Sinusoidal cycling swimming pattern of reservoir fishes

Abstract

A circular split‐beam transducer (7°, nominal angle), fixed on the bottom close to the deepest point (c. 36 m) of the Rimov Reservoir and beaming up towards the surface showed that most fishes stayed in the epilimnion during the summer. The majority (83%) of larger fish individuals (136–359 mm Ls; maximum target strength, TSmax, ranged from −42.2 to −34.1 dB) in open water performed very characteristic sinusoidal movements in the vertical plane during July and August. As they crossed the sonar beam (3–4 m wide), the fishes changed their depth several times (frequency 1.3–10.1 cycles min−1) with amplitude of 19—321 cm. The trajectory of fish vertical oscillations was in many cases greater than the trajectory of their horizontal movement. Fishes started to swim up‐and‐down after sunrise and continued doing so during daytime. The sinusoidal movement pattern was replaced by direct movement before sunset. A number of descriptive parameters of up‐and‐down movement is proposed and their ranges are given. Variation of target strength indicates active changes of fish tilt (30° on average) in transient phases of the sinusoidal cycle rather than changes of swim bladder volume. Up‐and‐down fish swimming is likely to be an efficient way of visual inspecting a larger volume of the epilimnion for prey, mainly large zooplankton (Daphnia, Leptodora), whose epilimnetic density in the Rimov Reservoir is low and patchy in summer time.