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Fishery Bulletin 98(2) 



tuna schools of 80, 100, and 130-cm fish respectively. 

 The decreasing trend in school target strength as 

 fish length and bladder volume increases, as shown 

 in Nero (1996), results primarily from the reduced 

 number of fish in a modeled 15-kg school. Some 

 decrease in Nero's reported school target strengths 

 could result from the smaller swimbladder volumes 

 that we measured for fish of similar lengths, depend- 

 ing upon the swimbladder's contribution to target 

 strength at high frequencies (Harden Jones and 

 Pearce, 1958; Foote, 1980). 



Nero ( 1996 ) estimated low-frequency target strengths 

 for schools of yellowfin tuna at various depths, fish 

 lengths, and packing densities but noted school target 

 strength was complicated because of interference and 

 coupled resonance effects dependent on the fish size, 

 numbers, aspect, and packing density. We expect Nero's 

 reported school target strengths, for low frequencies, 

 to decrease as a result of the smaller swimbladder vol- 

 umes we report. Because resonance frequency varies 

 inversely with swimbladder volume, overestimating 

 volume results in predicted lower resonance frequen- 

 cies. Direct measurement of resonance frequency and 

 target strength of in situ swimbladders would elimi- 

 nate the need to model these parameters and provide 

 better information to optimize an acoustic detection 

 system for large yellowfin tuna. 



Yellowfin tuna monitor their environment through 

 the use of sensory organs for visual, chemoreceptive, 

 and acoustic information. Although vision (Guthrie 

 and Muntz, 1993) and chemoreception (Hara, 1993) 

 are presumably important to yellowfin tuna in forag- 

 ing, sex, and social communication, acoustic sensory 

 capacities probably provide greater detection poten- 

 tial because of the light attenuation and chemical 

 dilution effects in the ocean ( Hawkins, 1993 ). Sounds 

 can travel great distances in the sea, depending upon 

 the sound propagation characteristics of the water 

 and the sound frequency and source level. Reception 

 and processing of sounds by fish presents the poten- 

 tial for detection at a greater distance than that by 

 either visual or chemoreceptive senses (Hawkins, 

 1993). Although the swimbladder of yellowfin tuna 

 may enhance their acoustic detection and their abil- 

 ity to detect sounds, the size and shape of their swim- 

 bladder does not appear to provide for any directional 

 information. Directionality in hearing, however, may 

 exist in yellowfin tuna based on the anatomy and 

 organization of the inner ears (Hawkins, 1993). 



The physiological behavior of yellowfin tuna and 

 the affect that it potentially has on the acoustic char- 

 acteristics of the swimbladder should be considered. 

 For instance, the swimming behavior of yellowfin 

 tuna, exemplified by vertical excursions, may enable 

 individuals to actively control the resonance frequen- 



cies of their swimbladders (Fig. 7) and to potentially 

 enhance their ability to sense their environment, as 

 previously proposed by Feuillade and Nero ( 1998) for 

 other fish with swimbladders. By varying the reso- 

 nance frequency of the swimbladder, yellowfin tuna 

 may be able not only to amplify acoustic signals but 

 also filter auditory signals and thus improve acous- 

 tic detection in the presence of high levels of ambi- 

 ent noise (Hawkins, 1993). 



Because estimates of yellowfin tuna swimbladder 

 resonance frequencies presented in our study were 

 within the range of frequencies audible to yellowfin 

 tuna (Iverson, 1967) and because swimbladders may 

 enhance yellowfin tuna hearing (Blaxter and Tytler, 

 1978; Blaxter, 1980), it is tempting to speculate 

 about the potential distance at which yellowfin tuna 

 could become aware of dolphins (Stenella spp. and 

 Delphinus delphis) or prey, predators, or conspecifics 

 through sound reception. Identification of a mech- 

 anism that facilitates the yellowfin tuna and por- 

 poise bond in the eastern Pacific (National Research 

 Council, 1992) may provide a means of breaking the 

 bond prior to setting nets that encircle dolphins, 

 thus enabling the capture of yellowfin tuna without 

 catching dolphins. If the mechanism is an attractant 

 (i.e. yellowfin tuna move towards the sounds of dol- 

 phins or other oceanic sounds, or towards the sounds 

 of both), then the possibility exists to attract larger 

 yellowfin tuna artificially with acoustical devices. 

 Active sounds produced by dolphins include clicks, 

 bangs, and whistles (Schevill, 1964; Tavolga, 1965; 

 Norris and Mohl, 1983; Watkins and Wartzok, 1985; 

 Marten et al., 1988) at peak frequencies as high as 

 160 kHz and peak source levels up to 228 dB re 

 IpPa (Au, 1993). Passive sounds resulting from tail- 

 slaps, breaches, and other behaviors have also been 

 described as loud (Hult, 1982; Smolker and Rich- 

 ards, 1988). The energy at frequencies between 50 

 and 1100 Hz is of particular interest because yellow- 

 fin tuna have been shown to respond to sounds in 

 this range — the most sensitive responses occurring 

 between 300 and 500 Hz (Iverson, 1967). 



Sound intensity decreases with range as a sound 

 propagates through the water, primarily because of 

 transmission loss associated with spherical spread- 

 ing of the wavefront and absorption (Richardson et 

 al., 1995). At 500 Hz, absorption loss is approxi- 

 mately 0.013 dB/km (Urick, 1983) and total trans- 

 mission loss can be approximated from spreading 

 loss alone over relatively short distances. We used 

 the best hearing sensitivity at 500 Hz reported by 

 Iversen (1967) for small yellowfin tuna (83 dB re 

 IpPa) as the minimum received source level (SL) a 

 tuna can hear. In the absence of published data on 

 SL at 500 Hz associated with low-frequency sounds 



