A Note on the Fisheries Condition of "Katuo" as a Function 

 of Several Oceanographic Factors 



Synopsis /^in English_/ 



Some tentative studies of the fishing conditions of "Katuo",* 

 Euthynnus vagans (Lesson), as a function of several oceanographic elements, 

 including surface temperature, are given here. 



The fishing locality of "Katuo" moves from the region off Nozimazaki 

 in May, passing off Tyosi in June, to the region off Kinkwazan in July. 

 The paths seem to follow the warm branches of currents, which are recog- 

 nizable by the tongues of isotherms either at the surface or at the sub- 

 surface of 100m depth. The concentration of the shoals seems to be 

 defined not only by the surface water temperatijre but also by the vertical 

 distribution of vertical temperature gradient in the uppermost 100m layer. 

 The paths also have close bearing to the migration of sardine shoals and 

 to the distribution of copepoda, both being the natural food fauna for 

 "Katuo". 



In the sea-region south off Kagosima Pref, the catch of "Katuo" 

 varies with salinity throughout the periods from March to September 

 (Figo 6) , The value of transparency and of salinity giving the mode for 

 the catch are shovm in Figs„ 5 and 7„ /f^End of English synopsis_7 



With regard to the relation between the skipjack fishing situation 

 and oceanographic conditions, a great number of reports have been published 

 in the past on surface water temperature, the present author himself 

 having published frequently on the subject in this journalj however, there 

 has not yet been any thorough study of factors other than surface tempera- 

 ture. For the determination as a practical problem of the zone which 

 connects the principal skipjacl: fishing grounds -- the apparent "fishway" — 

 the surface water temperature values alone do not suffice « Furthermore, 

 in explaining the skipjack fishing situation in the Satsunan and Zunan ■ 

 areas there is much that cannot be understood from the water temperatuires 

 alone. Further there is the general problem of the relationship between 

 oceanographic conditions and how well the skipjack take to the bait. If 

 we try to make a start at solving the problems in such an area, we 

 experience considerable difficulty because the threads of all of the 

 factors are entangled and there is a paucity of data other than surface 

 water temperatures. Nevertheless, feeling desirous of seizing a clue to 

 the solution of these problems, I have made two or three preliminary 

 investigations, the results of which are reported below. 



First of all, if we consider the catch /^ as a function of several 

 oceanographic factors, which are water temperatjre 5^, temporal changes in 

 water temperature 0^ the gradients ^-i^ of water temperatures in a horizontal 

 and a vertical direction, salinity'^, and transparency -2* we get the formula 

 Asj:(^, (^y(^<S',i), . , ,) . In the following let us consider cursorily some 



'Katuo," ( Euthynnus vagans (Lesson)) s katsuo . skipjack ( Katsuwonus 

 pelamis of authors) 



