Mitigation of Anoxia and Hypoxia in Enclosed Shallow Fishing Grounds Using a Water Jet Generation Device

Abstract

Intrusion of anoxic and hypoxic waters into shallow fishing grounds often leads to damage to fishery. In the Sanbanze shallow water area at the head of Tokyo Bay, Japan, a fishery union collaborating with local governments performed a field experiment to consider a mitigation measure against intrusion of anoxic waters caused by blue tides (upwelling of anoxic water) into shallow fishing grounds, using a water-jet generation device. In order to evaluate the measure, numerical hindcasting of hydrodynamic and water quality processes and numerical experiments on the measure, considering the difference in deployment and the number of devices, were performed employing the DHI Mike3 Ecolab model along with field observation for collecting information about boundary conditions. Observation results showed that vertical mixing was enhanced by the device in stratified periods, leading to an increase in dissolved oxygen (DO) concentrations in its downstream bottom layer and a decrease in the duration of anoxic condition during exposed to blue tides. These results suggested that this device could be a valuable oxygen supplier for benthic organisms. Numerical simulation results were found to be consistent with the observed results and show the effectiveness of the jets consisting of water and air rather than only water and of an increase of the number of devices deployed. These outcomes indicate possible usefulness of the measure aiming to protect enclosed shallow fishing grounds and to provide evacuation space for fish and benthic organisms during the intrusion of anoxic waters, which may contribute to accelerating the recovery of its local ecosystems after the occurrences of blue tides.