The growing body of research is not boding well for net pen salmon farms. Just a matter of time and more research to push these net pens onto the land where their negative impacts can be better managed!
Given that
P. orthoreovirus Strain PRV-1 causes HSMI in farmed Atlantic salmon, and likely causes jaundice/anemia in farmed chinook salmon, the absence of consistent sequence variation within PRV-1 in diseased Atlantic and chinook salmon
implies a risk of transmission of the virus from farmed salmon to wild Pacific salmon.As Atlantic salmon accounts for 97% of the total biomass of farmed salmon in BC, answering the question of whether the virus causing HSMI in Atlantic salmon causes disease in Pacific salmon is critical when assessing risk. PRV-1 occurred in 65%–75% of farm audit samples of Atlantic and chinook salmon, with approximately 25% of the salmon having high viral loads. In farms where HSMI disease ensues, BC Atlantic salmon show a morbidity rate (i.e., presence of histological lesions) of >80%, similarly to that reported in Norway (
Kongtorp et al. 2004a;
Kongtorp et al. 2006), and retain high loads of the virus and evidence of inflammatory lesions for a prolonged period of time (over 11 months in the study by
Di Cicco et al. 2017).
Hence, the environmental footprint and potential for viral transmission to wild salmon from disease-impacted farms is likely high. Although neither disease has yet been identified in wild fish to date, given the high sensitivity of the molecular viral disease development tool to early developing disease states, and of ISH to localize the virus across tissues, we can now apply these approaches to empirically determine whether any evidence exists that PRV-associated diseases may be occurring in migratory salmon in BC.
HSMI is one of the most common infectious diseases of Norwegian farmed salmon (
Garseth et al. 2017).
Moreover, disease modeling has identified that fish farming intensity in a region is a major risk factor for HSMI outbreaks (
Kristoffersen et al. 2013;
Morton et al. 2017)
suggesting that water-borne transmission may be an important factor in the spread of disease (
Hjeltnes et al. 2016).
These data also suggest that farming intensity could enhance the exposure of migrating smolts to PRV (
Garseth et al. 2017).
Escaped farmed salmon, which are most often infected with PRV, could also be a transmission vector for freshwater infections in wild fish if they enter rivers (
Madhun et al. 2015;
Hjeltnes et al. 2016,
2017).
Importantly, there have been recent reports of HSMI outbreaks occurring in freshwater hatcheries in Norway, which represents a shift in what was considered until recently a disease restricted to sea net pens (
Hjeltnes et al. 2016,
2017). There is also some indication that PRV infection in freshwater may be associated with earlier and more significant HSMI outbreaks in sea net pens and, as a result, some farmers are attempting to produce PRV-free smolts (E. Rimstad, personal communication, 2017;
Hjeltnes et al. 2017).
Prevalence of PRV is low (<3%) in wild migrating BC chinook and sockeye salmon (Oncorhynchus nerka (Walbaum, 1792)) smolts that have not been exposed to salmon farms (
Siah et al. 2015;
Morton et al. 2017;
Purcell et al. 2018;
Tucker et al. 2018).
Higher prevalence is observed in fall, especially in chinook salmon on the west coast of Vancouver Island that remain resident in the same bays and inlets occupied by salmon farms for prolonged periods of time (up to a year) (
Tucker et al. 2011,
2012). An epidemiological study is currently underway to assess the role of salmon farms in the transmission of PRV to wild stocks, based on both patterns of prevalence distributions and full genome sequencing of the virus across farmed and wild populations over multiple years and geographic locations.