170°W. longitude during March 1952which allow 

 the same conclusion, i.e., the longline gear 

 appears to sink deeper at several degrees north 

 of the Equator than in the Immediate vicinity of 

 the Equator. The deepest hooks at the Equator 

 reached 285 ft.; at I'Zl'N. , 300 ft,; at Z'SPN. , 

 455 ft. ; and at 4°10'N, , 520 ft. 



The depth of the deepest hooks varies 

 with time throughout the year. Figure 5 shows 

 trend lines drawn by eye through the average 

 depths of the deepest hooks of longline sets 

 with respect to time for three arbitrarily selected 

 areas, O'-Z'N. , 2''-4''N. , and 4"'-6°N. latitude. 

 In each of these areas there is an indication that 

 the variation with time is of generally similar 

 magnitude and seasonal pattern for the various 

 cruises. Although some of the variation within 

 months maybe the result of different operational 

 procedures by the different vessels, it would 

 seem that seasonal changes in current direction 

 and velocity might be primarily responsible for 

 the major variation in hook depth throughout 

 the year. 



There is some indication that the depth 

 of the deepest hooks on the longline gear is also 

 affected by peculiar current conditions immedi- 

 ately surrounding islands. In general the average 

 depth of sets made less than 10 miles from 

 islands was slightly deeper t h a n those farther 

 than 10 miles from land (fig. 4). We can only 

 speculate on the cause for this, but it seems 

 reasonable that in some locations near islands 

 the current velocity was reduced and this in 

 turn allowed the gear to reach a greater depth. 

 Such a situation could conceivably have existed 

 in the lees of the islands or perhaps upstream 

 where the islands had a damming effect. 



O 

 O 



X 



o 

 o 



7 - 



6 - 



^ 3 







T 



T 



T 



1 r 



• INSULAR STATIONS 

 o OCEANIC STATIONS 



\ 



3_ 



_1_ 



^ 8 

 ■?°o ° 1*8 



40 50 60 70 80 90 100 



AVERAGE DEPTH OF DEEPEST HOOK (FM.) 



Figure 6. --Relationship of the average depth of 

 the deepest hooks of longline sets and yellow- 

 fin catch rates. 



depths averaging between 60 and 80 fathoms. 

 Any further conclusions about fishing depths 

 that may be offered now would be mere con- 

 jecture, but perhaps with more data it would be 

 possible to more clearly delimit the optimum 

 fishing depth for yellowfin in this area. 



EFFECT OF DEPTH OF GEAR ON CATCH RATES 



While it has been known for some time 

 that the catch rates of yellowfin are generally 

 higher on the deeper fishing hooks, we have not 

 previously had nieasurements to determine the 

 absolute depth of the gear. Now that we have 

 these measurements at each longline station, 

 we have exannined the variation in catch rates 

 among sets to see if it were related to differ- 

 ences in hook depth. In figure 6 we have plotted 

 the yellowfin c a t c h rates against the average 

 depth of the deepest hooks of longline sets. Sta- 

 tions where there was only one depth determina- 

 tion available and stations where no yellowfin 

 were caught were not plotted. The data show 

 considerable variation in both depth of the sets 

 and catch rates. Looking first at the oceanic 

 stations, the highest catch rates were obtained 

 at stations where the deepest hooks fished at 



The insular catch rates were generally 

 higher than the oceanic catch rates regardless 

 of the depth of the gear. This suggests that 

 there are actually greater numbers of yellowfin 

 close to Islands 2uid that the catch rates are not 

 necessarily related to the depth of fishing, at 

 least within the depths that we sampled. 



CATCH COMPOSITION 



Different habitats were sampled by the 

 two fishing methods and the species and size 

 composition of the catches varied with the local- 

 ity and method of capture. Trolling near the 

 various islands yielded, in addition to tunas, 

 fishes associated with the reefs in the upper 

 layers of the sea. Longlining, on the other 

 hand, sampled the deeper strata in the ocean 

 at distances greater than 5 miles from land and 

 caught larger pelagic fishes for the most part. 



