Dissolved Oxygen and Nooksack Dace

The effects of DO on system capacity was 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, it should be noted that there remains great inter-specific variation around the mean response. Some species are relatively insensitive to hypoxia, particularly warmwater species, and others (particularly coolwater species and salmonids) showing higher sensitivity (see Fig. 1 below from Rosenfeld and Lee 2022).

We initially assumed that Nooksack dace would be relatively sensitive to low oxygen, as they are a riffle-dwelling species that occupy turbulent and well-oxygenated habitat. Consequently, our initial inference was to assign then the same sensitivity to reduced oxygen as salmonids, which have a relatively steeper decline in growth with reduced DO than the average for all species (Rosenfeld and Lee 2022, see Fig. 1b below). However, during sampling for fish in Aug. 2024 we made an observation which suggests that they are not as sensitive as salmonids. During a fish salvage in Pepin Creek, we placed 2 Nooksack dace in a 4l bucket along with one juvenile coho and 3 juvenile cutthroat trout (young-of-the-year). It was a warm day, and we inadvertently forgot about the bucket which was placed on the stream bank. When we checked up on it after an hour, the three yoy trout had asphyxiated, but the coho and the 2 dace were fine, and showed no sign of harm when held in the creek in a flow-through bin before release. This indicates that Nooksack dace are likely more tolerant of hypoxia than trout, despite being adapted to fast-water habitat, and we therefore used the average growth reduction vs. dissolved oxygen regression in Rosenfeld and Lee (2022; see their Fig. 1a below) as the stressor-response function for dissolved oxygen.