Classification of Post-Delousing Mortality in Farmed Atlantic Salmon: A Case Study of Standardised Causal Classification at Fish-Level.

The study investigated cause-specific, fish-level mortality of farmed Atlantic salmon following mechanical delousing. We visited three populations at two marine sites belonging to one company at four different time points, from 1 day before to 13 days after the mechanical delousing. A total of 453 dead fish were collected and necropsied during the four visits. The underlying mortality cause was determined by macroscopic pathology in fish that died peracutely, acutely and subacutely after the procedure. The macroscopic pathological changes were recorded, and a cause of death was assigned to each fish by applying a uniform code system to classify mortality and losses in farmed Atlantic salmon. The findings revealed that mechanical delousing procedures can lead to mortality and pathological changes up to 13 days after the procedure. We identified cerebral haemorrhages in approximately 25% of the necropsied fish that died peracutely. Comparing the cause-specific mortality assignments at necropsy to daily mortality recordings by farmers showed that farmers assigned a smaller proportion of the fish to injury/trauma than the necropsy indicated. This implies that current practices for recording mortality causes by farmers after delousing may overlook the long-term negative effects of the procedure. Therefore, guidelines for farmers to more accurately record mortality causes following delousing are necessary to improve the value and precision of these records.