Tuna cages developed and spread over the world in a short period of time and replaced classical fishing as the major source of tuna in the market. As one of the most profitable and globaly spred branches of aquaculture, it's impact on the environment is being increasingly called attention to. Even though no toxic substences are used to maintain fish cages, they produce large amounts of organic waste in the form of fish excrement and food leftovers, thus leading to a negative environmental impact. The areas around cages are generally characterised by increased levels of bioproduction and reduced concentrations of oxygen, an impact of fish cages that can be felt up to a kilometer away. Elevated concentrations of nitrogen and carbon have also been found in the water. Increased amounts of phophorus and sulfate ions, as a result of bacterial function in anaerobic conditions, are often found in the sediment. Other than the sediment, anaerobic conditions can occur in the sea in the case of currents being week or non-existing and the organic load being high enough, which can cause great environmental dammage. This sort of changes in the environment have a strong impact on the local biological community, changing it's composition and sometimes even erasing certain species altogether. One of the most endangered ecosystems are ones that depend on primary production of posidonia (Posidonia organica) which is extreemly sensitive to changes in the sendiment and reacts badly to them even at a distance of 400 meters from a fish cage. The retreat of posidonia means that a number of small fish and arthropods living in the plains of posidonia will have lost their source of food and shelter.