SUMMARY 



1. Nine Japanese mothership expeditions fished for tuna in the western equatorial Pacific 

 Ocean between June 1950 and October 1951. Fishing was carried on from 135 to 180 W. 

 longitude and from 1 to 10 N. latitude. 



2. Each catcher vessel maintained a daily log of its fishing activity. 



3. The two baits used, sardines and sauries, were about equal in terms of catch. 



4. With minor exceptions the vessels participating in the expeditions were about equal in 

 efficiency, indicating the catch rates could be used as indexes of abundance without ouijust- 

 ment. 



5. In the region just north of the Equator (1 - 5 N. ) the abundance of yellowfin tuna, bigeye 

 tuna, and black marlin does not change appreciably between 135 and 170 E. longitude. 



6. In the region just north of the Equator there were Sonne evidences of a mid-northern 

 summer and a mid-northern winter peak in abundance of yellowfin and bigeye tuna. Black 

 marlin did not show amy such peaks. 



7. In general, within the range of latitudes sampled, yellowfin tuna were nnost abundant between 

 the Equator and 4 N. latitude. Abundance of bigeye tuna appeared to increase north of 4 N. 

 latitude, as did that of black nnarlin also. 



8. The sizes of tunas caught indicate that the longline is sampling only a fraction of the 

 population- -the older, larger fish. Consideration of the size of longline skipjack leads to 

 the conclusion that there is an ecological separation by size of yellowfin, bigeye, and 

 albacore, with these species represented in the deeper waters mainly by large adults. 



9. Wind velocities, particularly those from the southeast quadrant, were of a low order of 

 magnitude when compared with those from the central Pacific. 



10. The pattern of latitudinal distribution of yellowfin tuna in the western Pacific, where 

 evidences of equatorial upwelling are lacking from the data at hand, is similar to the 

 pattern in the central Pacific, where evidences of equatorial upwelling are present on all 

 hydrographic sections examined. However, the absolute level of the population of yellowfin 

 was higher in the central Pacific in 1951 than in the western Pacific in 1950-51. 



11. Judging from pre-war published reports and post-war mothership data, tunas, in particular 

 the yellowfin, were considerably nnore abundant just prior to World War II than during 1950- 

 51 in the equatorial region of the western Pacific. 



