RECENT STUDIES ON TUNAS AND MARLINS IN JAPAN 167 



though the density is thin. This is supposed to be the spawning group 

 which came from the North, but this is not confirmed by accurate evi- 

 dence. One somewhat distinct group of large-sized fish usually goes south- 

 westward from October to February in a thick density, at 135°E, 12- 

 25 °N; but the relation with other groups is still not clear, 



Parathunnus mebachi. The previous works on the migration of 

 this tish discuss mainly the oceanographic conditions of the fishing 

 grounds, but Nakamura, by his density distribution maps, found the 

 following facts. The two separate fishing grounds exist in this area. 

 The one at about 40 °N in the Northwest Pacific in September is 

 stretched along the latitude line to the date line eastward, and then 

 goes down South, like G. germo; but the speed is slower than that, so 

 this fishing ground is always located at the North of the front of that. 

 Another group of fishing grounds exists all year round at 8 to 12°N, 

 a contact line of equatorial countercurrent and North equatorial cur- 

 rent. These are not homogeneously stretched as a line, but form some 

 isolated groups within them. He still has no definite idea on the rela- 

 tionship between the two. Besides these, a fishing ground appears in 

 20-25 °N during a certain season. The relationship of this to others is 

 unknown. Kamimura (not printed) recognized the phenomenon that 

 the modes in size-frequency curve differ every other year in the North- 

 west Pacific. 



Neothunnus macropterus. Past works are concerned mainly with 

 the oceanographic conditions of habitation, especially the temperature 

 of the water. However, Nakamura by his density distribution maps 

 observed the following facts. The size of fish is generally smaller in 

 the coastal area of Japan and gets larger in the grounds offshore to the 

 East. A remarkable change of size composition is also recognized ac- 

 cording to latitude in the same longitude in the same season, and the 

 border lines of this change are at 3-4 °N and 7-8 °N, which, he thinks, 

 are caused by the influence of the South equatorial current, the equa- 

 torial countercurrent, and the North equatorial current. Also, he thinks 

 that the different fish groups would come in and out by the influence 

 of the monsoon in the seas which are surrounded by land or islands 

 such as South China Sea, the Sulu Sea, the Celebes Sea, the Banda Sea 

 and others. The same fact would be supposed to exist at the coastal 

 waters off Timor, the Small Sundas and other equatorial areas in the 

 Pacific. Before Nakamura's work it was believed that most condensed 

 grounds are in the equatorial countercurrent, but he thinks it would 

 be somewhere South from the contacting area of the equatorial counter- 

 current and the South equatorial current. 



