Sentinels in the bay: using farm fish to monitor for pathogens in the environment

Farmed Atlantic salmon were used in British Columbia as sentinels to determine whether fallowing farms and previously infected channels would effectively eliminate infectious haematopoietic necrosis (IHN) virus from a region. The sentinel approach involved re-introducing salmon to a few strategically placed (sentinel) sites and subsequent monitoring for the presence of IHN virus over a specified period of time. Seven farmed Atlantic salmon sites with a history of infection were monitored for the presence of the virus for six months following a minimum fallow period of two months. After re-stocking, the virus was not detected in the sentinel fish, suggesting that the concentration of the virus in the area was not sufficient to cause disease in a species known to be highly susceptible to the virus. Specifically, transmission of the IHN virus from one year class to another may be prevented by fallowing sites between year classes. Similar models using farmed fish as sentinels may be useful for answering other environmental questions. Thus, fish farms themselves could act as monitors for the general health condition of a bay, inlet, strait, or other body of water in which the farms are situated.