Quality downgrading of fish with quality defects is a considerable cause for economic losses to farmers due to reduced marketability and sales prices, as well as increased processing costs. Several causes for quality downgrading can be linked to adverse welfare states during fish rearing and should therefore be considered as welfare indicators. In this study, we linked the production records of 24 sea cage sites to slaughter records from a processing plant in northern Norway. We quantified the prevalence of proximate causes for downgrading and used ordination and regression-based approaches to model the variation in quality traits due to selected production related drivers. The most important causes for downgrading were ulcers (39 % of downgraded fish), dark spots (17 %), deformities (12 %), and early maturation (10 %). The presence of ulcers was by far the most severe cause for downgrading. Important drivers for overall quality reduction were growth during the seawater stage, seawater temperature (mean and standard deviation), certain cause specific mortality counts, and day of harvest. Ulcers were linked to low seawater temperatures during production.
Our study demonstrates that proximate causes for quality downgrading at slaughter are relevant retrospective welfare performance indicators and provides an overview of how production related drivers can affect slaughter quality throughout the year. It highlights the link between improved welfare and improved production outcomes, as well as the utility of systematic data collection and analysis in aquaculture production.