The behaviour of skipjack shoals has often been studied since many years ago on the basis of the data of angle fishery. A s regards seine fishery data, in turn, they have also become available recently for skipjack and tuna made a rapid progress on the coast of the north eastern sea areas of Japan. Therefore, in this paper, the relation of behaviours of skipjack and tuna to their catch and also the relation of grouping of fish in shoals to their catch were studied from the data of seine

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... The difference in catching power was also pursued between one boat seine and two boats seine. The species of fish treated in this paper are blue-fin-tuna, young-tuna, yellow-fin-tuna, sword-fish, skipjack and their mixed shoal. 1) The values of the catching ratio (f) and catch (c), which were calculated for three ranks of grouping, namely, Great-, intermediate-, and small shoals, and for various species of shoals are indicated in Table 4 . Showing this table, the values of f of small shoals in various species are higher than those of large shoals. 2) The type of shoals has been sorted into nine classes from the viewpoint of their cha racteristic behaviours as described in Table 5 . The typle corresponding to good catching ratio was found one of the so-called "shoal with baits" indifferent of species to shoals and this finding forms a contrast against the case of angle fishery. 3) There was found a little difference in catch per hauling between one boat seine and two boats seine. Table 1. Number of fishing boats for each year ( ): Tonnage of seine boat