The Salish sucker (Catostomus sp. cf. catostomus) is a federally Threatened species in Canada, inhabiting small lowland streams along with juvenile Coho salmon (Oncorhynchus kisutch) in southwestern British Columbia and adjacent Washington State. Experiments were conducted in two ponds to determine the effects of reduced flow on dissolved oxygen (DO), growth of Salish sucker and juvenile Coho salmon, and fish movement using Passive Integrated Transponder (PIT) tags. Sequential flow reduction in the first experiment resulted in a significant decrease in DO (9.1 to 3.1 mg·l-1) and Salish sucker growth (23% decline), although lower DO and growth were confounded with decreased temperature. Flow reduction in a second split-pond experiment including Coho salmon resulted in significantly lower DO (6.9 to 2.9 mg·l-1), a 51% reduction in growth of Coho salmon, but no detectable change in Salish sucker growth. PIT tag detections demonstrated increased use by both species of a well-oxygenated refuge in the flow reduction treatment, indicating sublethal effects of hypoxia on fish behaviour and habitat use. These experiments demonstrate the induction of severe hypoxia by reduced flows under eutrophy, with the potential for negative impacts on Salish sucker and juvenile Coho salmon growth and habitat use.

To understand the effects of widespread urbanization and agricultural impacts on recovery of Salish sucker, a federally threatened Catostomid endemic to the lower Fraser Valley of British Columbia, we assessed i) the current extent and effects of hypoxia on the distribution of Salish sucker and juvenile salmonids, ii) potential drivers of hypoxia, and iii) management options for hypoxia mitigation. Over 40% of sucker critical habitat experiences hypoxia (dissolved oxygen (DO) < 4 mg·l-I) by late summer, indicating widespread non-compliance with water quality guidelines. The strong positive relationship between seasonal hypoxia and temperature (R2= 0.53) and negative relationship with streamflow (R2= 0.78) indicates that hypoxia is driven by a synergy between low summer flows, elevated temperatures, and high primary production associated with nutrient enrichment (eutrophication). Sucker show strong selection against high water temperatures and weaker negative selection against low DO; juvenile salmonids show very strong selection against both high temperatures and low DO. Climate projections for declining summer flows and elevated temperatures indicate worsening trends in DO without intensive watershed-scale management to reduce nutrient loads.

Substantial, acute mortality was observed in wild-caught Chinook salmon (Oncorhynchus tshawytscha) of various ages and sizes, from sub-adults to returning adults, held in tanks during two holding studies carried out in August and September 2022 at the Bamfield Marine Sciences Centre (BMSC), in Bamfield, British Columbia, Canada. Within days of capture, a substantial number of fish began presenting lethargy, loss of balance, abnormal swimming behavior, and skin ulcers involving the caudal peduncle, fins, belly, trunk and mouth. Molecular testing revealed high levels of Tenacibaculum dicentrarchi in skin swabs and gills, without appreciable consistent detections of other infectious agents. T. dicentrarchi was also isolated from the skin ulcers. Histological analysis confirmed the presence of ulcerative dermatitis and myositis, associated with mats of filamentous, rod-shaped bacteria. In two individuals, the infection became systemic, with a bacterial colony (identified as T. dicentrarchi) observed in the liver of one individual. In-situ hybridization against T. dicentrarchi and T. maritimum confirmed the presence of only the former agent in the gills and skin ulcers of the affected fish. This clinical report represents the first diagnosed case of tenacibaculosis in wild-caught (captive) Chinook salmon in British Columbia.

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