Abstract
Anadromous fishes such as steelhead trout, Oncorhynchus mykiss, are exposed to a suite of infectious agents and migratory challenges during their freshwater migrations. We assessed infectious agent load and richness and immune system gene expression in gill tissue of Bulkley River (British Columbia, CA) steelhead captured at and upstream of a migratory barrier to evaluate whether infectious burdens impacted migration success. We further considered the potential influences of water temperature, sex and fish size on host infectious agents and transcription profiles. There were eight infectious agents detected in steelhead gill tissue, with high prevalence of the bacteria Candidatus Branchiomonas cysticola (80%) and Flavobacterium psychrophilum (95%) and the microparasite Sphaerothecum destruens (53%). Fish sampled at the falls had significantly greater relative loads of Ca. B. cysticola and F. psychrophilum, higher infectious agent richness and differential gene expression compared to fish captured upstream. Flavobacterium psychrophilum was only associated with immune gene expression (particularly humoral immunity) of fish sampled at the falls, while water temperature was positively correlated with genes involved in the complement system, metabolic stress and oxidative stress for fish captured upstream. This work highlights interesting differences in agent-host interactions across fisheries and suggests that hydraulic barriers may reduce the passage of fish with the heaviest infectious agent burdens, emphasizing the selective role of areas of difficult passage. Further, this work expands our knowledge of infectious agent prevalence in wild salmonids and provides insight into the relationships between infectious agents and host physiology.
Highlights
Infectious agents are widespread in aquatic environments (Price, 1980) and play an integral role in shaping aquatic ecosystems through disease processes (e.g. Cotner and Biddanda, 2002; Suttle, 2005; Smith, 2007)
Flavobacterium psychrophilum was only associated with immune gene expression of fish sampled at the falls, while water temperature was positively correlated with genes involved in the complement system, metabolic stress and oxidative stress for fish captured upstream
Seven infectious agents were initially detected in Bulkley River steelhead gill tissue, though only two were sufficiently prevalent to evaluate drivers of their abundance and their influence on host physiology
Summary
Infectious agents (pathogenic microparasites including viruses, bacteria, fungi and protozoa) are widespread in aquatic environments (Price, 1980) and play an integral role in shaping aquatic ecosystems through disease processes (e.g. Cotner and Biddanda, 2002; Suttle, 2005; Smith, 2007). Recent advances in molecular genetics (Lawson Handley, 2015) have allowed for rapid and cost-effective quantification of gene expression, including the use of highthroughput quantitative reverse transcriptase polymerase chain reactions (PCR) on the BioMarkTM microfluidics platform, which has recently been validated for detection and quantification of salmon infectious agents (Miller et al, 2016) and detection of a molecular signature indicative of viral disease (Miller et al, 2017) This platform can help deduce when an infectious agent has moved from a carrier state to actively replicate and cause damage to the host, based on the host-immune response (Casadevall and Pirofski, 1999; Miller et al, 2014)
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