150 W. longitude; (3) Hugh M. Smith cruise 18, with lines of fishing stations across the Equator 

 along 120 W. and 130 W. longitude; and (4) John R. Manning cruise 13, with fishing sections 

 across the Equator on 150 W. and 169 W. longitude. 



The Cavalieri cruise warrants special tnention because it represented aji attempt at 

 commercially harvesting the equatorial longline tuna resource. During this cruise considerably 

 more gear was fished than during experimental surveys by POFI vessels. The design of the gear 

 was identical to that used by POFI, and the daily fishing schedule was comparable except that more 

 time was spent setting and hauling the greater number of baskets fished. 



The gear used on the four cruises covered by this report was similar in construction 

 to that used during previous POFI exploratory fishing, except that smeill amounts of specially de- 

 signed gear were fished during sonne trips. Complete descriptions of the "standard" gear that 

 formed the bulk of each set are furnished by Niska (1953) amd Murphy and Shomura (1953a). 



Briefly, each basket of the standard longline gear consisted of a main line 1, 260 feet 

 long suspended by 60-foot buoy lines and bearing six-hook droppers attached at 180-foot intervals. 

 These droppers made of cotton line and steel leaders were about 88 feet in overall length. At 

 each station the gear was set at dawn, and hauling commenced about noon. Setting took from 1 to 

 1.5 minutes per basket and hauling about 5 minutes per basket (tables 20-23). Thus, the opera- 

 tional aspect of these cruises was identical with past surveys except that different baits were used. 



On earlier cruises the bait had been almost exclusively sardine. During these cruises 

 other baits, such as herring and squid, and various methods for attaching the bait to the hook were 

 tested. The results of these experiments are not reported herein except to note that the experi- 

 mental baits and baiting methods did not appreciably alter the catch rates (Shomura MS), enabling 

 us to disregard this factor in our evaluation of the abundance of tunas. 



GEOGRAPHICAL VARIATION IN THE CATCH RATES 



The four cruises during the period August-November 1952 (fig. 1) provide estimates 

 of the abundance of deep-swimming tunas in the equatorial region over a wide range of longitudes 

 (120 W. to 170 W.). Previous fishing during this general season in 1950 and 1951 indicated a 

 concentration of yellowfin tuna between 1 N. eind 6 N. latitude at 150 W. to 160 W. longitude 

 (Murphy and Shomura 1953a). The results of the 1952 surveys provide a check on these earlier 

 findings and also extend the geographical coverage during the period August to Novennber. 



By way of review of previous results and the general concepts utilized in their inter- 

 pretation, it should first be pointed out that the prevailing winds in the mid-Pacific equatorial 

 region during the period August to Novennber are from the southeast quadrant (U. S. Pilot 

 Charts). According to Cromwell (1953), these winds create divergence and upwelling at the 

 Equator and tend to displace the upwelled water northward. This nutrient-rich upwelled water 

 supports a larger population of zooplankton than the waters to the north and south (King and De- 

 mond 1953), and it was expected that this increase in the basic animal food would be reflected 

 in the abundance of the larger fishes. Several fishing sections across the Equator indicated 

 that the tuna, ir. particular the yellowfin, were indeed more abundant in the zone of enrichment 

 (Murphy and Shomura 1953a, b). 



The results of these four latest cruises will be considered in the light of previous 

 findings, with attention focused on the yellowfin tuna catches, and the environmental circiim- 

 stances associated with these catches followed by a brief discussion of the apparent distribution 

 of albacore along the Equator. The catches of the other tunas are not singled out for special at- 

 tention, but are included in tabular form in order to indicate the relative levels of abundance of 

 the several species in the catch. 



