The ran^ of temperatiire In which apceliiiens w«re taken waa between 12^ aM 27*'C. The great- 

 eat number of fleh per haal (by long line) wae obtained at tenneraturee of 23° and 24''C, 

 but optlmoB temperataree apparently differed annually and with locality and aeason of year. 

 Kawana (1934, pp 11-1!^) obtained reaulta during drift net operations off aouthaaetern 

 Hokkaido which ahow not only that the amount of catch la correlated with water temperature 

 but that the optlnium temperature la lower for the northern latitudes. Black tun&a were 

 taken In thla area when the water temperature reached about 14^0. MazlmuiD catchea> however, 

 were obtained between 16° and 18^C. The greateat nuaber of flah waa eeen In the area where 

 arnm and cold currenta neet, for example, off Shirla Capei where the Taugaru warm current 

 eaarergee with the Hokkaido coaatal cold current, and In the neitzhborlng watera of Shlkotan 

 lalaodi where branchea of the Kuroahlo warm current, the OlAotak cold current, aad the 

 Hokjcaldo coaatal cold current meet. 



X«tilMi (1939, pp 14^-144), In a #tudy dealln<j with catch In relation to oceanographlc 

 ooxkdltloae off eouthem Kyuahu, elao obtained Information which Indloitee that T. orlentalla 

 occttrc sEilnly nr^* the line of convergence between the warm and cold currenta ana In the wa- 

 tera where conplex tidal currentn are creatfd by the area of contact. The greateat number 

 ef flah waa eau^t ts the eu-ea where warmer waters were In contact with the colder watera 

 to the north. During January the maximum catch waa along the 20°C laotherm. In February, 

 as the 19*^ laotherm (cold current) began to awing aouthward between the Kyuahu coaat and the 

 20" laotherm (warm current), the beet fishing grounds ahlfted, aind during March the optimum 

 catchea were obtained chiefly on the 19° isotherm In a locality further south and within a 

 pocket formed by a projection of the 19° laothem into the 20° isotherm. 



Kawar.a (1934, pp 16-lB) prtser.ted data vr.Lcr. snow that a definite relationship xey 

 exist between the amount of the catch and the difference between surface tetnperature and 

 tenperature at a depth of 50 meters. A. difference of 6" to 9° resulted la majtlmuj catch (by 

 drift neta) in Hokkaido waters. With differences of higher or lower ne^ltude the catch de- 

 creased markedly. 



Such data on oceanographic conditions are desirable in order to predict the location 

 of fishing grounds and the areas where the maximum catches can be obtained. The studies out- 

 lined above indicate that the Japanese scientists have mode a ba^nning toward obtaining such 

 information. However, much remains to be done before oceanographic data can be utilized with 

 some degree of aocuracy to predict the yearly mlgratlc^ and availability of the species. 



5. Seoent Changea in Fiablng Srounda 



Prior to 1937 the black tuna waa tcUcen in aufflclent abundance to warrant coomerclel 

 operationa aa far north aa Storofu I aland along the Pacific coaat and Sakhalin along the Sea 

 of Japan coast (Tlgure 6). During the laat decade, however, changea in the extent of the ai- 

 gratlons have taken place. The northern limit of abundance of the black tuna along the Pacif- 

 ic coast is now the Volcanic Bey area, Hokkaido. In the Sea of Japan only occasional catches 

 are made. Tlaheraen and reaeareh workera believe that the recent decreaae in numbers in thtf 

 more northern latltudea and along the west coast of Japan 1b due lest, to depletion thar to a 

 lowering of water temperature. Monthly oceanographic charts for 1936— 40, prepared by the Tokyo 

 Central Tlsheriea Sxperlment&l Station, show a gradual yearly lowering of water temperatjre 

 in the waters along the Japanese coaat with a reaulting shift aouthward in the course of the 

 warm Kuroshlo Current. Thia la believed to be the major factor in altering the migratory path 

 of the black tuna and thua its presence in known fiehing grounds. 



6. Habits 



Black tunas are migratory but often remain for a considerable period of time over 

 banks in waters as deep as 200 meters. Vfhen migrating the schools swim near the coast. 

 According to Klshinouye (1923, p 440) the black tuna feeds chiefly on different kinds of fish, 

 such aa the aardlno, emchovy, flying flah, acad. and aand-ael, which are more or less pelagic 

 in habit. Sometimes fishes living near the bottom are found in its stomach. Invertebrate i 

 such as o&lamarles, pteropods, pelagic crustaceans, larvae of brachyurans and atomatopods, and 

 anonoleue amphlpoda are alao eaten. Later atudlee (Hokkaido Fisheries Sxperimental Station. 

 1928; Hajll, 1932; Hyogoken Fisheries Kxperlmental Station, 1935; Suyehiro, 1942) confirm this 



23 



p-6307 



