Atlantic Salmon (Salmo salar) and Chinook Salmon (Oncorhynchus tshawytscha) develop a severe liver disease called net-pen liver disease (NPLD), which is characterized by hepatic lesions that include megalocytosis and loss of gross liver structure. Based on studies where salmonids have been exposed to microcystin (MC) via intraperitoneal injection, NPLD is believed to be caused by MC exposure, a hepatotoxin produced by cyanobacteria. Despite the link between MC and NPLD, it remains uncertain if environmentally relevant MC exposure is responsible for NPLD. To determine if we could produce histopathology consistent with NPLD, we compared the response of Atlantic and Chinook Salmon sub-lethal MC exposure. Salmon were orally gavaged with saline or MC containing algal paste and sampled over 2 weeks post-exposure. Liver lesions appeared by 6 h but were resolved 2-weeks post-exposure; histopathological changes observed in other tissues were not as widespread, nor was their severity as great as those in the liver. There was no evidence for NPLD due to the absence of hepatic megalocytosis. These results indicate that the development of NPLD is not due to acute MC exposure but may be associated with higher MC concentration occurring in food, long-term exposure through drinking of contaminated seawater and/or interactions with other marine toxins.

Piscine orthoreovirus genotype 1 (PRV-1) is an endemic virus to the Northeastern Pacific that infects both wild and farmed salmon. To better understand PRV-1 prevalence and transmission pathways in the region, we targeted out-migrating juvenile Pacific Salmon in the Strait of Georgia and Johnstone Strait in British Columbia, Canada, for PRV-1 molecular screening with an emphasis on Chinook (Oncorhynchus tshawytscha) and Coho (Oncorhynchus kisutch) salmon. A total of 4031 individuals were screened for PRV-1 and we identified an overall virus prevalence of 3.5% from 2011 to 2020. PRV-1 was absent in enhancement facilities and freshwater catchments and predominately found in the marine environment. The detection of PRV-1 varied greatly between species, year and stock of origin, but cumulatively identified that life history and migratory behaviors likely influenced viral prevalence. Specifically, Chinook salmon, which demonstrate long residence time in the Strait of Georgia relative to other species, had the highest PRV-1 prevalence in this study (7.4%). Varying stock composition and regional detection between year classes within the Strait of Georgia further indicated that the sources for Chinook infection were highly variable between years. These findings enhance our understanding for PRV-1 transmission in the region and more generally between/within salmon species.

Harmful algal blooms (HABs) in coastal British Columbia (BC), Canada, negatively impact the salmon aquaculture industry. One disease of interest to salmon aquaculture is Net Pen Liver Disease (NPLD), which induces severe liver damage and is believed to be caused by the exposure to microcystins (MCs). To address the lack of information about algal toxins in BC marine environments and the risk they pose, this study investigated the presence of MCs and other toxins at aquaculture sites. Sampling was carried out using discrete water samples and Solid Phase Adsorption Toxin Tracking (SPATT) samplers from 2017–2019. All 283 SPATT samples and all 81 water samples tested positive for MCs. Testing for okadaic acid (OA) and domoic acid (DA) occurred in 66 and 43 samples, respectively, and all samples were positive for the toxin tested. Testing for dinophysistoxin-1 (DTX-1) (20 samples), pectenotoxin-2 (PTX-2) (20 samples), and yessotoxin (YTX) (17 samples) revealed that all samples were positive for the tested toxins. This study revealed the presence of multiple co-occurring toxins in BC’s coastal waters and the levels detected in this study were below the regulatory limits for health and recreational use. This study expands our limited knowledge of algal toxins in coastal BC and shows that further studies are needed to understand the risks they pose to marine fisheries and ecosystems.