Abstract
The author studied the monthly and annual variation of surface water temperature and on the relation between those and the monthly and annual variation of hook rate (catch per 100 hooks) of yellowfin tuna in the tropical western Indian Ocean, and obtained following results: 1) The surface water temperature in sub-areas A, B and C was high in January to June and low in July to December, but the degree in the low temperature period in sub-area C was lower than in sub-area A and the degree in sub-area B was lower than in sub-area C. 2) The hook rate in sub-areas A, B and C was high in the high temperature period, but hook rate began to increase before the increase of surface water temperature, and the duration of such a preliminary high hook rate period was longer in sub-area C than in sub-area B, and insubarea A than in sub-area C, respectively. 3) The surface water temperature in sub-areas A, B and C was high in 1954 to 1955, in 1958 to 1959 and in 1962, and low in 1956 to 1957, in 1960 to 1961 and in 1963. 4) The relation between annual variation of hook rate and that of surface water temperature could not be seen.
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