A fishy tale: the impact of multiple stressors on host behaviour, physiology, and susceptibility to infectious disease

Aquatic habitats are facing increased anthropogenic stressors that are associated with multiple species demise. The loss of species is, sadly, part of a wider global crisis with current extinction levels estimated to be a thousand-fold higher than the background extinction rate. Freshwater habitats in particular are facing higher rates of degradation than any other habitat and within these, fish species are being lost faster than their terrestrial counterparts. Beyond the importance of fish as keystone species, they are an invaluable source of protein for humanity and stocks are facing a state of collapse. A key threat facing the fish industry, including wild stocks, is increased infectious disease burdens. A major reason we are witnessing a continued increase in losses to disease is because fish species are experiencing increased stressors that are compromising host welfare that in turn impacts disease susceptibility. This PhD project focussed on how fish welfare is impacted by different biotic and abiotic stressors, with an emphasis on host immunity and disease resistance. To accomplish this project, freshwater fish host-parasite models were utilised permitting long-term monitoring of infections in real time. Most stressors investigated negatively impacted fish disease resistance, with the first stressor investigated being mechanical disturbance associated with routine transportation practices. Beyond the increased susceptibility to disease seen in fish hosts exposed to stressors, this PhD also revealed that noise pollution significantly increases host mortality rates. However, in response to the ecological stressor, flow, no changes to fish immune gene expressions or pathogen burdens were seen. With regards to implementing simple measures for effective disease control, this project has revealed how the addition of structural enrichment to fish tanks significantly improved disease resistance while also reducing agnostic behaviour. The universal physiological cost of infection by significantly increasing host metabolic rates was also revealed. The final experimental study assessed how one of the most prevalent contaminants, microplastic, impacted host disease resistance, growth and mortality. Microplastic at variable concentrations significantly increased disease susceptibility and host mortality. Ultimately, this PhD project has furthered our understanding of how multiple emerging and widespread stressors are impacting fish host welfare through the lens of host-pathogen dynamics.