On the Surface Temperature of Water in the Tunny Fishing Grounds off Kusiro and Urakawa in Summer

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Kusiro and Urakawa are the two principal ports on the Pacific coast of Hokkaido notable for the fishery of tunny, Thynnus (Linné). The data of the present investigation are mainly based on the informations from the fishing boats sent on request to the HokkaidÔ Fishery Experimental Station during the summer of 1935. The results are srmmarised as follows:- 1) The most fav urable temperature ranges generally between 15°C and 17°C, and is higher at the beginning and becomes lower as the season advances. 2) The fishing ground is located by far nearer to the coast off Urakawa than off Kusiro, in accordance with the distance from the coast to a "Siwome" or the boundary be-tween two sorts of water in each region. 3) As for the size of the fish, the largest (weighing more than 112•5kg) is abundant in July, while the middle-sized (112•5-37•5 kg) and small-sized (less than 37•5 kg) in August and September. The larger the fish, the wider the range of water temperature of their fishing ground. 4) Certain connexion seems to exist between the incoming of shoals of sardine, skipper and gramps or of sea birds on one side and the tunny fishery on the other, which is still to be investigated.
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