continues over a very long period of time and throughout very- 

 extensive spawning grounds r There are hardly any data from the 

 southern hemisphere, so the situation there is not clear, but 

 there n^y be thought to be a possibility thatj, if we consider the 

 northern and southern hemispheres together, spawnj.ng may be taking 

 place somewhere throughout the whole yearo 



In the general viewp the higher the latitude of the sea areaj, 

 the shorter and more clearly delimited is the spawning season of 

 the fishes dwelling theroo There also seems to be a tendency for 

 the spawning grounds to be more sharply defined,, It goes without 

 saying that such p?ienomena are almost entirely controlled by tempera= 

 tureo In short « fishes of the cold seas have a short spawning 

 season and their spawning grounds are^ comparatively speakings 

 linjited to particular areas. In other wordSj, the propagation of 

 the resource is carried on within a restricted sea area during a 

 short period of time,- 



The best conditions for fertilization are that the greatest 

 possible number of ripe males and females should congregate at the 

 same time ^^dthin the smallest possible area„ The results of their 

 reproductive activity will be the production of a great number of 

 larvae in a short period of time. Unless the largest possible 

 number of these larvae attain maturity,, the basic objective of 

 reproduction will not be realized^ For this growth the most essen"= 

 tial and important condition is probably the presence of abundant 

 foodo It goes without saying that the ultimate food, not only of 

 larva© but of all fishes.„ is planktono 



The pi'oduction of plankton in cold seas is regulated by the 

 temperature^, and an explosive proliferation of plankton takes place 

 once a year» In temperate seas there are two such proliferations, 

 in the spring and autumnj, but the flowering is far less conspicuous 

 than it is in colder waters^ When we come to tropical waters it is 

 even less marked than it is in the temperate zone, and a particular 

 season of plankton abundance can hardly be detectedo 



If the spawning habits discussed above and the conditions of 

 plankton production in various sea areas are comparedj, it can be 

 seen that in each case the spawning habits are such as to provide 

 most adequately for the maintenance of the species„ 



The fact that the tunas spawning in the warm seas, where 

 seasonal quantitative changes in the plankton are slight and its 

 quantitative distribution is even and sparse^ spawn throughout broad 

 sea areas and long periods of time can be thought, from the ecologi- 

 cal point of vievTi, to be extremely reasonableo If„ like the fishes 

 of cold seas, the tunas spawned great numbers of eggs in a very 

 short period of time in a limited area, the larvae which hatched 

 out vrould quickly starve and it would be difficult to realize the 

 objective of maintaining the specieso 



77 



