Prolonging Azamethiphos bath increases the effectiveness of field treatments against Caligus rogercresseyi in Atlantic salmon in Chile (Salmo salar)

Azamethiphos is the most effective and most frequently used chemotherapeutant for the control of Caligus rogercresseyi in salmon aquaculture in Chile. Identification of Azamethiphos administration methods that maximize efficacy and reduce the risk of promoting the development of drug resistance will be critical to ensure continued treatment success. In this retrospective study of Atlantic salmon (Salmo salar) bath treatment practices, we used data from the Los Lagos and Aysen regions in southern Chile from January 2014-March 2017 to assess the impact of several factors on the effectiveness of Azamethiphos bath treatments against C. rogercresseyi with three treatment durations (30 min, >30 min and < 50 min, ≥50 min). To assess treatment efficacy, we modelled the counts of total adult Caligus (mobile adults and gravid females) following treatment. The results suggest that prolonging treatment significantly reduces the counts of adult Caligus post-treatment. As expected, the abundance of both juvenile and adult Caligus pre-treatment was positively associated with the abundance of adult Caligus post-treatment. Other factors such as the average fish weight and numbers can be determinants of treatment success. Results from this study have important implications for the management of Caligus infestations in Chilean salmon farming because of the current dependence on Azamethiphos for adequate Caligus control. Prolonging Azamethiphos bath treatments can be a powerful tool in situations where the risk of treatment failure is higher than expected. Ultimately, prolonging treatment-time with Azamethiphos appears to contribute to treatment success, reduce the number of treatments required throughout a production cycle, and reduce the risk of developing drug resistance.