Opercular Shortening and Deformities in Farmed Atlantic Salmon ( <i>Salmo salar</i> ) and Rainbow Trout ( <i>Oncorhynchus mykiss</i> )—A Qualitative Study

In the Norwegian aquaculture industry, opercular shortening has been ranked as an important cause of increased mortality, reduced welfare and growth. The aim of this study was to gather experience-based knowledge from fish health personnel and production staff involved in the commercial production of farmed Atlantic salmon and rainbow trout. A total of 29 semi-structured interviews with a predefined list of questions on the topic of opercular shortening were conducted in the period 22.04.24 to 10.12.24. Results showed that participants reported aggressive behaviour due to underfeeding, with subsequent nipping of gill lids as the main cause of shortened opercula in farmed salmonids. Inadequate feed allocation of fry was perceived to be the most important risk factor for aggression, in addition to starving fry. The prevalence of opercular shortening in the freshwater phase was reported to be between 0% and 70%. Opercular shortening was typically first detected in fry between 2 and 5 g, but sometimes already during first feeding (0.5 g) and in alevins. Damaged gill tissue, reduced growth rates and increased mortality were suggested as consequences of shortened opercula. Sufficient feed allocation in fry stages, optimization of environmental conditions, reducing fish density and culling of small fry were reported as preventive measures, while culling affected fish was the sole mitigating measure.