浅海養魚場における海水交流について-I 櫃石島ハマチ養魚場の場合

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Recently, the intensive pisciculture of Hamachi, the young of Seriola quinqueradiata, is extensively carried on in the Seto Inland Sea. It is needless to say, however, that there are only a few studies on the local marine environmental conditions in the Hamachi piscicultural farm. Without doubt, the dissolved oxygen content would be the most important one of various environmental factors influencing the productivity of fish farm. Accordingly, the water-exchange which plays the leading role in the dissolved oxygen budget should be at first clarified to determine the optimum number of fish cultured in the farm. This investigation has been carried out in the neap tide, because the water-exchange in the shallow marine fish farm becomes so small in this tide as to cause the deficit of the dissolved oxygen, which has been commonly experienced in some farms. In the Hitsuishi fish farm (Fig. 1), the water area and volume are 7.2×104m2 and 22.3×104m3 at high high water and 6.0×104m2 and 8.7×104m3 at low low water in the neap tide, respectively (Fig. 3). This shows that the farm resembles a shallow dish in shape and that the effective area in which Hamachi lives becomes very narrow at low water. To verify this fact, moreover, surveys were carried out on the horizontal distribution of the organic carbon and total nitrogen contents in superficial sediments (Fig. 6-a, b). There is a little sea water flowing in or out through a gate provided with in the middle of the southern embankment, but the water-exchange through the wall net stretched across about 350m between two islands is very remarkable owing to the tidal current (Fig. 4). The current flowing into the farm through the west half of the wall net runs directly to the southern embankment, but the one through the middle part of it turns round aloug Buto-jima and then goes out. This flow pattern is similar to that found in a fish farm where the wall net is stretched across toe mounth of a bay. The current in this farm runs at the speed of 5 to 10cm/sec. (Fig. 5). The quantity of sea water exchanged for each about 12 hours of M1 and M2 tide is 36.7×104m3 and 20.5×104m3, and consequently the water exchange rates give 3.7 and 2.0 times per about 12 hours, respectively (Fig. 4, Tab. 1). On the basis of the smaller water exchange rate in the neap tide, we can estimate the optimum number of fish cultured in this farm from the figure given in Ref. 2 by INOUE (1965); early in August, when Hamachi weighs about 300g, the number of fish cultured amounts to 6.5×104 for extra safety and 15.5×104 for the possible limit. These results were discussed considering the growth rates of Hamachi cultured at different numbers of fish in this farm.

浅海養魚場における海水交流について-I 櫃石島ハマチ養魚場の場合

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