TRAWLING FOR FORAGE ORGANISMS IN CENTRAL PACIFIC 



303 



Expressing the catch in terms of volume per 

 unit volume of water strained, we obtained an 

 averag:e catch for the 6-foot Isaacs-Kidd trawl of 

 1.86 ml./l,00() m.\ and an average of 1.65 ml./ 

 1 ,000 m.3 for the 10-foot Isaacs-Kidd trawl. These 

 volumes are approximately one-sixteenth to one- 

 eighteenth of the average zooplankton volume 

 obtained for the same general area. 



A nmch greater variety of organisms was found 

 in the trawl catches than in the tuna stomachs, 

 which might indicate some degree of selection by 

 the tunas. Most fishes in tuna stomachs were pe- 

 lagic — characteristic of the surface layer during 

 both day and night. The most abundant fishes in 

 the trawl catches were bathypelagic, occurring in 

 the middepths during the day and migrating to 

 tlie surface layer at night. A virtual lack of these 

 fishes in the tuna stomachs examined may indicate 

 that tunas feed principally in the daytime and in 

 the surface laj'er. Since the trawls were operated 

 almost entirely at night because the catches were 

 very poor during daylight and tuna fishing was 

 carried out during the day, it was not too sur- 

 prising to find a very poor correspondence between 

 the composition of the trawl catches and the con- 

 tents of tuna stomachs, with the similarities great- 

 ly outweighed by the diiferences. 



In 274 hauls made with the 4 types of trawls we 

 caught only 6 juvenile tunas, ranging in length 

 from 18 to 60 mm. On the basis of catches of 

 larval tunas made in zooplankton nets and our 

 knowledge of the distribution of the adults, we 

 believe juvenile tunas were present in these areas 

 at the time of the trawling. 



The greatest variety of organisms was captured 

 in the South Equatorial Current and in the 

 Countercurrent, the poorest variety in the North 

 Equatorial Current and Aleutian Current. Mid- 

 water trawls sampled principally primary and 

 secondary carnivores, some of which may also be 

 classed as herbivores and detritus eaters. Strictly 

 herbivorous animals were very scarce in the 

 catches. 



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