Assessing the impacts of habitat type and low instream flow on growth of an endangered stream-dwelling fish, Nooksack Dace (Rhinichthys cataractae)

Background/Question/Methods

Density based habitat associations are often the basis for species habitat protection and identification of minimum instream flow requirements of fish. However, the effects of low flow and alternative habitat use (i.e. pools versus riffles) are rarely quantified. Nooksack dace (Rhinichthys cataractae) are small benthic riffle-dwelling fish, endemic to northwest Washington, USA and south western British Columbia, Canada. Due to significant population declines associated with degradation of riffle habitat Nooksack Dace are listed as endangered in Canada under the Species at Risk Act. In addition to habitat destruction, recovery of this species is limited by extremely low discharge in summer months. As riffles dry up, these benthic fish may switch to using alternative pool habitat, and doing so is hypothesized to negatively impact fitness. Using four parallel in-ground experimental channels, I tested the hypothesis that low instream flow and forced use of pool habitat will have negative impacts on Nooksack Dace growth rate (a proxy for fitness).

Results/Conclusions

We found that Nooksack Dace growth rate was significantly lower in pools than in riffles at high flow, but at low flows growth was reduced regardless of habitat type. This supports our hypothesis that low instream flow, and isolation of these endangered fish in pool habitats at low flows, is detrimental to recovery efforts.