Abstract
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.