Based on the data gathered by the 11 Asahi catcher boat and the expedition of Tenyo tuna-fishing fleet in the South equatorial region during three months from August to Octover 1957, the following results are presumed as to the distribution of tuna and black marlin. (1) In the south equatorial Pacific Ocean, the distribution of tuna and black marlin differ clearly by latitude. In the low latitude areas, there is a remarkable difference of yellowfin tuna and bigeye tuna were investigated, but in the case of albacore, this phenomenon was different. Especially, in the southern region of 5°S, this tendency is more distinctly. (2) In the seas of 170°E to 175°E between 10°S to equator, the fishing-ground north of 5°S, the hooked-rate of yellowfin tuna, bigeye tuna and black marlin are higher than those in south of 5°S. But in the albacore, the hooked-rate is contrary. In the seas of 175°E to 175°W between 20°S to equator, the fishing-ground south of 10°S, the hooked-rate of yellowfin tuna, bigeye tuna and black marlin is higher than those in north of 10°S. (3) The highest hooked-rate was appeared few days before when the ground was first exploited and it drops gradually day by day. The hooked-rate, after the suspension of fishing, was more higher than that of the previous fishing period. (4) In regard to the tuna, as I reported before that difference in size of male and female was quite evident, and this tendency was also recognized in the south equatorial Pacific Ocean. The size of male tuna was bigger than that of female, but in black marlin is quite contrary. In the sex-ratio of yellowfin tuna, bigeye tuna and black marlin, the percentage of male is larger than that of female. But, in the albacore, the percentage of sex-ratio is quite contrary. Especially, in the black marlin, the sexual difference in size is most conspicuous. (5) In yellowfin tuna and bigeye tuna, the group of the smaller mode was dominant in 1957 and, on the contrary, in 1955 the larger mode was dominant. Therfore, I believe that these phenomena in the south equatorial regions are due to the decrease of its resources.

11旭丸操業を通じてソロモン海区のマグロ類及び,クロカヂキの分布に就て若干の知見を得たので之を纒めて見ると, (1) 南半球では一般にキハダ,メバチ,クロカヂキは低緯度に移行するにつれて分布密度の増大が見られる.ビンナガは赤道より南緯する程次第に分布密度は増大し,特に5°S以南は急激にその率が高まる. (2) 0°～10°S, 170°E～175°Eに於けるキハダ,メバチ,クロカヂキは南緯5°を境に北緯する程釣獲率は高くなり,之に反してビンナガは減少す.0°～20°S, 175°E～175°Wに於けるキハダ,メバチは南緯10°附近を境に南緯する程釣獲率は高くなり,逆にビンナガは低くなつている.従つて5°S～10°S, 175°E～180°Eを境にして変化する漁況は,この海区の夏季の特徴ではなかろうかと思われる. (3) 船団操業の如き大きな漁獲努力が加えられたこの海域では,漁撈開始日より数日後に最高の釣獲率を示し,それ以後は漸次低下し,休漁期間後再び操業を開始すると前期操業最終日の釣獲率より高い値を示した. (4) マグロ類カヂキ類にあつては,性に依る魚体差が判然としていて,マグロ類では雄が大きく,カヂキ類では雄が小さいという現象が認められる. (5) 性の組成に於てキハダ,メバチは雄の占める割合が大きく,ビンナガはそれと逆の現象を示していた. (6) クロカヂキは性比に依る魚体差ばかりでなく,分布や洄游等の生態も性比に依て相等の差があるのではないかと思はれる. (7) 1957年度に於けるキハダ,メバチでは明かに大型体長群が減少し,小型体長群の出現が増えている点より考察して資源の減少を意味するものと思われる.

長崎大学水産学部研究報告, v.7, pp.21-29; 1959