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In the morning of 7/17/2012, a few hundreds of whip ray were found going upstream in Isumi river of Chiba.
The surface of river (About 3km from estuary) was covered by brown and black whip rays. They disappeared before noon.
The mass of whip ray came back to the same point in the morning of 7/18/2012 again.
People near the area comment it is not rare to see whip ray coming upstream. On the other hand, a fish handler of Kamogawa seaworld talked it’s 30~40cm big, which is adult. It comes upstream from Spring to Summer occasionally but it is rare that the mass of whip ray come upstream at once. The reason is not known.
Mr. Horie from Tokai university states, female whip ray lay eggs near estuary sometimes but they are 80cm ~ 100cm without a tail. The ones of 40cm is too small to come upstream, it is not for laying eggs. Even if it is for laying eggs, it can’t be such a mass.
Fishery’s union in Taito fishing port also comments whip rays are released even if they are caught in the net. It feels strange to see such a mass of whip ray coming upstream.