genetic resistance to the disease. In the wild, diseases and parasites are normally at low levels, and kept in check by natural predation on weakened individuals. In crowded net pens, they can become epidemics. Diseases and parasites also transfer from farmed to wild salmon populations. A recent study in British Columbia links the spread of parasitic sea lice from river salmon farms to wild pink salmon in the same river. The European Commission (2002) concluded, "The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary." It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms. These predictions have been disputed by other scientists and recent harvests have indicated that the predictions were in error. In 2011, Scottish salmon farming introduced the use of farmed wrasse for the purpose of cleaning farmed salmon of ectoparasites. Globally, salmon production fell around 9% in 2015, in large part due to acute outbreaks of sea lice in Scotland and Norway. Lasers are used to reduce lice infections. In the mid 1980s to the 1990s, bacterial kidney disease (BKD) caused by Renibacterium salmoninarum heavily impacted Chinook hatcheries in Idaho. The disease causes granulomatous inflammation that can lead to abscesses in the liver, spleen, and kidney. Pollution and contaminants Salmonid farms are typically sited in marine ecosystems with good water quality, high water exchange rates, current speeds fast enough to prevent pollution of the bottom but slow enough to prevent pen damage, protection from major storms, reasonable water depth, and a reasonable distance from major infrastructure such as ports, processing plants, and logistical facilities such as airports. Logistical considerations are significant, and feed and maintenance labor must be transported to the facility and the product returned. Siting decisions are complicated by complex, politically driven permit problems in many countries that prevents optimal locations for the farms. In sites without adequate currents, heavy metals can accumulate on the benthos (seafloor) near the salmon farms, particularly copper and zinc. Contaminants are commonly found in the flesh of farmed and wild salmon. Health Canada in 2002 published measurements of PCBs, dioxins and furans and PDBEs in several varieties of fish. The farmed salmonids population had