feeding behavior in tunas. By feeding on this 

 animal, yellowfin and skipjack tunas probably con- 

 sume a much greater fraction of the organic mate- 

 rial produced by photosynthesis in this area than 

 they do in other areas where herbivores are small 

 and carnivores make up most of the tuna diet (see 

 King and Iversen, 1962, table 15, for probable 

 trophic levels of several groups of tuna prey listed 

 above). 



QUANTITATIVE COMPARISON OF 



CATCHES BY 1.5-M. AND 



HIGH-SPEED NETS 



Reference was made previously to qualitative 

 diifei'ences between the catches of the 1.5-m. net 

 and the high-speed net; the example given was 

 the greater representation of flyingfish in catches 

 of the high-speed net. The high-speed net was 

 hauled much faster and much closer to the sea 

 surface, on the average, than the 1.5-m. net. It is 

 of interest to compare quantitatively certain area! 

 distributions of micronekton as measured by the 

 two nets for the cruises (TO-59-2 and TO-60-2) 

 for which both sets of material have been fully 

 worked up. The volumes were standardized, as 

 explained earlier, so that all volumes for both nets 

 are comparable in ml. of micronekton per 1,000 m.^ 

 of water strained. 



Figure 17 summarizes the distribution of total 

 micronekton (fish, crustaceans, and cephalopods 

 combined) in standard night hauls of the 1.5-m. 

 net on cruise TO-60-2; the volumes are the sum 

 of the volumes shown for the same stations in 

 figiu'es 6, 7, and 8. Figure 18 gives similar infor- 

 mation for night hauls of the high-speed net (only 

 hauls lasting between 2i/^ and 31/^ hours) on the 

 same cruise. 



As far as they are comparable, figures 17 and 18 

 show similar trends in distribution of micronekton 

 from place to place within the area, especially from 

 onshore (high volumes) to offshore (low volumes) 

 for the ai'ea as a whole. Standardized volumes for 

 the 1.5-m. net, however, tend to be about 9 or 10 

 times higher than those of the high-speed net, in 

 the same area on the same night. 



Figure 19 shows a similar comparison, for part 

 of cruise TO-59-2 for catches of red crab (Pleu- 

 roncodes planipes) only. As far as the 1.5-m. 

 net and high-speed net catches occur in the same 



areas, they are broadly comparable in general dis- 

 tribution — particularly as between the area east 

 of long. 112° W., where the crabs were scarce, and 

 the rest of the region, where they were abundant. 

 It is again evident that the volumes from the 

 1.5-m. net were generally much higher than vol- 

 umes from the high-speed net when both were 

 available at the same time and place. 



The reason for the difference between the vol- 

 umes is uncertain. Possible reasons that have been 

 advanced are: (1) the catches in the high-speed 

 net tend to get shredded by the fast towing so 

 that much material is lost through the meshes; 

 (2) micronekton is actually scarcer at 10 m., where 

 the high-speed net fishes, than it is over the whole 

 water column 0-90 m., where the 1.5-m. net fishes; 

 and (3) the micronekton at 10 m. is disturbed by 

 the ship, and some of it svibmerges or escapes lat- 

 erally before the net reaches it. Inspection of 

 catches shows that the first point probably does 

 not apply to the extent that would be required to 

 explain the observed difference, although it does 

 apply to some extent; for instance, it is common 

 to find fish reduced to skeletons in the high-speed 

 net. There is no information on which to base a 

 judgment on the other points. Aron (1962b) noted 

 that catches of the Isaacs-Kidd midwater trawl 

 were invariably low when the net was towed at the 

 surface and attributed this effect to disturbance 

 by the ship. 



SUMMARY 



Micronekton (fishes, crustaceans, and cephalo- 

 pods about 1 to 10 cm. long) was collected on 

 oceanographic cruises in most parts of the eastern 

 tropical Pacific Ocean, mainly from 1958 to 1961. 

 The most commonly used collecting method was 

 to haul a large net (mouth 1.5 m. square; length 

 about 5.8 m.) of uniform mesh size obliquely 

 through the upper 90 m. of water at night, at a 

 ship sjjeed of 5 knots. Catches (total micronekton 

 and its family components) were measured by dis- 

 placement volume ; these volumes were then stand- 

 ardized to 1,000 m.'' of water strained, by using 

 data on the length of the haul and an empirical 

 filtration coefficient of 76 percent. These measure- 

 ments of standing crop of micronekton are supe- 

 rior to those that can be obtained with mixed-mesh 

 nets or trawls of the same general size. 



MICROXEKTON OF THE EASTERN TROPICAL PACIFIC OCEAN 



109 



