Temporal Variation in Prevalence of Viral Hemorrhagic Septicemia Virus Type IVb among Upper St. Lawrence River Smallmouth Bass

We investigated a newly introduced pathogen in a wild population of smallmouth bass Micropterus dolomieu in the St. Lawrence River to illuminate unknown temporal and demographic prevalence patterns. We used real‐time quantitative reverse‐transcription polymerase chain reaction to assess viral hemorrhagic septicemia virus (VHSV) genotype IVb (VHSV‐IVb) viral N‐gene levels. Our objectives were to characterize the distribution and abundance of this newly introduced pathogen within a naïve population of economically valuable sport fish through an annual cycle. We also sought to describe VHSV‐IVb levels across demographic groups (males and females; adults and subadults). Strong temporal variation in prevalence of VHSV‐IVb was evident through the annual cycle; peaks in prevalence corresponded to the smallmouth bass spawning period and a temperature range of 10–14°C. No difference in VHSV‐IVb prevalence between male and female smallmouth bass was detected. However, a markedly higher prevalence was observed in subadult fish than in adult fish. Owing to the observed temporal patterns in prevalence, surveillance programs for VHSV‐IVb in smallmouth bass should concentrate effort on springtime sampling and should include subadult fish.