The purpose of this paper is to investigate the auditory threshold of Japanese horse mackerel which is one of the important pelagic fish for Japanese fisheries. The hearing of fish was studied by means of a cardiac conditioning technique. Classical conditioning method was performed with the sound coupled with a delayed electric shock. The threshold was determined by analyzing the electrocardiogram. The sensable frequencies ranged widely from 70Hz to 3000Hz which was most stimulative to the Japanes horse mackerel. As the frequency becomes higher than 70Hz, the auditory threshold decreased. At 1000-1500Hz(sound pressure 81-75dB) when the fish's hearing was most sensitive, the threshold tends to increase more than one. Lastly, it was assumed that a round haul netter traffic noise caused an escape action for the fish shoal.

Natural phytoplankton blooms of the dinoflagellate Alexandrium minutum, milkfish (Chanos chanos) exposed to natural blooms, sediment and mangrove crab (Scylla serrata) were analysed for paralytic shellfish poisoning toxins by high-performance liquid chromatography. The toxin profiles of milkfish and mangrove crab were similar to that of A. minutum collected from blooming fishponds. In a laboratory A. minutum-blooming environment, the stomach and intestine of milkfish accumulated paralytic shellfish poisoning toxins during the exposure period. The non-visceral tissues were non-toxic. However, milkfish lost their entire body burden of toxin on the first day of transferring to a toxic algae-free environment. The result shows that milkfish concentrate paralytic shellfish poisoning toxins in digestive organs and did not retain toxins.

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The paper describes the period change, power, and continuation time of swimming sounds by fish .The underwater sounds measured in net cages at the Hayato culture ground were analyzed with the zero-crossing method.The results show that: 1) the swimming sound of fish schools had periods from 4ms to 100ms; 2) histograms of zero-cross periods for yellowtail and amberjack were sharp with one mode; red sea bream, mode with a shoulder; amberjack, bimode; and tiger puffer, mode; 3) high-speed swimming sounds by large-size fish consisted of pulse waves of narrow-range periods; 4) the swimming sounds of small and/or altering-speed fish largely changed in period; 5) the present method makes. it possible to separate swimming sounds of fish from ambient noise and to detect small power swimming sounds by fish such as tiger puffers.(1) Fig. 1. Waveforms of underwater sounds in net cages: a; ambient noise, b; yellowtail Seriola quinqueradiata, c; red sea bream Pagrus major.