Sea lice in the North Pacific: from sub-lethal effects on wild salmon to parasite management and policy

Since wild-capture fisheries production plateaued in the early 1990s, the world’s dependence on aquaculture has grown steadily. This ‘blue revolution’ may have helped the conservation of some wild aquatic species by decreasing fishing pressure, but for others it has depleted their populations through habitat degradation, harvest for feed, and the spread of infectious disease. This thesis examines how parasites from aquaculture facilities can indirectly influence wild host survival and assesses how improvements to policy could limit these effects. I explore these topics in British Columbia, Canada, where wild Pacific salmon (Oncorhynchus spp.) are commonly infested with parasitic sea lice (Lepeophtheirus salmonis and Caligus clemensi) from open-net salmon farms. In Chapter 2, I use a field experiment to demonstrate that heavy sea louse infestation is associated with decreased competitive foraging ability for juvenile sockeye salmon (O. nerka). In Chapter 3, I show that this louse-associated reduction in competitive ability leads to decreased foraging success for juvenile sockeye in the wild. In Chapter 4, I analyse the otoliths (i.e., ear stones) of juvenile sockeye to reveal that highly infested fish grow more slowly than uninfested individuals. Each of these responses – competitive ability, foraging success, and growth – has major implications for salmon survival. In Chapter 5, I then investigate the ways in which parasite control policy could be improved on salmon farms to limit transfer of sea lice to wild salmon. I demonstrate that there is considerable underestimation bias in self-reported sea lice counts from industry, which determine when delousing treatments are used to control sea lice outbreaks on farms. I also show that current parasite control policy is not resilient to changing environmental conditions and I assess the potential effectiveness of alternative policies. Ultimately, the sustainability and success of the blue revolution will depend on our understanding of the full impacts of disease on wildlife and our ability to limit them.