浜名湖における貝毒化の原因について : 第1報貝の人工的毒化の試み

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Regarding causes of yearly appearance of poisonous shell-fish at the Lake Hamana, an assumption proposed by NOGUCHI is that some harmful substances formed at the bottom layer of the central area of the lake in summer might rise to the surface by convection of water in autumn and flow down to the mouth of the lake to be incidentally taken by shell-fish. This assumption can succesfully explain the reason why appearance of poisonous shell-fish is restricted to the areas along the water-routes (cf. Fig. 1), as well as to winter and early spring. Another fact which likely supports the assumption is this. In summer water exchange by vertical mixing at the central area of the lake is so small that oxygen becomes deficient already at a depth of 6m. and absent below 8m. If the assumption is correct, (1) It might be possible to render experimentally shell-fish poisonous by transplanting them to the central part of the lake, and (2) There may exist any coincidence between the result of the transplantation in autumn and appearance of toxic shell-fish at the southern area in the following season. It is also important to know whether the shell-fish can take in poison from the surrounding water. Results 1. Oyster transplanted at the lake center in autumn 1951 was found poisonous when recovered, while another group of the shell-fish kept for the same period in another part (station F in Fig. 1) of the lake was not. From January to March 1952 high toxicity was detected in shell-fish sampled at the mouth of the lake (Table 1). Out of four experiments two cases showed the paralellism between the result of the transplantation and the subsequent occurrence of the poisonous shell-fish as expected. Toxicity of shell-fish in the mouth of the lake seems, however, to depend largly on hydrographical conditions of this area which in turn seem to be determined by the width of Imakiri Channel (Fig. 1 Station A). It must be born in mind, therefore, that if one fails to detect the toxic shell-fish in winter, transplantation experiments carried out in the previous autumn may not always provide a reliable criterion to the assumption. 2. In order to know whether the toxic substance is adsorbed on the bottom mud of the central area, short-neck clam was kept in a pot containing the mud for about two weeks, but they did not become poisonous (Table 2). No definite conclusion, however, can be drawn from this experiment because no toxicity was detected in the southern part in the following poison season. 3. Short-neck clam was kept in sea water containing shell-fish poison (Venerupin), the poison add ?? d into sea water was neither detected in the liver of the shell-fish, nor could be recovered from the water (Table 3). Decomposition of the poison by bacteria cannot explain the phenomenon, since no poison could as well be recovered, even immediately after being added, from sea water. It seems that the poison may undergo such a change as to lose the activity during isolation from sea water.
公益社団法人日本水産学会の論文

浜名湖における貝毒化の原因について : 第1報貝の人工的毒化の試み

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