NOTE Holland et al.: Residence times of Thunnus albacares and T obesus over a seamount 



393 



cause nighttime is when tuna often make significant 

 horizontal movements away from their daytime 

 haunts (Holland et al, 1990; Marsac et al, 1995), 

 attrition curves were constructed only for fish recap- 

 tured for time at liberty >24 h. 



Results 



Between August 1995 and November 1996, 835 big- 

 eye and 458 yellowfin tuna were released at Cross 

 Seamount. The tagged bigeye tuna were between 40 

 and 105 cm FL; yellowfin released were between 40 

 and 90 cm FL. There were no significant differences 

 between the two species in the size distribution of 

 fish tagged and released (Fig. 2) or in size distribu- 

 tion of the recaptures. Because no effort was made 

 to preferentially tag a particular species, the ratio 

 of releases (65'7f bigeye, 2>b'^7c yellowfin) reflected 

 the ratio of species actually caught. Release and re- 

 capture data are summarized in 

 Table 1. 



The numbers of yellowfin and 

 bigeye tuna recaptured at Cross 

 Seamount were aggregated into 

 30-day periods of time at liberty 

 and plotted as percentages of to- 

 tal number of fish recaptured. 

 The resultant regression curves 

 for the recapture of each species 

 are shown in Figure 3. An analy- 

 sis of covariance indicated that 

 the slopes of the attrition curves 

 for the two species are signifi- 

 cantly different (P=0.013). The 

 attrition rate (slope of the regres- 

 sion line) for yellowfin tuna is ap- 

 proximately twice the rate for 

 bigeye tuna, and the slopes indi- 

 cate a residence time ( SO'/r recap- 

 tured) of 15 days for yellowfin 

 tuna and 32 days for bigeye tuna. 

 By contrast, the tag attrition 



Figure 1 



Chart of the study area showing position of Cross 

 Seamount in relation to the main Hawaiian Islands. 



Hxi 



■-I . ^~l 



Fork length (cm) 



Figure 2 



Size distribution of bigeye tuna and yellowfin tuna tagged and released at Cross 

 Seamount. Solid bars = bigeye tuna; open bars = yellowfin tuna. 



