Marine aquaculture, among the others fishing practices and shellfish farming, is also practiced across Europe. Traditional extensive fish farming in lagoons and coastal ponds is one of the most ancient aquaculture methods. Sea cages are widely used for salmon, sea bass, sea bream, and to a lesser extent, trout. The cages are large pocket-shaped nets where the water flows through freely. These systems are vulnerable to external influences (i.e. pollution events or physical impact) and can pose threats to the adjacent environment with fish farm effluent. Shellfish farming is based primarily on naturally occurring species, which are cultivated from spats and utilize the nutrients in the surrounding environment. The main shellfish farmed in Europe are oysters and mussels.