Habitat selection and movement by spotted bass and shorthead redhorse downstream of a hydropeaking dam in Missouri
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Flow alteration caused by hydropeaking dams may disrupt the natural flow regime and impact physical habitat and biological community, thus threatening the health of freshwater ecosystems. Life history strategies of fish evolved with their environments, including mobility to access habitats for feeding, spawning, refuge, and rearing. However, behavioral responses among species to artificially extreme flow and the influence of spatial and temporal scale, are not clearly understood. We used radio telemetry (from April 2016 to June 2017) to determine the habitat selection and daily and seasonal movement of two native fishes downstream of a hydropeaking dam on the Osage River, Missouri, where river stage may fluctuate up to 5 m daily. We selected two fishes from different habitat and spawning guilds, Spotted Bass (Micropterus punctulatus) and Shorthead Redhorse (Moxostoma macrolepidotum), that are common throughout the Midwestern United States and Canada and represent diverse life history strategies of riverine fishes. We used a Bayesian discrete choice analysis to determine seasonal and flow-related habitat selection and linear regression to evaluate predictors of movement rate of fishes. We determined flow to be "steady" or "fluctuating" based on the range of discharge measured during the 24-hour period prior to a fish location, where the threshold corresponded to minimum or maximum daily discharge being within (steady) or exceeding (fluctuating) 30% of the mean daily flow. Fluctuating flow corresponded to increased movement rate of Spotted Bass and Shorthead Redhorse and selection of slower velocity (<0.5 m/s) nearshore habitat by Spotted Bass. Shorthead Redhorse did not select specific velocity during fluctuating flow, yet used fast velocities (>1 m/s) during steady flow. Both Spotted Bass and Shorthead Redhorse selected moderate depth (1.5 - 4.0 m) with submerged cover during both steady and fluctuating flow. Spotted Bass movement rate peaked when 3-day average discharge was 500 m3/s, which occurred during consecutive days of hydropeaking or flood management at the dam. This discharge occurred or was exceeded during 25% of the study period, primarily during spring and summer, but did not occur during winter. Smaller adult Spotted Bass had greater movement rates than larger bass, whereas increased movement rate of Shorthead Redhorse was related to increased barometric pressure and Julian day. Both Spotted Bass and Shorthead Redhorse movement rates were greatest during spring (10 - 23[degrees]C) and differed among seasons. The mean longitudinal dispersal by Spotted Bass and Shorthead Redhorse was approximately 20 and 30 rkm, respectively, although 50% and 60% of Spotted Bass and Shorthead Redhorse, respectively, made movements between 5 and 91 rkm from the tagging location. Spotted Bass and Shorthead Redhorse selected similar habitats among steady and fluctuating flow regimes, however, both species responded to flow disturbance by moving more during fluctuating flow than during steady flow, presumably to relocate to suitable habitat. These effects occurred on a short timescale (10 hours to 3 days) and should be could be considered when informing ecologically-sustainable river management in highly flow-altered systems.
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