coastal in character, that sixth-year fish are corparatively pelagic, and that 

 in their seventh and eighth years the fish again nilgrate into the coastal waters. 

 Consequently a good deal of caution is necessary in deducing the composition of 

 the stock from the data gathered at various fishing grounds. 



Since it is thought that fish in their first and second years remain close 

 to islands and in bays, the ratio of first-year to second-year fish will prob- 

 ably give the survival rate for each locality. For the waters adjacent to 

 Numazu this is |||*^ s.l98. Aft^r they enter their third yrar the fish enter 



upon a ifiigratory existence and there is prob- 

 ably no great age diffeirential in the subsequent survival rates. However, there 

 are differences between the ages of the fish which occur in coastal waters and 

 those found in pelagic waters and this gives rise to differences in the ago com- 

 position of the catch. The survival irate for sixth-, seventh-, and eighth-year 



^^^^ ^® S'sIsO^O "•'''^^ ^°^ *^® waters adjacent to Numazu, and 



(?§^t'??.^ (^4"^8'^^ . = .^6 for the waters east of Taiwan, The former is a 



coastal fishing ground while the latter is a pelagic ground so the average .5oo 

 for the two can be said to be the survival rate for ^ixth-year to eighth-year 

 fish. On the Shigedera fishing grounds the survival rate for fish of the fourth 

 year and older is ^j' l^ lj^'^^fj'^^lj'^^^^^e'^i g ^ •5;29, which indicates that the 



excessively small number of fifth-year fish in th^ vaters adjacent to Numazu 

 probably represented a condition restricted to tlje year 1937. Consequently it 

 will probably be satisfactory to consider the svyvival rate of the fish after 

 they have entered upon a migratory life as .57. 



On the Pacific coast of the American continent and from Southern California 

 to the Lquator a considerable quantity of yeHowfin is taken along with skipjack. 

 On the northern grounds migratory schopls^are fished during a three-month sea- 

 son in August, September, and October/ but on the southern grounds the fishing 

 continues throughout the year. It is thought that the schools fished are mainly 

 migratory schools of young fish. These yellowfin of the Fastern Pacific, like 

 those of our '.festom Pacific waters, are probably related to the small yellow- 

 fin >irtiich reside permanently around the various islands which are scattered 

 over a wide area north and south of the Equator, but since we do not have enou^ 

 data to pursije these questions any further tit present it is recommended that we 

 proceed on the assumption that the yellowfin tuna of the eastern and western 

 Pacific should be treated as separate stocks. 



As was mentioned above, in the South Seas young yellowfin occur mixed with 

 skipjack, but we have no exact knowledge of their numbers. In the course of his 

 investigations Kimura found that in the Ogasawaras a fairly large quantity of 

 snail yellowfin is taken along with the larger fish, however, detailed knowledge 

 is lacking regarding the numbers of small and large yellowfin taken at other 

 places so a thorough treatment of this question will have to be left until a 

 later date, What we wish to postulate here is that among the stock of fish ^rtiich 

 come into the waters adjacent to Numazu the proportion of small and large yel- 

 lowfin is about equal and th^it there is no great difference in the rate of catch 

 for the two size groups. Assuming this to be the case, it appears from the 

 amount landed at the Numazu market that the survival rate of young yellowfin in 

 the stock as a whole is „75, If we assume that the natural mortality rate for 

 yellowfin is the same as for the black tuna f Thunnus orientalis "]. vte get a 

 fishing rate of .06 for the young fish and o29 for the mature fish.* 



