Adam et al : Dynamics of Thunnus obesus and T albacares in Hawaii's pelagic fisheries 



221 



45 

 40 

 35 

 30 

 25 

 20 

 15 

 10 



160- 

 140- 

 120 

 100 



80 



60 



40 



20- 







20 



40 



60 



80 



100 



120 



140 



Yellowfin Tuna 



+ Observed 

 Predicted 



20 



lll|i <tl |iii II I I I I 



60 80 



Length (cm) 



100 



—I — •- 



120 



140 



Figure 3 



Observed (crosses) and predicted (continuous lines) tag returns by initial release length 

 from the Cross Seamount for bigeye and yellowfin tuna. 



average gross attrition for the entire geographic range of 

 the model area was not very different for the two species 

 (0.033/day for bigeye and 0.034/day for yellowfin tuna). 

 Similar results were obtained in a preliminary analysis of 

 the early recaptures (Holland et al., 1999). In other words, 

 there are consistent indications that yellowfin and bigeye 

 tuna behave differently at Cross Seamount. 



The attrition rate measures the rate of loss from the 

 system. A more intuitive measure may be calculated 

 from "half-life" (ln(2)/Z,^) which is a proxy for population 

 residence (Holland et al., 1999). Essentially, half-life is the 

 time required to reduce an existing size of the population 

 by half The half-life of about 18 days for yellowfin tuna at 

 Cross Seamount was roughly one half that of bigeye tuna 



(31 days). Although the half-life across the size classes for 

 yellowfin tuna was similar, the half-life for the large size 

 classes of bigeye tuna were significantly longer than those 

 for the smallest size class (Table 6). 



Table 7 shows the ratios of the attrition components 

 to the total gross attrition for both species on the Cross 

 Seamount. Roughly TO'^r of the total loss is due to emigra- 

 tion. Fishing mortality accounted for about 12'r for each 

 of the three size classes of bigeye tuna whereas yellowfin 

 tuna F estimates were 7**, SC^f , and 20'7c. respectively. The 

 contribution of natural mortality to overall attrition in the 

 smaller size classes was substantial. This was 24'?^ for big- 

 eye and 35'7r for yellowfin tuna. In the larger size classes, 

 the contributions were in the range 6-16%. 



