Using Agent-Based Modelling to Predict the Role of Wild Refugia in the Evolution of Resistance of Sea Lice to Chemotherapeutants

Abstract

A major challenge for Atlantic salmon farming in the northern hemisphere is infestation by the sea louse parasite Lepeophtheirus salmonis. The most frequent method of controlling these sea louse infestations is through the use of chemical treatments. However, most major salmon farming areas have observed resistance to common chemotherapeutants. In terrestrial environments, many strategies employed to manage the evolution of resistance involve the use of refugia, where a portion of the population is left untreated to maintain susceptibility. While refugia have not been deliberately used in Atlantic salmon farming, wild salmon populations that migrate close to salmon farms may act as natural refugia. In this paper we describe an agent-based model that explores the influence of different sizes of wild salmon populations on resistance evolution in sea lice on a salmon farm. Using the model, we demonstrate that wild salmon populations can act as refugia that limit the evolution of resistance in the sea louse populations. Additionally, we demonstrate that an increase in the size of the population of wild salmon results in an increased effect in slowing the evolution of resistance. We explore the effect of a population fitness cost associated with resistance, finding that in some cases it substantially reduces the speed of evolution to chemical treatments.

Lessons from sea louse and salmon epidemiology

“…[53]) and, in some areas, sea lice have evolved resistance to the chemicals used to control them [58,59]. Ordinary differential equation models and individual-based models show that the frequency of treatments and mixing rates between sea lice on farmed and wild untreated hosts can influence the rate of resistance evolution [60,61]. Mixing between wild-origin sea lice and farm sea lice can dilute the proportion of resistant individuals and provide an opportunity for resistant alleles to be removed by natural selection if they have fitness costs [60].…”

Managing aquatic parasites for reduced drug resistance: lessons from the land

“…Our model was modified from the model described in [18]. Both models evaluate sea louse infestations on populations of farmed fish and the evolution of chemical resistance in response to treatments.…”

“…Following the lead of [18], we use 'sea louse agents' to refer to modelled sea lice, and 'sea lice' to refer to real sea lice. Below, we use the overview, design and details protocol [23] to further describe the modifications made for this model.…”

“…the west coast of Canada [37]. The reason for this may be the presence of large wild populations of salmonids acting as refugia and fewer treatments being applied overall [18]. …”

“…Resistance to a number of commonly used chemical treatments has already evolved in sea lice in most major salmon producing areas including Norway, Scotland, the Faroe Islands, Ireland, Chile and eastern Canada (reviewed in [10]). Owing to the limited types of chemical treatments available for salmon infected with sea lice, many salmon farmers are running out of chemical options for effective control [7,8,10,18]. As only a few new active compounds for controlling sea lice have been developed during the past 30 years [19], it is imperative to maintain the efficacy of the existing compounds by using appropriate management techniques.…”