Characteristics of reaction field and the reactive distance of a planktivore, Pseudorasbora parva (Cyprinidae), in various environmental conditions

The characteristics of the reaction field and the reactive distance of the Stone moroko (Pseudorasbora parva) were studied under three environmental conditions (structural complexity, light intensity and turbidity) and three prey sizes. In optimal experimental conditions, under no structural complexity, light intensity of 200 lux and turbidity less than 1 NTU (Nephelometric Turbidity Units), the cross-section of the reaction field was found to be elliptic with a bearing angle larger than the elevation angle, but both angles changed slightly depending on environmental conditions. The reactive distance was large, and the fish frequently attacked prey that was located within 15–60 degrees to each side from the frontal direction of a fish (i.e., ± 15 degrees from the axis of the fish body) horizontally. In the light intensity below 50 lux or turbidity above 10 NTU, however, the attack frequency and the reactive distance in the frontal direction of a fish did not differ from other horizontal directions in the reaction field. The average reactive distance increased proportionally with increasing strand distance, but it gradually reached a constant value for strand distances greater than about 3.6 times the fish body length. The average reactive distance increased in the light intensity range of 10–200 lux and decreased negatively with turbidity increasing. The average reactive distance increased with larger prey size, but the rate of increase of the reactive distance gradually decreased.