Schaeffer and Oliver: Shape, volume, and resonance frequency of the swimbladder of Thunnus albacares 



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of swimbladder shapes and volumes from freshly 

 caught yellowfin tuna 50 to 150 cm in length, 2) 

 to compare the swimbladder displacement volumes 

 with volumes estimated from geometric reconstruc- 

 tion, 3) to compare the swimbladder volumes esti- 

 mated from geometric reconstruction for freshly 

 caught and frozen-and-thawed specimens, and 4) 

 to calculate swimbladder resonance frequencies as 

 functions offish length and depth. 



All four objectives are directly related to designing 

 a low-frequency, long-range acoustic detection system 

 to locate large yellowfin tuna. The first objective pro- 

 vides direct measurements of the relation between 

 swimbladder volume and fish length, upon which are 

 based acoustic target strength estimates. The previ- 

 ously modeled relationship was based upon smaller 

 yellowfin tuna (Magnuson, 1973; Nero, 1996). The 

 second and third objectives were to ascertain the 

 practicality of obtaining swimbladder volumes by 

 means other than volumetric displacement of the 

 swimbladder from freshly caught fish, thus providing 

 both a simpler method and wider sampling opportu- 

 nities. The last objective allows incorporation of fish- 

 ery information (fish size and swimming depth) to 

 select a particular frequency, or frequency range, in 

 order to optimize an acoustic detection system. 



5 mL. The swimbladder was then punctured and the 

 volumetric displacement of the tissue measured. The 

 estimated volume of gas in the swimbladder was calcu- 

 lated as the difference between displacement volumes 

 of the inflated and deflated swimbladder, including 

 the attached extraneous tissues. The volume of the 

 swimbladder wall was not determined and should be 

 considered insignificant, being extremely thin, with 

 respect to volumetric displacement. 



Estimates of swimbladder volumes were also com- 

 puted by a geometric reconstruction from the blad- 

 der's length and width measurements. Based on 

 the above four measurements of each bladder, an 

 algorithm was employed to estimate the volumes 

 between several cross sections. Cross sections of the 

 swimbladder were assumed to be elliptical. The ros- 

 tral and caudal ends of the bladders were assumed 

 to be hemispheres. The total estimate of the volume 

 of gas within the swimbladder was the sum of all 

 the geometrical units. Estimates of swimbladder vol- 

 umes by a geometric reconstruction for 62 frozen and 

 thawed yellowfin tuna, ranging in length from 35 to 

 149 cm (Schaefer, 1999), were included in this study 

 for both comparative and comprehensive analyses. 



Results 



Materials and methods 



Yellowfin tuna specimens were caught by rod and 

 reel. Seventeen specimens, 57 to 70 cm in length, 

 were collected from a skiff during October 1997, in 

 the vicinity of the Frailes Islands (lat. 7°20N, long. 

 80°08'W). An additional 29 specimens, 71 to 157 cm in 

 length, were collected aboard the MW Royal Polaris, a 

 San Diego-based long-range sportfishingboat, during 

 January and February 1998, primarily in the vicini- 

 ties of Alijos Bank (lat. 24=49N, long. 115°56'W) and 

 Hurricane Bank (lat. 16°52'N, long. 117°30'W). 



Freshly caught specimens were assigned an iden- 

 tification number. Fish length was measured with a 

 caliper to the nearest millimeter and fish weight was 

 measured with an electronic balance to the nearest 

 pound. Cutting the abdominal cavity open from the 

 anus to the isthmus and removing most of the vis- 

 cera exposed the swimbladder. A photograph of the 

 intact swimbladder was then taken with a digital 

 camera. Morphometric measurements of length and 

 three widths (rostral, medial, and caudal) were taken 

 for each intact swimbladder, with a dial caliper, to the 

 nearest tenth of a millimeter. The inflated swimblad- 

 der and some extraneous tissue were excised from the 

 abdominal cavity, and the volumetric displacement 

 was measured in a graduated cylinder to the nearest 



Swimbladder shape and volume 



Photographic images of the ventral, dorsal, left, and 

 right profiles of an excised intact swimbladder of 

 yellowfin tuna provided definitive views of the over- 

 all shape (Fig. 1). The swimbladder is cylindrically 

 shaped and has medial bulging and hemispheric 

 ends. There are paired protuberances on the rostral- 

 dorsal surface. The protuberances are commonly of 

 unequal size (the left is larger than the right) and 

 they fit into sockets located on each side of the ver- 

 tebral column. As size increased in the yellowfin 

 specimens we examined (Table 1), the ratio of the 

 swimbladder length to the width remained fairly 

 constant, around a mean of 3.1 (range: 2.2-4.8). The 

 swimbladder shape, however, changed noticeably, 

 particularly at the caudal end (Fig. 2). 



The swimbladder has thin elastic walls except for 

 the thicker walls of the protuberances. The dorsal 

 surface of the swimbladder is attached to a sheet 

 of thick connective tissue, situated along the dorsal 

 wall of the abdominal cavity adjacent to the verte- 

 bral column. In larger specimens, there is a promi- 

 nent cord of connective tissue originating from the 

 posterior area of this tissue and extending anteriorly 

 to the area of the sockets at the rostral end of the 

 swimbladder. This distinct cord of connective tissue 



