On the Sstimatiori of Favorable I'enperature for i^ng-Line ^ishing of -Cunny 



|_English synopsisj Owing tc the lack of observations both of the accurate 

 depth d and of temperature of the layer in which tunnies off the Pacific 

 coasts of Japan are angled with long-lines^, the catch has custoniarily been 

 discussed, reference being made only to the surface temperature 6q c 



From July to Dec, 1930., off the North-eastern Japan^ and from Dec. 

 1930 to Iwarch 1931, off south to Bosyu.„ several fishery research boats 

 carried out the said observations for the tunny„ including Thunnus thunnus 

 Lo, Thunnus alalunga (.Gmelin)^, Para thunn us sibi (To & S„), and Germo 

 macropterus (.Jo Jc S „) o Their reports enabled the writer to examine the 

 difference between the distributions of total catch N as well as of the 

 frequency of catch _f j, referred to the temperature and thos e referred to 



Off the North-eastern Japanj, both N and f show the mode at 19-20°C of 

 Oq, while at 18-19°C of 9 (Fig„l)o Evidently the difference is due to the 

 remarkable stratification of the water during summer timeo No definite 

 explanation can be given to the appearance of a secondai^ mode at 13-15° 

 of Oc. Off south to Bosyu*, both the mode of f at 19-20°C and of n (= N/f) 

 at 17-19°C are not changed in 9 and 9q, showing that' the stratification~in 

 temperature is but slight during winter (Figo2)o 



Estimation, if not very" accurate, of 9 without direct observation can 

 yet be made by calculating preliminarily the value of 9q-8 from other serial 

 observations of temperature for each region and for each monthj, provided d 

 ip known o From the records of 9^, submitted by a fishing company "Tokai- 

 Enyo Kabushiki Kaisha/' for Thunnus alalunga (iGmelin) off south to Bosyi? in 

 1930, was calculated in such a way as just stated^, with the result that 

 the mode of N lies at about 18°C, d having been taken as 50-lOOm or 100m 

 (Fig„3 ajo Remarkably enough,, the variation of N in 9 is much smaller than 

 that in 9q A mode at the same temperature is seen in the frequency distri- 

 bution of 0, at which the catch was maximum in every month ('''igo 3 b)„ 

 Some other records were also treated similarly and again the mode of N at 

 18°C was obtainedo [end of English synopsi^] 



In discussing favorable temperatures for the capture of fishes such 

 as the tuna and mackerel j, which swim in the middle layers, we should seek 

 to find out the temperature of the water at the depths where the fish 

 swiml.'o Hitherto, however, it has not been clear in many cases at what 

 depth the fish were swimming and few observations have been made of any- 

 thing but surface water temperatures so all that could be said was that tuna 

 ere taken most frequently and in the greatest numbers where the surface 



'Aikawa, Hiroakii Situation in the Fisheries for Tuna^ Skipjack, and 

 Saury on the f'acific Coast, Suisan Gakkai Ho, Volo5, Noo4j, ppo364-365. 



draws attention to this o 



