Abstract
The evolution of pest resistance to management strategies is a major challenge for farmed systems. Mitigating the effects of pest adaptation requires identifying the selective pressures imposed by these strategies. In Atlantic salmon (Salmo salar) aquaculture, barriers are used to prevent salmon louse (Lepeophtheirus salmonis) larvae (copepodids) from entering salmon cages. These barriers are effective against shallow‐swimming copepodids, but those swimming deeper can pass underneath and infest salmon. Laboratory experiments suggest that depth regulation in copepodids is a variable behavioural trait with a genetic basis. We used biological–hydrodynamic dispersal models to assess how this trait variation alters the dispersion of lice through the ocean environment and into farms. The dispersal of copepodids with 3 behavioural phenotypes (deep, mean or shallow) was modelled over winter–spring and spring–summer periods in a Norwegian fjord system with intensive aquaculture. The infestation pressure of each phenotype on barrier cages was estimated from their modelled depth distributions: copepodids deeper than 10 m were predicted to successfully pass underneath barriers. The deep phenotype was the most abundant below 10 m and reached infestation pressures 3 times higher than that of the mean phenotype. In contrast, the shallow phenotype infestation pressure reached less than half that of the mean phenotype. These differences in relative fitness indicate that barriers can impose strong directional selection on the swimming behaviour of copepodids. The strength of this selection varied seasonally and geographically, with selection for the deep phenotype stronger in winter–spring and at coastal locations than in spring–summer and within fjords. These findings can be applied across farms to slow louse adaptation, by limiting barriers during situations of strong selection, although this must be balanced against trade‐offs to short‐term efficacy. More broadly, our study highlights new ways in which dispersal models can address evolutionary questions crucial for sustainable parasite management in aquaculture.
Highlights
Farming is inextricably tied to evolutionary processes, and this is true for pest management (Thrall et al, 2011)
Our simulations suggest that variation in the swimming behaviour of larval salmon lice, as observed in small-scale experimental columns (Coates et al, 2020), translates to variation in the depth of lice in the natural environment
Our model builds off a previously validated lice dispersal model (Sandvik et al, 2020), covers a physical and temporal scale that dwarfs that possible in the laboratory and included key environmental factors that influence the distribution of copepodid at different depths
Summary
Farming is inextricably tied to evolutionary processes, and this is true for pest management (Thrall et al, 2011). Assuming a genetic element to this behavioural variation, if depth-based preventions select for louse copepodids that occur deeper in the water column (those that pass underneath barriers), adaptive responses are likely to ensue To assess this possibility, it is first essential to determine whether the inter-family variation observed in experimental columns (Coates et al, 2020) translates to differences in depth in the natural environment. We converted behavioural data (from Coates et al, 2020) into new parameters for a Norwegian lice dispersal model and tracked the effect of different behaviours on the spatial distribution of larvae in three dimensions We used these outputs to estimate the strength of selection that depth-based preventions might impose on louse behavioural. We examine how the strength of this selection varies over time and space
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