School of BioSciences - Theses
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ItemThe evolution of salmon lice (Lepeophtheirus salmonis) in response to parasite management strategies in salmon aquaculture( 2022)Parasites, pathogens and other pests have a major impact on global agriculture. Pest populations are able to adapt to novel selection pressures on farms, making long-term pest management a challenge. This is exemplified by the repeated evolution of pesticide resistance over the last century. Resistance can be slowed or avoided by applying evolutionary principles to pest management. This first requires understanding the evolutionary dynamics of the pest under contemporary farming conditions. Such knowledge is especially needed in rapidly emerging and changing farm systems, such as aquaculture. Salmon farming – among the biggest aquaculture industries – is significantly affected by parasitic sea lice (especially the salmon louse Lepeophtheirus salmonis). Efforts to control this parasite have been undermined by salmon lice evolving resistance to most chemical pesticides. An abundance of non-chemical technologies have been introduced as alternatives, but there has been little research into whether they may also drive louse evolution. This thesis explores the potential for L. salmonis to adapt to the pressures faced in salmon aquaculture today; in particular, possible resistance to non-chemical management strategies. Chapter 1 outlines the need for evolutionary perspectives in pest management, and provides an overview of the thesis. In Chapter 2, I review the literature and conclude that salmon lice could, theoretically, adapt to non-chemical methods. This chapter explores how non-chemical resistance might arise, how it might have ripple effects on wild salmon, and how it could be prevented using evolutionarily-enlightened pest management. Chapters 3 and 4 focus on one non-chemical strategy: using physical barriers to prevent lice from entering salmon cages. In Chapter 3 I describe an empirical study that identified significant inter-family variation in louse swimming behaviour, with some families preferring deeper water than others. Chapter 4 addresses whether preventative cage barriers could impose selection on this behaviour, by inadvertently allowing deeper-swimming lice to pass underneath barriers to infest salmon. This was tested using a particle-tracking model, which predicted strong directional selection on swimming behaviour. Taken together, Chapters 3 and 4 offer the first evidence that lice could adapt to physical barriers. Chapter 5 describes a metapopulation model designed to predict evolutionary dynamics of lice across the larger, interconnected farm network. This model will be a versatile tool for future research exploring how various management regimes could limit the spread of resistance through the population. Finally, Chapter 6 ties the threads of this thesis together into a research framework that can be applied to the wider aquaculture sector to assist with managing resistance. This thesis combines theoretical, empirical and modelling techniques to predict the trajectory of L. salmonis evolution in response to different pest management strategies. Key areas for future study are highlighted – more research is crucial if the salmon industry wishes to find ways to stymie louse resistance. More broadly, this thesis illustrates that pests can adapt to management strategies – especially non-chemical methods – in surprising ways. By looking at the world from the parasite’s point of view, we can discover new techniques for safeguarding farms against undesirable evolutionary processes.