II. Characteristics of the Tuna Longline Fishery 



It is probably no great mistake to consider that the character of a fishery is deter- 

 mined by the character of the resource which is its object, and that the character of the re- 

 source is detern-iined by the species which compose it and their environmental conditions. Con- 

 sequently, if it is possible to reveal and grasp the biological characteristics of the species which 

 is the object of the fishery and the environmental factors which control the occurrence, proga- 

 gation, and migration of that species, the character of the fishery can thereby be analyzed. 



As was stated earlier, there are a number of different fisheries which have the tunas 

 as their objective, but among thenn the longline fishery is the most important both in terms of the 

 scale on which it is prosecuted and in terms of the catch. I shall, therefore, take up the longline 

 fishery as a representative of the tuna fisheries and shall give some consideration to its charac- 

 ter. The number of species of tunas, spearfishes, and sharks which are the principal catch of 

 the tuna longline fishery is, as will be detailed later, rather great. The majority of them 2ire, 

 from the biological point of view, distributed throughout the open sea from the Equator to the 

 vicinity of 35 - 40 N. latitude. The distribution in the Southern Hemisphere is not yet well 

 known, but it appears that the species are distributed to latitudes about as high as those at which 

 they occur in the Northern Hemisphere. The species have this extremely extensive range of 

 occurrence, however, the density of their occurrence is not the same everywhere within this 

 range, being subject to great fluctuations seasonally and presenting differences in the location 

 of the center of distribution of the various species. In other words, in different regions of the 

 sea the composition of the catch is different, and seasonal changes give rise to other variations. 



Put in very broad terms, the differences in the density of occurrence of the majority 

 of the species making up the resource of the tuna fishery appear to be mainly related to latitude. 

 This probably indicates that among the environmental factors which control the distribution of 

 these fishes, temperature is a great limiting condition. The foregoing, of course, only states 

 a general tendency and there are cases in which some species show a marked trend toward 

 variations in an east-west direction. In other cases it is thought that the density of distribution 

 is affected by topographical conditions. In still other cases, changes in the density of distribu- 

 tion from one region to another appear to accompany growth. However, an extremely broad 

 range of occurrence, a high degree of nnobility of the fish, and nnigrations extending over broad 

 areas of the sea are common characteristics of these species which make up the resource of the 

 tuna fishery, giving them a very different character from the resources of the coastal fisheries 

 or the bottom fisheries. 



The information which we have been able to gather concerning the spawning and 

 growth of these fishes is as yet extremely poor. Among the fishes taken by the tuna fishery, 

 the sharks are viviparous and therefore are already possessed of fairly strong powers of sur- 

 vival at the time they leave their mothers' body. The tunas and spearfishes are oviparous and 

 for sometime after hatching they have extremely poor powers of resistance. In the case of the 

 sharks, if we know the number of times they produce young in one year and the number of young 

 produced at one time, it is possible to clarify the question of their reproductive potential com- 

 paratively easily. In the case of the oviparous tunas and spearfishes, however, even though we 

 nnay be able to calculate the number of eggs contained in a matured ovary, we cannot easily as- 

 certain the proportion of those eggs that are fertilized and hatched. Furthermore, judging from 

 the fact that the larval and juvenile fish aire extrennely weak, it is not difficult to innagine that 

 attrition during the early stages of their life is at a rather high rate. In actuality it is by no 

 means rare to find tunas eind spearfishes of 10 to 40 cnn. in length ajnong the stomach contents 

 of adult fishes of the same species. From our present knowledge of several species, it appears 

 that the number of eggs spawned at one time by these species may range frona several hundred 

 thousand to several million, but it is difficult to believe that very many of these eggs grow into 

 mature individuals, and the attrition accompauiying growth is probably rather drastic. 



In the viviparous fishes like the sharks variations in reproductive potential from 

 year to year are probably comparatively sn-iall. We do not know much as yet about the number 

 of times the young are produced each year, but the nunnber of young sharks produced each time 



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