The fluctaution in the monthly skipjack-catch by the seven pole-and-line boat belonging to the Minato-ura Fishermen's Cooperative (Tanabe City, Wakayama Pref.) was studied in relation to the distribution pattern of the surface temperature over the fishing ground. The study dealt with the 'spring skipjack seasons' (i.e., April through July) of 1953-1960. It was safely assumed that the combined fishing capacity of the seven fishing boats remained almost unchanged during the period in question. These boats, each measuring 15 to 16 gross tons and manned with a crew of 18 or 19, made a one-day trip over the fishing ground centering around Shiono-misaki and extending to Muroto-zaki
and Miki-zaki and the trip was in less than 50 sea-miles offshore of Shiono-misaki (Text-fig. 1). Water temperature data were taken from the "Ten-day Mainre Report" of the Japan Meteorogical Agency.
The results obtained are summarized as follows:
1) When a cold water mass was located in the vicinity of Shiono-misaki bordered with a pronounced 'shiozakai' (a boundary zone characterized by a sharp horizontal temperature gradient), a good fishing ground for skipjack was ususally found in the neighbourhood of the 'shiozaki.' The catch of skipjack was abundant as long as the cold water mass continued to stay to the east or the southeast of Shiono-misaki (e.g, 1954, '55). On the other hand, the cold water mass was found to the south of Shiono-misaki, however, the catch was poor due to the unfavorable location of the 'shiozaki' (e.g., 1960).
2) When a cold water mass was absent to the east of Shiono-misaki, with the isotherms laid parallel to one another either in a general direction of east-and-west or southwest-and-northeast with their protruding northward either to the immediate east of Shiono-misaki or father eastwards (e.g., 1953, '56, '57, '58 and '59), skipjack catch was poor in the vicinity of Shiono-misaki in the majority of cases. With the isotherms of 22·c or about protruding southward to the east of Shiono-misaki, skipjack fishing was good in the neighbourhood of Shiono-misaki ; the temperature distribution of this type was encountered chiefly in case of a cold water mass was present.
3) As was indicated above, the fluctuation in the monthly catch of skipjack was explainable from the surface temperature distribution with exception of few cases. In an effort to find explanations for the exceptional cases, the increment of surface temperature in the lapse of ten days was computed for different parts of the fishing ground and its geographic distribution was plotted on the chart. It was found that the distribution of temperature increment for a ten-day period gave only a partial explanation to the above-mentioned exceptional cases (Text-fig. 5 (2)). The increment of surface temperature in the lapse of ten days was computed also for those months, in which the fluctuation of the catch was adequately explained on the basis of the distribution of surface temperature. There upon it was finally found that the distribution of temperature increment did not explain the fluctuation of the catch in some of these months so lucidly as the distribution of surface temperature.