Percent Riffle in Reach and Nooksack Dace

The shape and threshold for the final curve is based on Pearson et al. (2007) and Pearson (2004), which shows a large decline in percent reaches occupied by Nooksack Dace when the percent riffle in a reach in below 5-10% (Figure 1). The definition of Critical Habitat for Nooksack Dace includes reaches that are greater than 10% riffle, because reaches with less than 10% riffle tend to have lower riffle occupancy, presumably because the less hospitable habitat between riffles (i.e., low suitability pool habitat) reduces the likelihood of the overall reach supporting a local dace population. Occupancy drops noticeably at 5% riffle, and then approaches zero occupancy at 0% riffle (see Fig. 2 below). However, the logic that reaches above 10% riffle have equivalent habitat capacity - implicit in the plateau at a habitat suitability of 1 in the Edwards et al. (1983) habitat suitability curve (Fig. 2), and Pearson 2007 (Fig. 1) is flawed. Logically, the abundance of dace should be directly proportional to riffle area in a reach (i.e. % riffle), all else being equal. This means that there should be a linear relationships between % riffle and habitat capacity, i.e. a direct 1:1 relationship, rather than the direct implication of the Edwards et al. line that dace abundance is equal in reaches ranging from 25-80% riffle, which is somewhat nonsensical.

The SR function has therefore been altered so that habitat capacity (relative dace abundance) is zero at 0% riffle, increases steeply from 5% to 10% riffle at the reach scale, and then at a reduced rate at higher riffle abundance until it plateaus at 100% capacity at 30% riffle. 100% capacity (the plateau breakpoint) was not set at 80% or 100% riffle because the low gradient of most Nooksack Dace reaches (aside from in the Brunette River) means that achieving more than 25-30% riffle would be geomorphically impossible. In addition, the highest reference densities of dace occur in Bertrand Creek which has a maximum % riffle in the range of 30%; setting 100% capacity at 30% riffle therefor ensures that current maximum capacity will be achieved in the reaches with maximum possible % riffle. This is broadly realistic, because some reaches in Bertrand creek with ~15% riffle currently support healthy populations of dace, and the steep slope between 5-15% riffle will allow for rapidly increased carrying capacity over this range. While the curve allows for increase capacity at a higher proportion of riffle habitat up to 30%, the incremental benefits are likely lower. NOTE, however, that this SR function only applies to "Joe" model applications with relative habitat capacity where the reference state is set at 100% capacity (in this case, corresponding to 30% riffle). If dace abundance is being directly estimated based on the product of maximum density and wetted area of the reach, then the relationship between % riffle habitat and habitat capacity (or population size) should be treated as linear in direct calculation of population abundance.