Sinusoidal swimming in fishes: the role of season, fish length and solar radiation

The sinusoidal swimming of fish, the swim-search feeding behaviour (Cech & Kubecka 2002, Journal of Fish Biology 61, 456-471), was studied with respect to season, fish length and solar radiation in Římov Reservoir, Czech Republic, using a bottom-mounted, split-beam transducer (7^o, nominal angle; frequency 120 kHz). The proportion of sinusoidally swimming fish increased from April to August while this behaviour was absent in October. The occurrence of sinusoidal swimming showed an apparent pattern throughout the day; it increased sharply around sunrise, was highest within 5-6 hours around solar noon and sharply decreased around sunset. Significantly less frequent occurrence of sinusoidal swimming was recorded during cloudy days compared to sunny days. The vast majority of records came from fish of standard length ranging from 100 to 400 mm, which represents the typical size range of common bream Abramis brama and roach Rutilus rutilus of age >1+, the main zooplanktivores in the reservoir. The presence of larger fish in the open water of the reservoir, as well as the presence of sinusoidal swimming, apparently correlates with the presence of larger zooplankton (Daphnia, Leptodora, Cyclops vicinus) in the epilimnion.