Elevated Seawater Temperature and Infection with Exacerbate Complex Gill Disease in Farmed Atlantic Salmon () in British Columbia, Canada.

Gill disorders and diseases are emergent health concerns affecting marine-farmed salmon, for which the causal factors are poorly understood in British Columbia (BC), Canada. This study sought to describe and compare spatial and temporal patterns of infection with , the causal agent of amoebic gill disease, and visually assessed gill health scores in farmed Atlantic salmon. Gill tissue obtained during the Fisheries and Oceans Canada's Fish Health Audit and Intelligence Program (DFO-FHAIP) between 2016 and 2020 were screened for by qPCR. Semi-quantitative visual gill health assessments were conducted during the audits, and farms were assigned clinical AGD status based on microscopic visualization of together with histopathological lesions. Seawater temperature and salinity data were collected from all active farms in the region during the study period. Trends in gill scores and associations with infections were described and tested using an ordinal logistic mixed model. The amoeba was detected in 21% of 345 audited farms and in 12% of 1925 fish samples. Most (56%, n = 1898) samples had no visible gill damage (score = 0), and 23% had scores ≥ 2 (high). Distinct patterns of spatial and temporal variability in the rates of high gill scores and infections are demonstrated. The model supported the statistically significant relationship observed between seawater temperature and the proportion of samples with elevated gill scores. The model also revealed a direct relationship between salinity and gill score but only in the presence of . While the data suggest that histopathological lesions contributed to the gill scores, temperature and, to a lesser extent, salinity were significant risk factors of increased gill score. The results are discussed in the context of recently frequent thermal anomalies in the northeastern Pacific Ocean.