TRAWLING FOR FORAGE ORGANISMS IN CENTRAL PACIFIC 



287 



sideration Keid's (1959) report of a weak easterly 

 countercurrent near 10° S. 



Variation in Size of Catch 



Latitudinal cariatiotu. — Since the major en- 

 vironmental boundaries or discontinuities in the 

 central Pacific are zonal, i.e., east-west in aline- 

 ment, we assumed that variations in trawl catch 

 associated with longitude would be of much less 

 significance than the latitudinal variations. 

 Therefore, in order to examine major variations 

 in the data with respect to the current system, the 

 catch data for the 6- and 10-foot Isaacs-Kidd 

 trawls were combined over the longitudes sampled 

 but segregated according to the latitudinal zones 

 we have defined. 



The results, shown in figure 13, demonstrate a 

 marked variation with latitude in the abundance 

 of forage organisms sampled with the Isaacs-Kidd 

 trawls. With respect to volume,^ the curves were 

 similar for both trawls, with peak values recorded 

 for each trawl in the same zones. With respect 

 to number of organisms, the catch was similar for 

 the two trawls except in the most northern zones, 

 where tlie data were strongly influenced by a few 

 large catches of euphausiids and barnacle larvae. 

 These organisms were particularly abundant in 

 catches of the 6-foot trawl on Manning cruise 22 

 in September 1954 and account for the major dif- 

 ference between the two trawls in the Aleutian 

 Current and in the North Pacific Current. Both 

 trawls, however, captured the largest numbers of 

 organisms in the northern zones, with a secondary 

 peak at the Equator. 



The least productive areas, with respect to both 

 volume and number of organisms, were the South 

 Equatorial Current south of latitude 5° S., the 

 North Equatorial Current between about latitudes 

 10° N. and 18° N., and waters around the Ha- 

 waiian Islands. 



One interesting feature of the data, illustrated 

 in figure 13, is the increase of variance, particu- 

 larly in numbers of organisms, in the northern 

 zones. This increase is indicated by the width of 

 the 0.95 fiducial interval shown for each mean. 

 McGary et al. (1958) have pointed out that this 

 nortliern region is an area of great complexity and 



^ The volumes used In this comparison were the total catches 

 minus unusually large or bulky organisms constitutinK approxi- 

 mately 50 percent or more of the catch. 



^ 6000 



z 



I 



^ 5000 



o 



I 

 a. 



'KJOO 



O 3OO0 



•4556 



2 I 



6 - FOOT ISAACS-KIDD TRAWL 

 10 -FOOT ISAACS-KIDD TRAWL 



0} 



9409 



I9« 5» 5" 10' IS* 29* 35* 50" 



S— LATITUOe-N 



Figure 13. — Variation with latitude in average volumes 

 and numbers of organisms captured per hour of hauling 

 with the 6- and 10-foot Isaacs-Kidd trawls. (The 

 limits of the 0.95 fiducial interval are indicated for each 

 mean ; the number of samples for each area is shown 

 in parentheses. Zones defined in figure 12.) 



of wide fluctuations, hotli seasonally and other- 

 wise, in such features as temperature, salinity, 

 pliosphate concentration, and zooplankton abim- 

 dance. The high variance in the trawl catches 

 may be related to these fltict nations in the environ- 

 ment in the northern waters, as contrasted with 

 the comparatively stable conditions in tropical and 

 subtropical waters. 



Although the data obtained from the 1-meter 

 ring tiawl and the G-foot beam trawl were not of 

 sufficient geographic coverage to treat in a de- 

 tailed manner, the catches of the beam trawl did 

 sliow a marked ditference between the two major 

 ai-eas sampled. For live night hauls of Manning 



