Studies on harbour porpoise Phocoena phocoena incidentally caught by fishing gears around Japan and the mechanism of bycatch

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The stranding records of Harbour porpoise Phocena phocoena collected by Institute of Cetacean Research was analyzed and the high proportion of the stranding was reported on April and May. The most frequently reported stranding place was Usujiri (36 individuals) and the next was Shiraoi (7 individuals). The number of bycatch individuals confirmed by our survey was 16. From the observation of ovary, they were immature. The passability experiment was conducted in the pool of Otaru Aquarium for an individual and found that the 50% passable aperture width was 39cm and become narrower as the habituation. The tau dot analysis was conducted to find the approach speed control to the target. Tau was increased linearly and tau-dot was estimated as 0.772. There were two inflection points of tau at 2sec and 0.7sec before the target. From the behaviour observation, there were two peaks of inter crick intervals at 2ms and 5ms, which means the target of the echo-location was 1.5m and 4m. These result suggest that the target recognitions of harbour porpoise was occurred at 4m and 1.5m before the target using echo-location.