The invention relates to a collection device for collecting sludge such as sunken particulate waste and dead fish from an aquaculture cultivation unit such as a fish cage positioned in the sea. The collection device comprises a collection head at the bottom of the cage for collection of sunken sludge and sunken farmed organisms, the collection head comprises a funnel formed top plate wherein the sunken sludge and fish are collected in or in vicinity to the lowest point of the funnel. It further comprises a first removal system for sunken sludge, having a first inlet for sludge and small particles at the lowest point of the funnel-formed top plate, the first inlet being provided with a retaining system for holding back the farmed fish, and in that the first removal system further comprises a first pumping system for removal of said sunken sludge from the first inlet. Dead fish accumulated on the top plate are removed by a second removal system and a second pumping system.
