Evolution of chemical resistance in sea lice
In open-net aquaculture of Atlantic salmon (Salmo salar), high stocking densities lead to infestations of sea lice (Lepeophtheirus salmonis and Caligus spp.). This costs the industry hundreds of millions of dollars annually and poses serious risk to populations of wild Pacific salmon (Oncorhynchus spp.), affecting out-migrating smolts that would otherwise escape heavy infection. Fortunately, control measures have been largely successful at curbing louse infestations. Emamectin benzoate (SLICETM), the primary chemical treatment used, remains effective in BC, although lice have evolved resistance to this drug in most other salmon-farming jurisdictions. The prospect of resistance in BC is an issue of great concern to both industry and conservationists.
Wild-origin lice “spill over” onto farmed salmon when wild adult salmon return to spawn in rivers, and farm-origin lice later “spill back” onto seaward-migrating smolts. Chemical treatment on farms selects for resistance in lice but also reduces spill-back, preserving a wild-salmon refuge for lice, where physiological costs likely select against chemical resistance. Wild-origin lice may displace resistant lice on farms, or dilute the advantages of resistance alleles through genetic mixing, maintaining or prolonging the efficacy of chemical treatment. Thus, treatment on farms protects wild salmon populations, and wild salmon populations may provide an ecosystem service for farms.
I use mathematical models to explore strategies for avoiding chemical resistance in sea lice. The aim is to understand the eco-evolutionary dynamics involved, with a view to making management recommendations that could reduce conflict surrounding the contentious salmon-farming industry.
Population and group dynamics of cooperative breeders
Allee effects (declining reproduction, survival, or fitness with decreases in population size or density) have been widely suggested to occur in cooperative species, where individuals rely on each other to find food, defend against predators, or raise offspring. My PhD work on meerkats (Suricata suricatta) found little support for Allee effects at the social group level. While mortality rates are elevated in small groups, reproduction and emigration dominate group dynamics and produce conventional density dependence.
I did find evidence for conventional density dependence in meerkat groups and delayed effects of rainfall. It seems that changing patterns of reproductive conflict contribute to shifts in age structure and result in steep declines in group size following dry years in the Kalahari, especially in larger groups.
If groups do not “suffer” when small, and large groups tend to decline faster than small ones, the demographic and selective benefits of group-living likely accrue outside of the social group context. Evidence does suggest that larger groups produce larger cohorts of dispersing individuals, which are better able to found new groups. I am modelling this process to understand the fitness consequences and implications for population dynamics.
An ongoing aspect of work with meerkats is the consideration of spatial processes in their population dynamics. I have worked on adapting partial differential equation home-range models to describe territorial dynamics, and I am involved with a developing project, led by Arpat Ozgul, to track dispersal using GPS collars.