Dissolved Oxygen and Salish Sucker System Capacity

The effects of DO on system capacity was mainly inferred based on a meta-analysis examining the effect of Dissolved Oxygen (DO) concentration on specific growth rate of fish, which evaluated data from 38 studies on 30 freshwater and marine species (Rosenfeld and Lee 2022). The shape of the curve and threshold value (5.1 mg/L) is based on segmented regression of specific growth rate as a function of dissolved oxygen averaged across 30 species. The inference implicit in using growth as the response variable is that threshold effects of low DO on individual growth will be similar to population-level effects. Although the average segmented regression across all species is highly significant and the threshold or 5.1 mg/L is consistent with previous analyses and regulatory guidelines for water quality, there remains great inter-specific variation around the mean response. Salish sucker is not a cold-water species and therefore, less sensitive to hypoxia compared to salmonids (Rosenfeld et al. 2021; Zinn et al. 2021). Based on observation and experience, and the assumption that Salish sucker are selected for some degree of hypoxia tolerance because they have evolved in small streams subject to seasonal drying and low flows (and also prefer deep pool or wetland habitat that is subject to intermittent hypoxia), the threshold DO for sublethal effects on Salish sucker has been set at 4 mg/L (Fisheries and Oceans Canada 2019). We set the x-intercept at 0.5 mg/L to reflect the higher known tolerance of Salish sucker compared to salmonids (x-intercept ≈ 3 mg/L) and the average across all species (0.8 mg/l DO; Rosenfeld et al. 2021, Rosenfeld and Lee 2022). The habitat suitability index model for Longnose Sucker (Edwards, 1983) generally supports the shape of the function, but has a higher threshold (6 mg/L) and x-intercept (4.5 mg/L), reflecting the fact that Longnose sucker (the sister-species to Salish sucker) are a cool-water species.