varies greatly with the species, being always two in the case of thresher sharks, less than 10 in 

 the case of make shaxks, and from 10 to 40 in the case of great blue sharks. In the case of the 

 oviparous tunas and spearfishes, it can be imagined that the effect of environmental conditions 

 is very great and that fluctuations in reproductive power from year to year are marked. In 

 other words, the number of eggs spawned is enormous and therefore even slight variations in 

 environmental factors will be reflected on a large scale in the total stock through their action on 

 fertilization, hatching, and growth. 



The spawning habits of the tunas and spearfishes are not yet well known, but the 

 degree of maturity of various individuals in a single school is extremely variable, the distribu- 

 tion of sexually mature individuals is extremely extensive, and it appears that the occurrence of 

 juvenile fish is also remarkably broad but very sparse so that they cannot be collected in large 

 numbers. Furthermore, juvenile fish of various sizes can be taken in the same area at the same 

 season, and it is thought that all of these phenomena indicate that it is the habit of these fishes to 

 spawn over a wide area throughout a rather long period of time. 



When we consider the oceanographic characteristics of the low latitude seas which are 

 the spawning grounds of these fishes, we see that the content of nutrient salts in the sea water is 

 small and that it varies little throughout the year. This means that the quantitative distribution 

 of plankton is small and that its seasonal variation is also small. Accordingly, it nnay be 

 thought that the food for juvenile tunas and spearfishes is not very abundant and that the seasonal 

 changes in its quantity are not very pronounced. Such characteristics are markedly different 

 from those of the waters of high latitudes. In the seas of high latitudes we find seasons of con- 

 spicuous proliferation of plankton accompanying changes in temperature. In coastal waters 

 nutrient salts are constantly supplied from the land by rivers and by other agents. However, in 

 the very middle of the ocean there is no source of supply for these nutrient salts. Consequently, 

 in the seas of the low latitudes where these fishes spawn and pass their juvenile stages, the con- 

 ditions are like those in overworked soil with an insufficient supply of fertilizer. It is thought 

 that the reason that the spawning of tunas and spearfishes in such waters extends over such a 

 broad area and such a long period of time is that this is the most reasonable way of attaining the 

 objective of maintaining the species. In short, spawning over a broad area and throughout a long 

 period is in better agreennent with the environmental conditions in the seas in which these fishes 

 spawn, from the point of view of the maintenance of the race, than would be spawning in a com- 

 paratively restricted area and a short period of time, as is the case with fishes of the high lati- 

 tudes . 



No connected study of the food habits of the tunas and spearfishes has been made ajid 

 they are not well understood. There is no roona for doubt that their food in the juvenile stages 

 consists of various kinds of plankton. The natural food of the adult is indeed heterogenous, and 

 it is very doubtful whether they exercise a great deal of selection with regard to their food. It 

 is thought rather that they eat whatever is most abundant in the area where they occur or what- 

 ever is comparatively easv to catch. The spearfishes and sharks eat mainly large fish and 

 squid. The tunas eat, in addition various kinds of large plankton. The plankton feeding character 

 seems to be most pronounced in the albacore. At sonne places and seasons Alepisaurus, deepsea 

 fishes, and fishes such as Bentenia, which are thought of as rare species, are very commonly 

 found in their stomachs. 



If we take all of these factors into consideration, we may think that the resource of 

 the tuna longline fishery is propagated and maintained throughout an extremely broad expanse of 

 ocean. If we consider this resource in terms of the sea which supports it, the density of distri- 

 bution expressed as fish per unit of area can naturally be thought to be rather low in comparison 

 with the resources of the coastal fisheries or of the fisheries in northern waters. However, as 

 regards the absolute quantity of the resource, we cannot conclude that they are necessarily in- 

 ferior to other fisheries resources, if we consider the area of the seas in which they occur. 



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