FISHERY BULLETIN: VOL, 77, NO 4 



Table 4. — Swim bladder size and calculated depths for 12 kHz resonance, assuming regulation of swim bladder 

 volume for the specimens of Triphoturus mexiLunus, Protomyctophum crockeri, Argyropelecus sladeni, and 

 Vinciguerria lucetia in our collections. 



'Where swim bladder would resonate at 12 kHz, assuming constant volume at all depths. 



high value of - 60.6 dB at 284 m to a low value of 

 -71.3dBat206m. 



The change in calculated target strength with 

 depth could be due to two factors: either the densi- 

 ty of fishes per cubic meter decreased with time or 

 the target strength decreased due to the changing 

 orientation of the migrating fishes. The second 

 explanation is more likely for two reasons. First, 

 the thickness of the scattering layer appears to be 

 constant over the period where target strengths 

 were calculated (1716-1741 h, Figure 2). Second, 

 the increase in the migratory rate of the layer over 

 time (Figures 2, 3) implies a more rapid, upward 

 swimming of the fishes. This would result in a 

 more vertical orientation of the fish in the water 

 column. 



The calculated target strengths for the juvenile 

 T. mexicanus at 102 kHz (Table 2, Figure 4) can 

 only be compared with theoretical values since no 

 measured values could be found in the literature. 

 Love ( 1977) presented formulas for predicting the 

 target strength of an individual fish at any aspect 

 as a function offish size and insonify ing frequency. 

 His equations are valid for the range 1 sL/\ 'S 100 

 where L is the fish length and k is the acoustic 

 wavelength. Our data on T. mexicanus for a mean 

 standard length of 24.5 mm (Table 1) and at a 

 frequency of 102 kHz would show aL/K ratio of 1 .7. 



Using his formulas on our data, calculated target 

 strengths for dorsal aspect vary from -55.6 dB to 

 - 56.6 dB and for anterior aspect from -67.1 dB to 

 -67.7 dB. Thus, the target strength would be de- 

 creased by 10 to 12 dB as the orientation of the fish 

 changed from dorsal aspect to anterior aspect. The 

 change in target strength values from our data 

 ( 10.7 dB) indicates that such a change in the orien- 

 tation of the fish might have occurred. 



The absolute values of our calculated tsirget 

 strengths are about 4.5 dB less than the predicted 

 values. Since the data used by Love ( 1977) to de- 

 termine his equations did not include myctophids, 

 it is possible that juvenile T. mexicanus (and 

 lanternfishes in general) may be poorer scatterers 

 than the larger, nearshore, and surface fishes used 

 for his study. 



The migratory pattern shown for this layer is 

 not unique to this study. The increased migratory 

 rate of scattering layers during the middle of the 

 sunset migration has been shown by a number of 

 authors (e.g., Kampa and Boden 1954). Kampa 

 and Boden (1954) also correlated this type of mi- 

 gratory pattern to a similar pattern in the isolume 

 at the scattering layer depths. The interrelation- 

 ship between isolumes, scattering layer mi- 

 grations, and vertical water mass structure is not 

 well understood. Thus, the observed change in mi- 



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