Salmon farms often confine hundreds of thousands of fish in highly restrictive net pens for all of their adult life. Salmon farms, consequently, are ideal incubators for parasites and infectious diseases that are then spread to adjacent farms and to wild fish. These outbreaks are impossible to quarantine; mass escapes from salmon farms and the normal flow of tides and currents spread diseases and parasites to other fish over very wide areas.
Sea lice chew on salmon, creating open lesions that weaken their ability to maintain a healthy salt-to-water balance. A recent study found that sea lice concentrations at one Canadian salmon farm were 30,000 times higher than normal. Lice are dispersed around a farm at concentrations 73 times higher than normally occurring levels. They spread easily to migrating juvenile wild salmon.
One of the most common diseases in large-scale fish farming is infectious salmon anaemia (ISA). In salmon farms where this occurs, death rates may approach 100 percent. Source: Pure Salmon Campaign, photograph: Bill Keay