Schaeffer and Oliver: Shape, volume, and resonance frequency of the swimbladder of Thunnus albacares 371 



inflation and deflation of the swimbladder is 

 controlled through special glands that function 

 in the secretion or resorption of gases from, or 

 to, the blood (Alexander, 1993). Virtually noth- 

 ing is known about inflation and deflation rates 

 in swimbladders of tunas, but it would appear 

 from studies of other fishes (Alexander, 1993) 

 that swimbladder volume adjustments would 

 be extremely slow in relation to the rapid swim- 

 ming speeds during vertical forays by these 

 species (Holland et al., 1990). 



Geometric reconstruction of swimbladders in 

 yellowfin tuna was previously derived from radio- 

 graphs to estimate volumes, which were vali- 

 dated by volumetric displacement (Chang and 

 Magnuson, 1968). It is apparent from the results 

 of that study and our study that the geometric 

 reconstruction method is sufficiently accurate 

 for deriving estimates of volumes of swimblad- 

 ders of yellowfin tuna, and possibly other tunas 

 as well. Obtaining swimbladder volumetric data 

 by geometric reconstruction is more practical 

 than by volumetric displacement because of the 

 additional time required and potential of punc- 

 turing the swambladder when excising for deter- 

 mination of volumetric displacement. Furthemiore, 

 unless there are instances where it is not feasible to 

 cut open the abdominal cavity of specimens, it does 

 not appear to be necessary to employ an x-ray unit for 

 obtaining these estimates. 



Magnuson (1973) reported that swimbladder vol- 

 umes for 11 yellowfin tuna specimens, 44 to 82 cm 

 in length, ranged from around 0.25'^ to 4.0'7c of body 

 volume (estimated from Fig. 4a of Magnuson, 1973). 

 The swimbladder volumes in our study, derived from 

 volumetric displacements, expressed as a percentage 

 of the estimated body volumes (Fig. 4) had a mean of 

 about 1.3*^, with a range of about 0.3*^ to 2.849f, and 

 almost no relation with increasing mass. Swimbladder 

 volumes, fi-om the present study, for yellowfin tuna in 

 the length range presented in Magnuson ( 1973 ) appear 

 to be significantly lower (Fig. 4). The data of Magnu- 

 son (1973) were based upon measured volumetric dis- 

 placements of the fish, whereas in the present study 

 body volume was estimated, without adjusting for fish 

 density because those values were not available for 

 these specimens. However, we calculated the body vol- 

 umes from weights for specimens from the present 

 study, using an adjustment factor for density of 1.05 

 g/mL (Magnuson, 1973) and found swimbladder vol- 

 umes, expressed as a percentage of body weight, would 

 be increased by only 0.079f on average. This small 

 increase in volume does not account for the apparent 

 differences in swimbladder volumes between the pres- 

 ent study and those in Magnuson (1973). In addition. 



•2000 



1500 



600 



Figure 7 



Relation between estimated swimbladder resonance frequency, 

 fish length, and fish depth for yellowfin tuna. 



although Magnuson (1973) reported that specimens 

 of 2 kg or less have no gas in the bladder, Schaefer 

 (1999) reported yellowfin swimbladders are inflated 

 with measurable quantities of gas in specimens as 

 small as 0.85 kg (353 mm) (Fig. 6). 



Swimbladder resonance frequency 



Acoustic tracking studies have shown that yellowfin 

 tuna occupy the lower mixed layer during daylight 

 and waters closer to the surface at night (Carey and 

 Olson, 1982; Holland et al., 1990; Block et al., 1997). 

 Although they appear to make frequent short excur- 

 sions toward the surface, they spend very little time 

 at the surface. In the area of the eastern Pacific sur- 

 face fishery (Bayliff, 1998), the thermocline depth 

 ranges from about 40 to 120 meters (Fiedler, 1992). 

 Resonance frequency will change with depth because 

 volume is the primary determinant of the resonance 

 frequency of a swimbladder. Thus, the acoustic target 

 strength of a tuna, or school of tunas, will vary as 

 the swimbladder volumes vary at depth for low-fre- 

 quency acoustic detection systems. 



Nero (1996) modeled target strengths for schools 

 of larger yellowfin tuna for both high frequencies 

 (2-200 kHz), and low frequencies (below 2 kHz), 

 using an assumed swimbladder volume equal to 5% 

 offish volume for calculating resonance frequencies. 

 Nero's ( 1996) high-frequency model predicted target 

 strengths of 2.5, 1.6, and 0.9 dB re IpPa for yellowfin 



