Farmed fish are affected by environmental variability and farming practices, such as farming density, feed composition and feeding regime. We developed four feeding activity indices to assess how environmental variability, ration size and feeding time lags affect the feeding activity of farmed Salmo salar. We used a real-time video monitoring system to record eight 1800-m3 cages between November 2017 and August 2018. Each cage contained approximately 55,000 S. salar post-smolts (initial weight of 1253 g +/- 195). We found that ration size and UV radiation positively and negatively affected feeding activity indices, respectively.