Assessing the biological process of Hydrilla verticillata predation in a eutrophic pond using high-resolution acoustic imaging sonar

Abstract

Most Hydrilla verticillata disappeared soon after planting in unsuccessful restoration efforts carried out at Lake Izunuma (northeastern Japan). Furthermore, the disappearance of aquatic plants has been speculated to be a result of crayfish predation. However, the precise cause remains unclear because of the low transparency of the lake water. In this study, to identify the source of Hydrilla verticillata predation and characterize the potential behaviors of predators, we used an acoustic monitoring system based on an image-processing program that we developed. A field experiment was performed in a small eutrophic pond adjacent to Lake Izunuma, and 24 Hydrilla verticillata were planted on the pond bottom. A DIDSON unit was mounted on an original frame with a 3° concentrator lens. Data were collected at 1.8 MHz and at a maximum range of 3.0 m. All Hydrilla verticillata disappeared within a day of planting. Using the image tracking approach, we estimated the walking speeds of two potential predators, namely crayfish (Procambarus clarkii) and pond snails (Cipangopaludina japonica), and distinguished them accordingly. Using this method, we identified crayfish as the main source of predation and characterized their foraging behavior in terms of the decreasing amount of Hydrilla verticillata. The foraging behavior of crayfish mostly ended after 17 h of planting, possibly because all plants had been eaten by that time.