Estimation of the assimilative capacity of fish-farm environments based on the current velocity measured by plaster balls

Current flow under fish-farm cages was investigated in eight net-pen fish farms in Gokasho Bay and Owase Bay, Pacific coast of central Japan, to assess the assimilative capacity of the water body. Time-averaged current velocities during neap- and spring-tide periods were determined using plaster balls and a concomitant current meter set 1 m above the seabed. An index of suitable location for fish farms (ISL), which is given by DV 2, where D is the water depth (m) and V is the mean current velocity (m/s), is proposed. Field data on biotic and abiotic factors that have been obtained from the same stations were used to validate ISL. Values of biotic and abiotic factors changed along a gradient of ISL versus annual fish production. As ISL values decreased and fish-production levels increased, values of dissolved oxygen of the bottom water decreased, values of sediment total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD) and acid-volatile sulfide (AVS) increased, and values of biomass, density and number of species of the macrobenthos decreased. Three zones of fish-farm environments, i.e., healthy, cautionary and critical zones, which had been defined on the basis of the macrofauna and chemical factors, were also arranged in a gradient of ISL and fish production. The healthy zone was located at areas having large ISL values, whereas the critical zone was located at areas having small ISL values and a high level of fish production. As fish-production levels increased, the cautionary zone shifted to areas having larger ISL values. These findings suggest that bathymetry and water flow control waste dispersal and loading, and ISL is an effective indicator of the assimilative capacity. Plaster balls can be used as a pragmatic tool in determining time-averaged current velocities.