Assessment of a semiquantitative scoring system for mild-to-moderate gill lesions in Atlantic salmon reared in recirculating aquaculture systems in Norway

Compromised gill health is a critical cause of forfeited welfare in Atlantic salmon farming. Detecting and quantifying the early onset of gill disease is important to reveal initial inflicting stimuli. We collected gill samples of 45 Atlantic salmon from 2 commercial recirculating aquaculture systems (RASs) spanning fry-to-market-size fish with no clinical signs of gill disease. Gill samples were assessed histologically by 3 independent raters with different levels of experience. Semiquantitative scoring for 7 types of gill changes was carried out for 10 filaments per gill (450 filaments total) over 3 rounds on anonymized samples. Scores were summarized for each type of gill change. The assumed clinical relevance for each change was transformed into a category score, followed by an assessment of agreement within (intra) and between (inter) raters. A generalized linear model estimated the difference in score levels between raters. For each rater, intra-rater agreement was high for 6 gill changes and moderate for 1 gill change. Inter-rater agreement was moderate to almost-perfect, except for 2 gill changes; generalized linear model regression revealed systematic differences in score usage between the raters. Our scoring protocol worked satisfactorily for mucous cell amount, lamellar clubbing, lamellar hypertrophy and/or hyperplasia, and aneurysms, despite different levels of expertise in histologic evaluation. Intra-rater agreement was consistent, but differences existed for interlamellar hypercellularity, lamellar inflammation, and degeneration. Scoring subclinical gill changes is a challenge, and our scoring system for mild-to-moderate lesions may enable early intervention to limit the detrimental effects of poor gill health in RAS farming.