Evaluating the spatial range of the effect of synchronized antiparasitic treatments on the abundance of sea lice Caligus rogercresseyi (Boxshall & Bravo, 2000) in Chile

Abstract

Sea lice are the most important ectoparasites affecting farmed salmonids in marine water worldwide and pharmacological treatments are widely used to control their abundance. Synchronization of antiparasitic treatments among closely located salmon farms has been recently evaluated as a promising strategy for improving treatment performance. However, the optimum distance at which farms should synchronize their treatments is not clear. We used a repeated measures linear mixed effects model to evaluate the impact of two nationwide treatment synchronization procedures conducted in Chile in 2014 and 2015 on subsequent adult lice counts up to 13 weeks after the procedure. Each treatment synchronization procedure consisted of two 2-week synchronization windows, in which farms were required to treat their fish. A period of 3 weeks elapsed between the two windows. Treatment synchronization was measured as the number of neighbouring farms that treated their fish within a certain geographical threshold, each of them weighted by its distance from the farm of interest. We tested four different thresholds: 5, 10, 20 and 30 km. The results indicate that the abundance of adult lice on farms that synchronized treatments with their neighbours within a distance of 5 km was lower than the abundance on non-synchronized farms from weeks 4 to 11 after the procedure. Our findings suggest the treatment synchronization effect was distance dependent and greater when neighbouring farms up to 5 km joined the procedure.