^» 5iS condition factor in skipjack o The condition factor 

 is the so-called "qijiality indxcator" (10^/l3) ,, Kimuraj/ has 

 used it to find one of the characterastics of sardine populationso 

 Tiis condition factors of the skipjack schools of the Rjnikyu Sea 

 Area are broadly distributed between 13-35 (figo 3) and the 

 differences due to age are slight (table 5)o If we divide the 

 schools into those in which the average condition factor j, or at 

 least the mode, is under 20 (shown with black circles in all 

 figures) and those in which it is over 20 (shown with white circles 

 in the figures), and then consider the distribution of the posi'^- 

 tions at which catches were made of each (flgo k) s the lean fish 

 beloTT 20 are taken in large numbers around the small islets and 

 shoals (banks) stretching from Yakushima and Tanegashima to 

 Ajnand Oshima in the north, and to the south many of them are 

 taken on the shoal groimds between Miyakojima and Kumeshimag but 

 they are generally scarce throughout the offshore fishing grounds 

 northwest of Okinawao The fat fish^ however j are taken at all 

 grounds at the -game rate. The proportion of catches made from 

 each group in a three-year total was 21 times for the lean group 

 to 19 times for the fat group on the northern shoal grounds j and 

 11 times for the lean group to IiS times for the fat groip on the 

 central offshore grounds, ifrtiere the number of catches from the 

 lean group was conspicuously lower o On the southern shoal grounds 

 the proportion of catches from the lean group again increases 

 with 32 catches of lean fish to 20 catches of fat fisho There is 

 no departure from the trend for each fishing ground in any year 

 (table 6)o This indicates that the lean schools are somehow 

 closely related to islets and shoals o The skipjack schools of 

 the Ryukyu Sea Area are divided ecologically into the two cate- 

 gories of migratory schools and sedentary schools o In the migra- 

 tory schools fat schools make up 63^.j and in sedentary schools the 

 lean schools make up 68^ of the whole (table 6) o Accordingly it 

 can be said that the migtatory schools are those that have a 

 condition factor of over 20 and have no selectivity within the 

 range of their migrations^ while the sedentary schools are those 

 that have a condition factor of less than 20 and reside permanently 

 chiefly in the waters adjacent to islets and shoalso As two or 

 three characteristics irixLch should be addeds (1) mth the migra- 

 tory schools, in the total landings for one fishing season the 

 fifth-year fish are more numerous than the fourth-year fish, but 

 with the sedentary schools the opposite is trueo (2) In the 

 migratory schools the condition factor of the fifth-year fish 

 is lower than that of the fourth-year fish, but in the sedentary 

 schools it is higher, (3) Both the average and the mode of the 

 body length are greater for the migratory schools than for the 

 sedentary schools (table 7) Csic)o However, the difference in 



3/ Kimura, Kinosukes Bullo Japo Soc„ Sci o Fish 3 (6),1935o 



39 



