Salish Sea Marine Survival Project

68 SUMMARY OF RESULTS TO DATE Over 50 infectious agents have been observed in juvenile salmon in the Salish Sea. Analytical models derived from 8-10 years of infectious agent monitoring, acoustic tracking and predation studies that relate agents with fate, and physiological assessments have been applied to reveal infectious agents showing the greatest impacts during downstream freshwater migration and in the Salish Sea. The most notable parasites include Myxobolus arcticus, Ichthyoptherius multifiliis, Ichthyophonus hoferii, Paranucleospora theridion, Loma and all three species of Parvicapsula. Notable bacteria include Tenacibaculum maratimus, Rickettsia-like organism and Flavobacterium psychrophilum. Notable viruses include infectious haemotopoetic necrosis virus (IHNV ), piscine orthoreo- virus (PRV ) and erythrocytic necrosis virus (ENV ). We are continuing to study the 15 novel viruses discovered in the SSHI (e.g., Figure 2). Linkages with fate vary by salmon species, season, age and environment. Some infectious agents correlated with marine survival come from freshwater, confirming earlier understanding that health and condition at the time of ocean entry can impact subsequent survival. This finding provides a potential means to enhance survival by carefully monitoring and optimising the health and condition of hatchery fish. Models relating infection probabilities with environmen- tal (e.g., temperature, sea surface salinity, depth sampled, distance from aquaculture, stock latitude) and intrinsic (origin—hatchery or wild, life-history type, stock) factors reveal that temperature in the Salish Sea is a major driver of infection (Figure 3). In fact, the majority of infectious agents are more prevalent in years of higher coastal marine temperatures. Stress challenge studies (mostly on adults) have also revealed many pathogens with effects enhanced under elevated temperatures. Pathogen impacts on survival may arise through direct effects, where fish die from disease, or indirect effects, where infection affects physiological performance and behaviour, enhancing risk of predation and susceptibility to other stressors. It is likely that the latter is the more important for wild fish, where even modest compromise in performance (especially swimming and visual) can lead to death. Tracking studies of Chilko Lake sockeye smolts identified linkages between IHNV and migratory survival. Subsequent study showed that smolts with IHNV detec- tion had 16-24 times greater odds of being consumed by bull trout. In the marine environment, Rhinoceros auklets preferentially consume smolts infected with Parvicapsula parasites. This research demonstrates linkages between infection state and risk of predation, and the potential role that predators may play in removing infected indi- viduals, thereby increasing the health of the populations. Figure 2. Investigating infectivity of CTV-2 (red), localised to the optic lobe of an Atlantic Salmon brain. Credit: Emiliano Di Cicco. After the discovery of HSMI (Heart and Skeletal Muscle Inflammation) on an Atlantic Salmon farm in BC, SSHI researchers began conducting further analysis on Pacific Salmon to determine if the virus, PRV, is associated with, and ultimately causative of, disease in Pacific Salmon. Analysis of Chinook Salmon from farm audit data suggested an association between PRV and jaundice/ anemia (jaundice syndrome), a disease highly similar to PRV-caused diseases in Rainbow Trout in Norway, and Coho Salmon in Japan and Chile, as well as a disease originally described in farmed Atlantic Salmon in eastern Canada (hemorrhagic kidney syndrome). In 2018, the SSHI provided evidence that the same virus (PRV ) is likely causing different expression of diseases in Atlantic and Chinook Salmon in BC, increasing the evidence that PRV transmission from farmed salmon poses a risk to wild Chinook Salmon. Genetic analyses currently being final- ized show PRV exchanges between aquaculture and wild salmon do occur on the BC coast. Moreover, research in 2019 on physiological associations with infective agents in wild-caught Chinook Salmon revealed the strongest linkages for PRV. PRV-infected Chinook showed both a powerful immune response and histological changes consistent with early development of jaundice/anemia. It is important to note that PRV is far more common, and perhaps more relevant, in Chinook Salmon on the west coast of Vancouver Island than in the Salish Sea, with disease manifestation (farmed and potentially wild) occurring over cool fall/winter periods.

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