UBC Theses and Dissertations
Determining the impacts of hydrological drought on endangered Nooksack dace (Rhinichthys cataractae) at the population and individual level : Implications for minimum environmental flow requirements Avery-Gomm, Stephanie
Understanding the impacts of hydrological drought is crucial to the conservation of freshwater fishes. In British Columbia, Nooksack dace (Cyprinidae: Rhinichthys cataractae) are an endangered riffle specialist and are threatened by extremely low summer flows. The purpose of this thesis was to explore the impacts of drought on Nooksack dace, whether pool habitats may act as refugia to mitigate these impacts, and to define minimum environmental flow requirements. The first two objectives were addressed using a combination of field survey and experimental manipulations. A reduction in Nooksack dace population size with declining summer flow in Bertrand Creek, and a marked decrease in growth at low discharge in experimental riffles, indicated that low discharge has negative impacts on dace at both population and individual levels. Pool habitats were found to play a minor role in mitigating the negative impacts of hydrological drought (e.g., decreased growth rate), save as a refuge from stranding when riffles dewater. The third objective was addressed using the Instream Flow Incremental Methodology (IFIM). Because this study involved an endangered species an emphasis was placed on evaluating two fundamental assumptions of the methodology. Experimental results for Nooksack dace growth at different depths and velocities provided support for the first assumption, that density-based Habitat Suitability Curves (HSCs) accurately reflect habitat quality, but only for the lower limits of the HSCs. Next, a significant positive relationship between Weighted Usable Area (WUA) and dace biomass was found, supporting the assumption that such a relationship exists. However, this relationship was weak indicating a high degree of uncertainty in how Nooksack dace biomass will respond at high discharges. The IFIM model predicted that habitat availability for Nooksack dace begins to decline most rapidly at discharges of 0.12 m³.s-¹. As there is low confidence in upper ranges of the HSCs this low flow threshold may underestimate declines with discharge, and therefore protection of at least 0.12 m³.s-¹ is considered necessary for the persistence of Nooksack dace individuals and populations. Compared to conventional instream flow criteria, 0.12 m³.s-¹ represents ~10% mean annual discharge which is the threshold for severely degraded habitat.
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