The 2017 assessment described the status and trends of external fish disease in common dab at monitoring stations in waters around the UK. A fish disease index (FDI) that combines observations of disease incidence and severity, disease weighting scores (raw FDI), and adjusts the output to reflect factors including sex, size and time of year (standardised FDI) was calculated at an individual level. This was combined to assess each monitoring station at a single time point and further synthesised to assess status at the biogeographic regional level for each sampling year.