Investigating the role of harmful environmental organisms in multifactorial gill pathology in salmonids

Abstract

In this thesis, histopathology and molecular research tools were utilised to investigate gill health in salmonids. Of interest were the roles of the harmful environmental organism’s cnidarian jellyfish and toxic phytoplankton in gill pathologies of farmed fish. Multifactorial (complex) gill diseases are poorly understood disorders of fish and problematic for aquaculture, particularly in the marine environment. This thesis attempts to address the involvement of several factors in initiation and potentiation of altered health states within gills, to enhance the collective understanding of gill disease in salmonids. Results chapters address the question of the involvement of these harmful environmental organisms in gill disease and explore the microbial communities of salmonid gills with the aim of understanding the process of dysbiosis. Chapters explore the microbiomes of cnidarian jellyfish for microbial pathogens that might be transmitted, the gill transcriptome response to toxic phytoplankton exposure, and alterations to the gill microbiome with histopathological change.

The work of this thesis identified the presence of potentially harmful microbes within the microbiome of sampled cnidarian jellyfish. Alongside the previously demonstrated ability of cnidarian organisms to elicit traumatic damage to fish, it seems vector transmission might represent an additional avenue of harm through introduction of bacterial agents. The transcriptomic response of gills to phytoplankton was also explored, representing the first study of altered genetic expression of fish gills in response to toxin production phytoplankton. Results provide useful insight into the mechanisms of tissue damage by phytoplankton Prymesium parvum, as well as potentially informing future mitigation strategies for algal blooms through study of the pathways of tissue response. Final chapters of this thesis provide insight into the microbial community structure of farmed Atlantic salmon and factors influencing altered or ‘dysbiotic’ microbial populations. Temporal and pathology-associated alterations were observed within gill microbial communities in a farmed situation, as well as the impact of on-farm activities such as hydrogen peroxide treatment. Significant variation in results were identified with varied sampling methodology, highlighting the requirement for careful consideration of experimental design in the study of the microbiome, as well as providing early insight into the potential for niche partitioning of the gill microbiota.

Overall, results of this thesis emphasise the potential for harm from cnidarian jellyfish and phytoplankton in both farmed and wild fisheries. The gills are a delicate organ constantly exposed to environmental insult that suffer complex, mixed aetiology disorders as a result. These disorders are problematic to diagnose and treat, and prevention therefore seems preferable to cure. Results identify the potential initiating and propagating role of various factors in gill disorders, as well as exploring additional avenues of health status monitoring. The final chapter of this thesis discusses the main findings of this research and the potential applications both in further study and directly to enhance the aquaculture production of salmonids.