EVALUATION OF THE IMPACT OF FISH NEST SIZE ON THE DISCHARGE RATE OF FILAMENTOUS ALGAE

Abstract

 A method was developed to analyze the scale of artificial fish nests that prevents filamentous algae, which detach from the canal bed and flow downstream (detached algae), from accumulating in rectangular artificial fish nests. Previously, it has been reported that a large amount of sediment, including detached algae, accumulating within fish nests can reduce the living space for fish, thereby impairing the effectiveness of the habitats. In this study, existing fish nests in the agricultural drainage canal in Ibaraki Prefecture were the focus, and numerical computations of three-dimensional flow and the three-dimensional behavior of cylindrical detached algae were conducted using iRIC NaysCUBE. Five different scales for the fish nests were set, and the "discharge rate," which is the percentage of detached algae that flows out of the nest due to water flow, was calculated. The results showed that when the entrance width and depth of the fish nest are reduced beyond a certain threshold from their current dimensions, the discharge rate of detached algae decreases.