Behaviour of tigerfish Hydrocynus brevis in the River Niger, Mali, as revealed by simultaneous telemetry of activity and swimming depth

The ecology and behaviour of tigerfishes (Hydrocynus spp., Alestiidae) remains largely unknown despite these piscivores play a crucial role in tropical ecosystems. A 44-cm tigerfish H. brevis was equipped with two transmitters using accelerometer and depth sensors, the signals of which were simultaneously recorded while tracking the fish over four 24-h cycles during February and March 2001 in the River Niger near Bamako, Mali. This methodology permitted discriminating between different behaviours, including hunting sequences. Over 1 month, the tigerfish occupied an area of no more than 3 ha over one third of the river width, and showed consistent fidelity to the same activity core ( circa 0.1 ha), which consisted in a calm (< 10 cm s(-1)) glide with depth ranging from 60 to 100 cm, and comprising many large immersed rocks. Activity was essentially diurnal, with no hunting sequence observed at light intensities < 0.01 Lux. It decreased under gusting winds or when boat traffic was intense and forced the fish to deeper ( c. 1.5 m) waters. These preliminary results support the idea that H. brevis is essentially a diurnal, visually orientated predator with strong sedentary habits during the mid-dry season, and that its predatory activity might be compromised by the intensification of boat traffic.