Particulate waste outflow from fish-farming cages. How much is uneaten feed?

•Particulate waste sampler was designed to study solid wastes exiting from fish cages.•Wasted feed fraction was measured combining in situ measurements and modelling.•Particulate wastes flow out all around the cages, and mainly through the bottom.•Wasted feed is not predictable and strongly depends on feeding practices.

Particulate wastes drive benthic organic enrichment from cage fish farming. Differentiation between faeces and uneaten feed estimates at cage level are of great value to both economize the feeding process and reduce waste. This study estimates the particulate waste outflowing cages at different depths and orientations, and the wasted feed component by combining in situ measurements and modelling. Particulate matter flux (PMF) was greater vertically through the cage bottoms (60.89%), but lateral outflow was also substantial (39.11%). PMF occurs all around the cages, and the influence of the mainstream current was low. Wasted feed was greatly variable, reaching high values (about 50% of supplied feed. The self-application of feed wastage monitoring and estimates by fish farmers is recommended to improve sustainability.