Streamflow Reductions and Habitat Drying Affect Growth, Survival, and Recruitment of Brassy Minnow across a Great Plains Riverscape

Abstract

Flow alterations caused by reservoir storage, groundwater pumping, diversions, and drought are widespread in North American Great Plains streams and have altered and fragmented habitats and reduced native fish biodiversity. Early life stages of fish are particularly sensitive to altered flow regimes, and reduced growth and survival may negatively affect the persistence of native species and assemblages. We investigated how growth and survival of brassy minnow Hybognathus hankinsoni larvae in the Arikaree River, Colorado, varied among three 6.4-km river segments that differed in hydrology and how climate influenced drying rates of spawning and rearing habitats in these segments over 3 years. We found that brassy minnow spawned in backwater habitats within a discrete period from mid-April to late May, based on otolith increment analysis. The timing of spawning and growth of larvae were influenced by climate and the hydrologic context of the river segment. Brassy minnow spawned 2 weeks earlier under warm, dry conditions in 1 year, and both growth rates and survival were significantly lower than during two wetter years (growth: 0.25 mm/d versus 0.30 and 0.41 mm/d; survival: 0.8391/d versus 0.894 and 0.897/d). For cohorts of larvae in individual backwaters, survival was higher in spawning habitats that were larger and that dried more slowly, and among cohorts that hatched in the middle of the spawning period under a moderate thermal regime. Overall, we found that brassy minnow spawning and recruitment were strongly influenced by habitat drying driven by interactions among stream geomorphology, groundwater pumping, and climate across multiple spatial scales. We suggest that conservation efforts explicitly consider the adaptations of this fish to harsh environments and focus on providing flows to maintain the spawning, rearing, and refuge habitats that are critical to brassy minnow population persistence.