Sudden increased mortality in large seemingly healthy farmed Atlantic salmon (Salmo salar L.) was associated with environmental and dietary changes

Abstract

Mortality of seemingly healthy farmed Atlantic salmon is a large problem in Norwegian aquaculture, and has been linked to infectious and noninfectious cardiovascular diseases. In this study, an event of sudden mortality of seemingly healthy farmed salmon during the winter period in northern Norway is reported. The experimental fish reared in net-pens were fed two dietary treatments; control and test experimental diets in duplicates. An increased mortality of 6% and 10% was only observed within the two net-pens receiving the test diets. The moribund fish had significantly higher lipid content in the liver, altered liver fatty acid composition, and increased levels of alanine aminotransferase and alkaline phosphatase in the blood plasma compared to non-dying fish. Instant and significant reduction in mortality was observed when the fish fed the test diet were starved. The observed mortality was associated with dietary and environmental changes. Possible mechanism for the increased mortality is discussed.