The HIGH and MIDDLE accumulation systems contained 4 tanks of 450 L (flow rate: 23 L/min), with high densities group-housed fish. The group-housed fish were used only as a possible source of fish-related GIF. As it is not known if fish-related substances that accumulate in RAS are produced by fish of a certain size class, 3 size classes were used in the group-housed fish (Table 2). These fish were fed restrictively using automatic feeding belts. The feed used was a commercial Tilapia feed (3mm floating pellets; 44% crude protein, 10% fat, 25% carbohydrates,
11.5% ash; Skretting, France).