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It is of considerable biological and commercial importance 

 to know at what depths the deep=swimniing tunas are most abundant. 

 Several Japanese workers (eogoj, Nakamura 1943,, O'-.hi 1952) have noted 

 that the catch rate of yellowfin and bigeye tuna is higher on the 

 deeper fishing hooks though they have been unable to ascertain the 

 precise level at which the hooks were fishingo 



The construction of the POFI longline gear permits 

 classifying the hooks according to three depth level So The potential 

 depths that these three levels can attain is fixed by the distance 

 between buoyso On Smith Cruise 11 the buoys were generally between 

 700 and 900 feet apeirto Utilizing the regression lines in figure 5, 

 the hooks could have been fishing at approximately the following 

 depths 8 



Shallow 310=>320 feet 

 Intermediate 450-490 feet 

 Deep 540=610 feet 



Actually the line probably streams out in response to the current 

 gradient between the surface and the thermoclinep for at one station 

 the depth of the main line on one basket was measured at 336 feetz/o 

 The potential depth of this line was 550 feeto The day was calm and 

 so the streaming effect must have been due to a differential currento 



Because the exact level of the hooks is not known, the 

 hook positions on the longlines are taken as an indication of rela= 

 tive deptho Tables 4 to 8 are chi=square analyses of the most 

 pertinent data from Smith cruises 7 and 11 (Snedecor 1946, po 188)c, 

 The data were separated into five groups by speoies, cruisesp and 

 type of gearo Tables 4 and 5 are analyses of the yellowfin tuna 

 catches for each of the two types of gear used on Smith Cruise 7o 

 Table 6 is an analysis of the bigeye catches for one type of gear 

 used on Smith Cruise 7„ The catches on the other type of gear were 

 not large enough to warrant analysiso Tables ? and 8 are analyses 

 of the yellowfin and bigeye catches of Smith Cruise llo The results 

 show very clearly that the deep hooks generally caught more bigeye 

 and yellowfin (tables 5'=8)j, although the shallow fishing gear used 

 on stations 1A.^ 15, 16, and 19 does not show this tendency (table 

 4, figo 4)0 It is of interest to note that the mean lengths of tuna 

 caught at the three levels of depth on Cruise 11 are almost identical 

 (1400 miiio J, 1406 mm„ , and 1399 mmo ), indicating that the three fishing 

 levels probably were sampling the same populationo 



Y 



The depth of the main line was measured by To Cromwello He 

 attached putty balls at regular intervals to a line and hung 

 it from a skiff using a heavy weight to keep it verticalc The 

 skiff was then drifted over the center of the main line,, The 

 sounding line was then retrieved and the position of the 

 shallowest dislodged putty ball notedo 



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