On the Difference of the Stomach Contents of Tuna and Black Marlin in the South Equatorial Pacific Ocean

Examining the stomach contents of tuna and black marlin obtained in the south equatorial Pacific Ocean during three months from August to Octover 1957, the following results were obtained; 1) Food organisms obtained in the stomach were enumerated about 40. (c.f. Table 4) It seems that compositions of food resemble in equatorial regions. 2) I found the difference between yellowfin tuna and bigeye tuna, and also between the all species of tuna and black marlin in respects of certain kinds of preys. I fonnd some species of the deep sea Pisces, such as Sternoptychidae, Myctophidae and Bramidae and, these Pisces were more abundant in the stomach of bigeye tuna than that of yellowfin tuna. These Pisces were not seen in Black Marlin. On the contrary, Megalopa and Alima, the planktonic larvae of the Decapoda and Stomatopoda in Crustacea, were found more abundant in the stomach of yellowfin tuna and black marlin than that of bigeye tuna. I have rarely seen the reversed stomach in some individual of the bigeye tuna. As the result, it is presumed that bigeye tuna mostly swims in slightly deeper layer than yellowfin tuna and black marlin, and at the same time, black marlin mostly swims in shallower layer than tuna group. 3) Food organisms were composed of more than 10% by the following preys, such as, Plagyodontidae, Sphyranenidae, Triacanthidae, Acinaceidae and Juvenile Skipjack, Decapoda in Crustacea and Decapoda in Cephalopoda. These preys are seemed to be the most important as food of tuna and black marlin in the equatorial regions. 4) In the south equatorial Pacific Ocean, the stomachs were hardly seen empty, therefore, this area will be rich in food organisms. I presumed that the fishing ground for yellowfin tuna, the south equatorial Pacific Ocean is a considerably important situation for the Japanese tuna long-line. The species of the compositions of food in yellowfin tuna were richer than that of other species of tuna.(1) 東ソロモン海区で操業した11旭丸の資料から,食餌組成に就て検討して見た結果,現在迄マグロ類の餌の選択範囲は夫々非常に広く,その組成の傾向も相似であると云われてきた従来の見解と略々一致する.(2) 数種のpreyに就て,キハダ対メバチ,マグロ類対クロカヂキの食性に顕著な相異が見られた.これは全く渡辺(1958)の見解と一致する.即,ムネエソ,ハダカイワシ,シマガツオの深海魚はキハダよりメバチの胃中に多く見られた.ムネエソ,ハダカイワシはクロカヂキの胃中に出現しなかつた.且,クロカヂキの胃中に見られたシマガツオの出現率はメバチより低かつた.次に表層性のトビウオがメバチの胃中に出現せず,且キハダに比してクロカヂキの胃中に多く出現した.次に甲殻類中MegalopaやAlimaの如き浮遊性preyの出現率がメバチに比して,キハダ,クロカヂキに高い.又,釣獲されたマグロ類中,胃が反転してpreyを吐出した状態で揚つてくる現象はメバチにのみ見られた.以上の事から,メバチはキハダより深い層を遊泳している.又,クロカヂキはマグロ類より棲息分布は浅い層にあると云う事が推察される.(3) 鮪類,梶木類に執つて出現率10%以上のpreyは餌料として最も重要であることは論を俟たないが,西部印度洋海域と南太平洋赤道海域に於て,共通に然も高い出現率を示したミズウオ,カマス,ギマ,クロタチカマス,カツオ若年魚の6種と甲殻類の,Decapoda,頭足類のDecapodaの餌生物は,マグロ,カヂキ類のpreyとして最も重要な位置を占めるものと思われる.(4) 南太平洋赤道海域と西部印度洋海域に共通して見られたことは,各魚種共空胃が極めて少い.次にキハダは他の魚類に比して食餌組成の中が広いと云う事である.以上の事から前者の理由に依り,両海区共preyが豊富で好漁場だと考えられ,後者の理由に依つて赤道海域はキハダの好漁場であることが推察される