On Water Exchange in a Shallow Marine Fishfarm-II

The Tanoura fishfarm, located on the southern coast of Syozu-sima in Kagawa Prefecture, has a long bay-mouth stretch (about 560m wide) and is partitioned with double sheets of netting. The water area and its volume are 16.4×104m2 and 166×104m3, respectively, at high tide. In 1965, 46×104 fish (Hamachi, the young of Seriola quinqueradiata) were stocked initially and 42×104 of fish harvested. On the whole, sea water (the upper layer) flows into the farm and from the flshfarm the lower layer flows out through the western half of the netting fence, but the water is little exchanged through the eastern half. Surface currents flowing into the farm through the western half of the netting fence run directly to the interior, but the current through the middle part makes an eastward turn. In the eastern region of the farm the current flows veryslowly. The quantity of sea water exchanged is about 56×104m3 during half a tidal cycle in the neap tide. On the basis of the water exchange rate during the neap tide, it may be estimated that the optimum number of Hamachi stocked in the Tanoura fishfarm amounts to 40×104, late in August, when each Hamachi weighs about 400g. The results are discussed with due consideration given to the growth rates of Hamachi cultured in this farm for the past several years.