insignificant in comparison with the depth of the sea, it goes without saying that it can take only 

 those fish which swim at the very surface. Consequently unless this gear is used at a place and 

 at a time where the tunas swim in the surface layer, it will not attain its objective of catching 

 fish. Also, the tunas naturally have the power of sight, and when they encounter a net, they will 

 try to escape, so this gear is useless unless employed at night, when the meshes of the net can- 

 not be seen. 



The factors which control the vertical distribution of the tunas are thought to be 

 principally such things as light, temperature, and water pressure. As far as light is concerned, 

 fishes in general share the characteristic of inhabiting deeper levels during the day than during 

 the night. This characteristic appears also in the tunas, and is particularly marked in the case 

 of the bigeye tuna. 



With respect to temperature, the majority of tuna species are basically warm-water 

 fishes, preferring to inliabit sea areas having warm waters. They are thought to be extremely 

 sensitive to changes in tennperature, but this means only that the linnits of the tennperature at 

 which they will live are clearly nnarked, and does not mean that the range of temperatures which 

 they can withstand is narrow. It is thought that the range of temperatures which they can endure 

 is rather broad. It is known in the waters of the low latitudes they generally swim at fairly deep 

 levels. In such areas temperatures at which the tunas can live are found at the surface, at the 

 100-meter level, and in some cases even deeper. Fronn an examination of a considerable 

 amount of data, it appears that in the waters of the low latitudes, the tunas are generally most 

 abundant in the vicinity of the 100-meter level. Compared with the temperatures which are 

 thought to be most favorable for the occurrence of tuna in Japanese waters, the water tempera- 

 tures at the 100-meter level in tropical seas are rather high. This fact is thought to show that 

 in some circumstances the effect of water temperature on vertical distribution is not very great, 

 indicating that the action of light is more important. The above-expressed opinion that the 

 density of distribution is greatest in the vicinity of the 100- meter level is not actually based on 

 any study of the density of distribution, but rather on the circumstances under which catches are 

 made in the longline fishery, and therefore it may naturally be thought that the strength of the 

 light rays may be a factor in making the fish easier to catch. However, at present it is inn- 

 possible to conclude whether light or temperature is the nnost important factor in determining the 

 vertical distribution of the tunas. 



As far as water pressure is concerned, the adaptability of the tunas to changes in 

 this factor is thought to be extraordinarily great. In tropical waters, for exan-iple in the Celebes 

 Sea, they occur in considerable density as deep as well over 100 meters, and they also occur 

 practically on the surface. In short, it is thought that they can withstand water pressures rang- 

 ing from about 1 to well over 10 atmospheres. 



Other fishes are known which can withstand considerable changes in water pressure 

 where the changes occur gradually, but ordinarily they cannot withstand sudden changes. The 

 tunas, however, are thought to have remarkable resistance against sudden changes in water 

 pressure. For example, if we look at the behavior of tunas when they are hooked, we see that 

 they sometimes make one deep dive, going so deep that the glass ball which is used as a float is 

 crushed by the water pressure. As was noted above, in tropical sea areas the identical species 

 is distributed at markedly greater depths than it is in waters of higher^ latitudes, Jind is thought 

 to be able to withstand more violent changes in external pressure, for which reason it can be 

 concluded that the effects of external pressure are not very importauit in determining the vertical 

 distribution of the tunas. 



It has already been stated that temperature and light are the main causes of the 

 vertical movement of the fish. In Japanese waters, off Hokkaido and the northeastern coast of 

 Honshu, a warnn current forms a shallow layer on the surface, below which runs the cold current 

 of the Oyashio. In waters presenting oceanogr aphic conditions of this sort, the tunas naturally 

 are distributed only in the surface layer, affording favorable conditions for the drift-net 

 fishery. 



