 Other polluting processes include the use of chemicals on the farms, which leach into the surrounding environment.These include anti-foulants to keep the cages free from colonial algae and animals, and therapeutants to deal with disease and parasitism.Toxicants, such as mercury and PCBs (polychlorinated biphenyls), can build up over time, particularly through the tuna feed, with some evidence of contaminants being more elevated in farmed fish than in wild stocks.