommended concentrations. The 100% of the parasites were affected for azamethiphos in all the bioassays. With cypermethrin and deltamethrin the results were highly variable between seasons. In general the antiparasitics were observed to be more effective in spring-summer than in autumn-winter. The results indicate a high variability of parasite sensitivity to antiparasitics and seasons. However, the most important finding is that the average EC50 value for the main treatment used (azamethiphos) continues to increase from 6 ± 3 ppb in 2014 to 14 ± 14 ppb in 2015, suggesting a decrease in sensitivity of a fraction of the population. This suggests that other chemicals or non-pharmacological mechanisms should be incorporated to control caligidosis, and that surveillance of resistance to current treatments must be maintained as part of Caligus integrated management.