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Fishery Bulletin 101(2) 



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Time at liberty (days) 



Figure 2 



Observed (crosses) and predicted (continuous lines) tag returns by time at liberty from the 

 Cross Seamount for bigeye and yellowfin tuna. 



The estimates of natural mortality rate for both species 

 were highest in the smallest size class. The estimates de- 

 creased gradually for both species, but for yellowfin tuna 

 there was a slight increase in the largest size class, yielding 

 a "U" shaped curve (Fig. 4). The estimates for bigeye tuna 

 were 0.00576, 0.00372, and 0.00181/day (2.102, 1.356, and 

 0.660/yr) for 29-55, 56-70, and >71 cm, respectively, and for 

 yellowfin tuna they were 0.01425, 0.00221, and 0.0036 1/day 

 (5.203, 0.806, and 1.316/yr) for 20-45, 46-55, and >56 cm, 

 respectively (Fig. 4 and Table 6). These estimates are with- 

 in the range of the values estimated by Hampton (2000) 

 from analyses conducted for fisheries in other regions of 

 the Pacific. 



Fishing mortality estimates are highly variable both 

 within the three size classes and between the sites (Fig. 5). 



For the Cross Seamount, F was nearly the same for bigeye 

 tuna over the three size classes (=0.0026/day) Yellowfin 

 tuna F, estimated for Cross Seamount, was higher for the 

 medium size than for the smaller and large size class (i.e. 

 0.0027, 0.0115, and 0.0067/day). 



The total attrition rate by size k and by site i can be cal- 

 culated from Z,^ = Mf, + F^f.+ A + ST,^, from which the aver- 

 ages for the size class or site may be obtained. Alternatively 

 these could be estimated from a model in which Z is kept 

 constant over the size classes. Although there were large 

 variations in the estimates for different sites, the estimates 

 were not appreciably different for each of the three size 

 classes at any particular site. At the Cross Seamount, the 

 gross attrition rate for yellowfin of 0.038/day was roughly 

 twice that for bigeye tuna (0.022/day). However, the 



