a partial report on studies of the ecology of tuna and plankton. This 

 material is drawn from a report of biological research done by the Palau 

 Tropical Biological Research Station for the South Seas Government-General 

 Fisheries Experiment Station. 



(A) Relationships between types and quantities of plankton and ocean 

 currents 



Studies of this type in tropical waters, particularly the Inner South 

 Seas area and equatorial waters, have been very few. About the only ones 

 which have been made are the quantitative studies of Motoda (1938) and 

 Marukawa (1939) and the qualitative study by Haneda (1938). 



1„ The marine plankton of the South Seas region consists mostly 

 of copepods, schizopods, chaetognaths, pteropods, mollusks, larval crabs, 

 polychaetes, jellyfish, radiolarians, dinoflagellates, diatoms, and 

 blue-green algae <> In the Northern Equatorial Current the copepods are 

 most conspicuous while the diatoms predominate in the Counter-current. 



2. Larvae of bottom-dwelling animals which are the mature forms 

 of coastal plankton are rarely found in pelagic waterSo (Revealed by the 

 inspection of plankton material) 



3c The togenashi eboshimi.jinko / a daphniid_7 is taken only in 

 the Northern Equatorial Current area. 



U„ Diatoms are poor both in quantity and number of species in 

 the Northern Equatorial Current, but in the Counter-current area they are 

 remarkably abundant in both respects, a fact which shows a conspicuous 

 difference between the two areas. 



5o The blue -green alga Lyngbga /^?_7 is distributed throughout 

 the whole area. 



6o Seasonal variations in composition and quantity of the marine 

 plankton of the South Seas area can be detected. The various larval 

 forms appear more abundantly in the plankton in the autumn than in the 

 spring. The quantity of plankton increases in the spring and autumn, 



(B) Stomach contents of the yellowfin tuna 



A study extending over one year was made of the stomach contents, but 

 most of the material recovered was so far digested that it could not be 

 identified below the genus. 



It is difficult to distinguish any difference between the sexes with 

 regard to the total quantity of the stomach contents. Judging from the 

 time at which the fish were taken and the progress of digestion it appears 

 that the fish feed at night. If we classify the stomach contents we find 

 that they all consist solely of oelagic planktonic animals. It is especially 

 surprising that in many cases large numbers of fish such as a species of 

 trunk-fish and hard-shelled mollusks like the paper-nautilus are found. In 



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