Collector 1 for waste at a bottom of a net cage (2, fig 1), having an upper face designed to collect the waste and connected to a pump for removal of the waste. A balloon 6 is fastened to the upper face of the collector. The balloon 6 is connected to a hose 8 for supply and removal of a fluid having a density of less than water. The collector is positioned at the bottom of the net cage by utilising control lines in conjunction with supplying and removing fluid, in particular air, from the balloon in order to control the position of the collector
