364 



Abstract.— The shapes and volumes of 

 swimbladders of yellowfin tuna, Thun- 

 nus albacares, were measured from 

 freshly caught fish from the eastern 

 Pacific Ocean. Direct measurements of 

 swimbladder volumes were obtained 

 from a geometric reconstruction with 

 morphometric measurements of intact 

 bladders and by volumetric displace- 

 ments of the same intact bladders 

 excised from 46 fish (57 to 157 cm in 

 length). The estimates of the swimblad- 

 der volumes obtained from geometric 

 reconstruction were not significantly dif- 

 ferent from those obtained with the cor- 

 responding volumetric displacements. 

 There is a nonlinear relationship be- 

 tween yellowfin swimbladder volumes 

 and fish lengths. The mean swimblad- 

 der volume was 1.339!: of body volume 

 vnth a minimum of 0.30% and a maxi- 

 mum of 2.84%. A comprehensive model, 

 based on the data from this study and 

 those from a previous investigation, is 

 presented for the relationship of yel- 

 lowfin swimbladder volumes, estimated 

 from geometric reconstruction and fish 

 lengths for 108 specimens (35 to 157 cm). 

 This predictive model was then used 

 with other formulae to estimate yel- 

 lowfin tuna swimbladder resonance fre- 

 quencies for fish lengths and fish depths. 

 Because these resonance frequencies are 

 within the range of frequencies audible 

 to yellowfin tuna, we speculate on the 

 potential distance at which dolphins 

 could be detected by yellowfin tuna. 



Shape, volume, and resonance frequency 

 of the swimbladder of yellowfin tuna, 

 Thunnus albacares 



Kurt M. Schaefer 



Inter-Amencan Tropical Tuna Commission 

 8604 La Jolla Shores Drive 

 La Jolla, Cahlornia 92037-1508 

 E-mail address kschaefer.a lattc ucsd edu 



Charles W. Oliver 



Southwest Fisheries Science Center 

 National Manne Fisheries Service, NOAA 

 PO. Box 271, La Jolla, California 92038 



Manuscript accepted 19 November 1999. 

 Fish. Bull. 98:364-374 (2000). 



The shape and volume of the swim- 

 bladder of yellowfin tuna, Thun- 

 nus albacares, is obviously impor- 

 tant because it functions as a 

 hydrostatic organ, which lowers the 

 energy costs of locomotion (Magnu- 

 son, 1973; Alexander, 1993). The yel- 

 lowfin swimbladder may also func- 

 tion in acoustic detection, providing 

 increased sensitivity in hearing, thus 

 enhancing the ability of fishes to 

 detect other organisms, such as 

 dolphins and prey (Iverson, 1967; 

 Hawkins, 1993). In addition, the 

 swimbladder may also function in 

 acoustical detection of tuna by other 

 species. At low frequencies (< 2 

 kHz ), the maximum acoustic target 

 strength occurs at a resonance fre- 

 quency determined by the volume 

 of the swimbladder (Love, 1978). At 

 high frequencies (2-200 kHz), the 

 swimbladder has been reported to 

 account for ?>0'7c (Harden Jones and 

 Pearce, 1958) to as much as 95% 

 (Foote, 1980) of the acoustic target 

 strength for some fish. 



Low-frequency acoustic detection 

 and tracking of yellowfin tuna 

 schools is being investigated by 

 the U.S. National Marine Fisheries 

 Service as an alternative method 

 of locating yellowfin tuna indepen- 

 dent of dolphins. Studies suggest 

 that yellowfin tuna schools could 

 be detected at much greater ranges 



(20 to 40 km) than are currently 

 feasible (Rees, 1998). Development 

 of an acoustic detection system 

 could greatly increase the efficiency 

 of commercial fishing and might 

 also provide a fishery-independent 

 method for assessing yellowfin tuna 

 or other large pelagic fish. 



Nero^ used two acoustic-scatter- 

 ing models to estimate the target 

 strengths of yellowfin tuna schools: 

 a model for very low frequencies 

 (50-1000 Hz) assumed to be near 

 swimbladder resonance (Feuillade 

 et al., 1996; Feuillade and Nero, 

 1998), and a high-frequency (2-200 

 kHz ) model for frequencies well above 

 swimbladder resonance (Love, 1977; 

 Love, 1981). Yellowfin swimbladders 

 were modeled as gas-filled spheres 

 ( Feuillade et al. , 1996 ). Nero's models 

 ( 1996) included swimbladder volume 

 estimates of approximately 5*?^ offish 

 volume for yellowfin tima in excess 

 of 80 cm in length, extrapolated fi-om 

 Magnuson's (1973) relationship of 

 swimbladder volume to fish length for 

 yellowfin tuna 44 to 82 cm in length. 



The objectives of our study were 

 1) to obtain direct measurements 



• Nero, R. W. 1996. Model estimates of 

 acoustic scattering from schools of large yel- 

 lowfin tuna. Report NRL/MRy774-95-7708. 

 Naval Research Laboratory, Ocean Acous- 

 tics Branch, Acoustics Division, Stennis 

 Space Center, MS 39529-5004, 21 p. 



