Predatory behavior of the backswimmer (Anisops ogasawarensis) on larval medaka (Oryzias latipes)

To establish ecologically and environmentally safe countermeasures to prevent the predation of aquatic insects on larval fish in aquaculture systems, the predatory behavior of a backswimmer (Anisops ogasawarensis) on larval Medaka (Oryzias latipes) was observed under laboratory conditions. Observations were conducted to describe the vertical distribution and predatory behaviors of A. ogasawarensis. Additionally, a predation experiment evaluated the effect of various light levels (0, 3, 30, 300, and 3000 lx) on predation. Both the vertical distribution of A. ogasawarensis and the occurrences of predatory behavior was the highest near the water surface. A. ogasawarensis showed ambush predatory behavior and initiated chases when the larval O. latipes were within 5 cm of the predator. In the experiment, the predation rate increased with an increase in light intensity from 0 to 300 lx and decreased at 3000 lx. The present study indicates that predation by A. ogasawarensis on larval O. latipes changes under conditions of visual obstruction for the larvae such as decreasing of light intensity in the fish pond. Encounters with predators during the seed production may be higher in shallower.