370 



Fishery Bulletin 98(2) 



volumes at the surface for fish lengths (Fig. 

 6), and (ietermineti the percent increase in the 

 expected resonance frequency. The regression 

 coefficient (6=1.053) for the linear relation- 

 ship of swimbladder volume estimated by geo- 

 metric reconstruction to the volume estimated 

 from the equation for a prolate spheroid is not 



significantly different from 1 it, 

 P>0.05). Solving for b yields: 



0.05(21.106' 



V- 



-"depth 



depth 



-rta 



=1.84; 



(7) 



Resonance frequencies for swimbladders of 

 yellowfin tuna of various lengths and at vari- 

 ous depths were estimated (Fig. 7), using the 

 above equations and the predictive regres- 

 sion function for swimbladder volumes for 

 fish lengths (Fig. 6). Applying this correction 

 increased resonance frequencies between 5% 

 and 26% for yellowfin tuna from 40 to 150 cm 

 at depths from to 120 m. 



Discussion 



Swimbladder shape and volume 



Godsil and Byers (1944) described the shape 

 of the swimbladder of yellowfin tuna. Addi- 

 tional information regarding yellowfin tuna 

 swimbladder shape is provided through the 

 digital images of the various views of the 

 swimbladder (Figs. 1 and 2) and the morpho- 

 metric information presented in Table 1. 



The swimbladder volume estimates derived 

 by geometric reconstruction in this study and 

 in Schaefer ( 1999) appear to provide realistic 

 representations for swimbladders of live yel- 

 lowfin tuna swimming at the surface of the 

 ocean. All of the intact swimbladders within 

 the abdominal cavities for those specimens 

 were sufficiently inflated so that the bladders 

 were taut, rather than flaccid. Many of the 

 yellowfin tuna stomachs were relatively full, 

 and there was a broad range in the stage of 

 gonadal development. There was no extrusion 

 of the bladder or other organs from the ini- 

 tial incision into the abdominal cavity for any 

 specimen. Thus, it does not appear there were 

 any deformations or reduction in swimblad- 

 der volumes within the body cavity similar to 

 those described for cod (Ona, 1990). In addi- 

 tion, yellowfin tuna are physoclistous, and the 



