394 



Fishery Bulletin 97(2), 1999 



curves for statewide returns (that is, time at liberty 

 for all recaptured Cross Seamount yellowfin and big- 

 eye tuna, regardless of recapture location) were not 

 significantly different (P=0.45, Fig. 3). 



In addition, there were more recaptures of bigeye 

 tuna with longer periods at liberty than of yellowfin 

 tuna at the seamount and the longest time at liberty 

 was 169 days for bigeye tuna compared with the long- 

 est point-of-release recapture of 93 days for yellow- 

 fin tuna. 



100 



500 



10,0- 



50- 



1.0 



05 



Q) 



Q. 

 CO 

 CJ 

 OJ 



(D 

 o 



^_ 



CD 

 CL 



50 



100 



150 



Days at liberty 



1000 



500 



10.0 



5.0- 



1.0 



0,5 



A 



Bigeye 

 Yellowfin 



50 



100 



150 



Figure 3 



Tag recapture attrition cur\'es (.semi-log plots with 9.5'^/ confidence contours) for 

 bigeye and yellowfin tuna tagged and released at Cross Seamount. (A) All recap- 

 tures from all locations; (B) Recaptured at Cross Seamount. 



Discussion 



The main thrust of this analysis was to compare the 

 residence times of the two tuna species at Cross Sea- 

 mount. Consequently, many complicating factors of- 

 ten associated with analysis of tag-and-recapture 

 data could be avoided (especially the impact of vari- 

 ability of fishing effort on the temporal pattern of 

 recaptures) because effort could be assumed to be 

 equal for both species. Therefore, the difference in 

 the tag attrition curves, and 

 the resultant difference in resi- 

 dence times ("half-life"), prob- 

 ably reflect real differences in 

 the behavior of these two spe- 

 cies at Cross Seamount. The 

 similarity of the recapture 

 curves for the entire area (Fig. 

 3) further suggests that meth- 

 odological or experimental bi- 

 ases are not responsible for the 

 differences in attrition curves 

 obtained for these two species at 

 Cross Seamount. 



These results differ from 

 those of Fonteneau (1991) who 

 observed no differences in the 

 temporal characteristics of re- 

 captures of tagged bigeye, yel- 

 lowfin, and skipjack tunas re- 

 leased at a seamount in the 

 tropical Atlantic. 



Fisheries data (Hanamoto, 

 1976, 1987) and acoustic track- 

 ing (Holland et al., 1990) both 

 indicate that the open ocean 

 behavior of bigeye and yellow- 

 fin tuna is different. Bigeye tuna 

 select colder waters and are 

 therefore usually found deeper 

 than yellowfin tuna, which ori- 

 ent principally to the top of the 

 thermocline and the mixed sur- 

 face layer. However, this verti- 

 cal separation breaks down 

 around fish aggregating devices 

 (FADs, Holland et al., 1990) and 

 floating objects where bigeye 

 tuna move closer to the surface 

 and overlap in vertical distribu- 

 tion with yellowfin tuna. This 

 same effect occurs at Cross Sea- 

 mount where both species are 

 caught in surface schools and 

 bigeye tuna outnumber yellow- 



200 



B 



Bigeye 



— Yellowfin 



200 



