290 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



NORTHERN BOUNDARY OF COUNTERCURRENT 



/ ._.. 



• TRawL STATIONS 



140° 130 



WEST LONGITUDE 



Figure 17. — Variation In trawl catch (volume in milliliters 

 per hour of hauling), of the 10-foot Isaacs-Kidd trawl 

 along the northern boundary of the Countercurrent, 

 Hugh M. Smith cruise 31, * September-October 1955. 



with the catch volumes increasing to the westward 

 between longitudes 135° W. and 151° W. 



Austin and Kinkel (1958) have shown that there 

 is an east-west variation in upwelling at the Equa- 

 tor with the maximum occurring in the eastern 

 Pacific and the period of most active upwelling 

 being August through October, which is the period 

 of this cruise. King and Hida (1957a) found a 

 gradient of decreasing zooplankton abundance 

 along the Equator between longitudes 140° W. 

 and 180°. Austin (1958) has shown that as the 

 surface currents carry the newly upwelled water 

 to the westward it "ages," i.e., it becomes wanner, 

 its content of inorganic phosphate is reduced, and 

 the thermocline deepens. We realize that the 

 following explanation is an oversimplification of 

 a very complex series of events, but we hypothesize 

 that the reduction in the zooplankton standing 

 crop to the west of 140° W. may be the result of 

 heavy predation by an increasing population of 

 forage organisms. 



Differences betiveen o'uises. — The midwater 

 trawling study was not conducted in a manner to 

 permit the proper evaluation of differences related 

 to seasons for any of the areas sampled. In three 



instances, however, an area was visited on two 

 cruises at different times of the year, or in different 

 years, which provided some comparison of differ- 

 ences between cruises that most likely were related 

 to seasonal or annual changes in the environment. 



The mean volumes obtained with the 6-foot 

 Isaacs-Kidd trawl on Planning cruise 20 (April- 

 May 1954) and Manning cruise 24 (March-April 

 1955) to the equatorial Pacific were not signifi- 

 cantly different {t = 0.769, P>OA). 



The catch volumes obtained with the 6-foot 

 Isaacs-Kidd trawl on Manning cruise 22 (Septem- 

 ber-October 1954) and on Smith cruise 27 (Jan- 

 uary-February 1955) to tlifi central North Pacific 



0° 

 LATITUDE - 



Figure 18. — Variation in trawl catch (volume in milliliters 

 per hour of hauling) of the lO-foot Isaacs-Kidd trawl 

 on four north-south sections, Iliiyh M. Smith cruise 35, 

 August-October 1956. 



