Studies on the Yosanaikai, its Oceanographic Conditions and their Improvement for Fisheries Purposes

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The Yosanaikai is located on the southwestern coast of the Japan sea and it represents a lagoon formed by a sand bar about 2.5km lo_??_g. This lagoon has been noted for its delicious fat sardin. In recent years, however, the fish and molluscan production became rapidly lower. By our oceanographic survey, the following facts were revealed: 1) The salinity of surface and bottom waters differs considerably, and in the deeper layer no oxygen is contained in the warmer season and only a trace in the colder season. 2) Benthic animals are found on shallow bottoms only. The deposits of deeper bottom, which are jet black in color, are azoic throughout the year, but contain a large amount of bivalve shells chiefly of Theora lubrica and Raeta yokohamaensis, which are well known euryoxybionts. 3) The living specimens of these bivalves, though found on shallow bottom, are very scarce. The transportation of dead shells from shallow bottom to deeper bottom can seldom occur, because the lagoon is so calm that the stagnation causes the putrefaction of water. Thus the thanatocoenoses of this lagoon must be au ?? ochthonous, being the remains of molluscs which lived abundantly for some years before. 5) It is said by the fishermen that the sar ?? in was used to be caught from the upper layer in the warmer season and from the lower layer in the colder season. In recent years, however, nothing could be caught in the deeper strata throughout the year. From these facts we reached to a conclusion that ecologi ?? al conditions of this lagoon were so changed that the stagnation of water became more serious than ever and fatal to those living in the deeper zone. The outlet of the lagoon became narrower as well as shallower than before by the accumulation of sand, and both the outflow of less saline surface water and the inflow of the more saline sea water were prevented. It seemed to us that this topographic change is the chief cause of the trouble. Upon our advice, the dredging of the outlet was made with the results of better aeration of deeper water, wider distribution of benthic animals and larger production of sardin, other fishes and useful molluscs. The writers believe that, by similar procedure, the lagoons on our coast may be made more productive.

Studies on the Yosanaikai, its Oceanographic Conditions and their Improvement for Fisheries Purposes

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