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Diel movement of smallmouth yellowfish Labeobarbus aeneus in the Vaal River, South Africa

Diel movements of Orange–Vaal smallmouth yellowfish Labeobarbus aeneus (Burchell, 1822) in the Vaal River, South Africa, were determined by externally attaching radio transmitters to 11 adult fish and manually tracking them between March and May 2012. Twenty-four radio telemetry monitoring surveys produced 2 304 diel tracks. At night, yellowfish displayed a preference for slow shallow (<0.3 m s−1, <0.5 m) and fast shallow habitats (>0.3 m s−1, <0.3 m), whereas by day they avoided these habitats, preferring fast deep areas (>0.3 m s−1, >0.3 m). The average total distance of 272 m moved per 24-hour period was three times greater than the diel range, and the average maximum displacement per minute was significantly higher in daytime (4 m) than at night (1.5 m). These findings suggest that L. aeneus is active primarily during the day in fast-flowing, deeper waters, and relatively inactive at night, when it occupies shallower habitats. This behaviour should be further explored to identify causal mechanisms underlying the diel habitat shifts in this species such as water temperature, foraging tactics and/or predator avoidance.