FISHERY BULLETIN: VOL. 70, NO. 3 



after 1953 focused on special problems (Iversen 

 and Yoshida, 1956) and are not considered in 

 this report. 



This paper on the equatorial tuna is in part 

 documentary and summarizes the results of the 

 Laboratory's experimental fishing and scouting; 

 however, the major effort is a review of the dis- 

 tribution of yellowfin and skipjack tunas as it 

 relates to the distribution of properties in the 

 environment. To a lesser extent, we discuss two 

 other tuna species, the albacore and the bigeye 

 tuna. The results suggest that the abundance 

 of yellowfin and skipjack tunas in the region var- 

 ies according to the properties in the environ- 

 ment. The variation in abundance of deep- 

 swimming yellowfin tuna in space and time is 

 relatable to variations in the wind-driven ocean 

 circulation that alter the rate of enrichment of 

 the euphotic zone. How albacore and bigeye 

 tuna are related to the environment is less clear. 

 The occurrence of these species in deeper water 

 than yellowfin tuna suggests that other factors 

 may be of greater importance. The apparent 

 abundance of surface tunas is not believed to be 

 closely related to basic enrichment; rather, it 

 appears that secondary factors, such as food- 

 concentrating mechanisms, are more important. 



Most of the data in this report have been 

 published. The fish distributions have been de- 

 scribed by Ikehara (1953), Murphy and Sho- 

 mura (1953a, 1953b, 1955) , Murphy and Ikehara 

 (1955), and Shomura and Murphy (1955). The 

 oceanographic data were published by Cromwell 

 (1951, 1953), Austin (1954), Cromwell and 

 Austin (1954), Cromwell, Montgomery, and 

 Stroup (1954), Stroup (1954), and Stroup and 

 Austin (1955). The plankton catches were re- 

 ported by King and Demond (1953), Hida and 

 King (1955), and King and Hida (1957). 



SAMPLING METHODS AND MATERIAL 



Longlining, trolling, and surface sighting, the 

 latter supplemented by occasional live-bait fish- 

 ing, were used to assess the abundance of tunas. 

 None of these was completely satisfactory, chief- 

 ly because no single type of gear sampled all spe- 

 cies and sizes of fish within each species with 



equal efl'ectiveness. Further studies will un- 

 doubtedly reveal additional weaknesses in our 

 general assumption that catches represent rel- 

 ative density of tunas. 



Longlining 



More effort was devoted to longlining (Fig- 

 ure 1) than to the other sampling methods, 

 mainly because it proved to be the best way to 

 capture significant numbers of tuna systematic- 

 ally. 



Figure 2 shows the type of longline gear used 

 during the survey. Niska (1953) gave details 

 of its construction and operation. Basically the 

 gear consists of (1) a mainline suspended at in- 

 tervals from buoys and (2) branch lines that 

 hang from the mainline, each with a baited hook. 

 The amount of tackle between two buoys is called 

 a "basket." 



The mainline hangs slack in the water, pre- 

 sumably in the shape of a catenary (Figure 2). 

 Since the mainline was 384 m long and the buoys 

 were generally spaced about 274 m apart, the 

 deepest hooks should have fished at about 152 m 

 and the shallowest at about 76 m. The gear, 

 however, fished at shallower depths than the ex- 

 pected depth because of diflferences in the forces 

 exerted on the buoys by wind rnd surface cur- 

 rents and on the mainline by the deeper, some- 

 times opposing currents. Consequently the deep 

 hooks may frequently have fished as shallow as 

 76 m (Murphy and Shomura, 1955). 



At a typical longline station, 40 to 60 baskets 

 of gear were set in the early morning and re- 

 trieved in the afternoon. 



The longline catches are presented as numbers 

 of fish per 100 hooks. Valid comparisons of 

 availability can be based on this statistic, pro- 

 vided the construction of the gear and the oper- 

 ational details remain constant. Fortunately all 

 of the catches were obtained in a nearly stan- 

 dardized manner that satisfied most of the as- 

 sumptions, e.g., construction did not vary, rough- 

 ly the same amount of gear was fished each day, 

 and the fishing schedule was the same each day. 



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