I'lEPER and BAKGO: ACOUSTIC MEASUREMENTS OF MIGRATING MEXICAN LAMPFISH 



where fpf^ = resonant frequency (Hz) 



a = equivalent spherical radius (/xm) 

 z = depth (m). 



In the above formula, /Vr is 12 kHz. The a is the 

 radius of a sphere equal in volume to that of the 

 swim bladder at the surface. 



RESULTS 



The deepest of the three 12 kHz scattering 

 layers (Figure 1) was also recorded at 102 kHz 

 (Figure 2). The calculated volume scattering 

 strengths at 102 kHz and at different times and 

 depths are also shown in Figure 2. The biological 

 sample from this layer was composed almost ex- 



clusively of juvenile T. rnexicanus (Table 1). The 

 movement of the scattering layer with time 

 showed an increasing rate of migration up to a 

 depth of around 180 m which corresponded to a 

 change in the temperature-salinity characteris- 

 tics of the water (Figure 3). 



Calculated target strengths for T. rnexicanus 

 (Table 2) were highest at the deepest depth ( -60.6 

 dB at 284 m) and slowly decreased as the layer 

 migrated upwards ( -71.3 dB at 206 m). The de- 

 crease in calculated target strengths corresponded 

 to the increased migratory rate of the layer (Fig- 



Table 2.— Volume scattering strengths (S^.) for the 102 kHz 

 scattering layer and calculated target strengths ITS) for 

 Tnphoturus rnexicanus. Data from San Clemente basin, south- 

 em California, 26 January 1977. 



Time 

 (PST) 



Type of 

 sea tiering 



Deplti 

 (m) 



Sv 



(dB/m^) 



Fish Sv Fish rS 

 (dB/m3) (dB) 



Q. 

 UJ 

 Q 



50- 



100- 



150 — 



200 — 



250- 





T-- 



(a) /^ 



I 



I 



Interference- 



-> 



(b) 



(c) 



— 50 



— 100 



— 150 



— 200 



■250 



TIME 



l-H — I — I — I — I — I — I — I — I — I — I — \ — I — l-H 



1714 



1722 



1730 



1738 



1746 



1754 



1802 



1810 



Figure l. — A 12 kHz echogram from the San Clemente basin, southern California, 26 January 1977. Three scattering layers are 

 shown, first appearing at depths around 150 m, and then migrating towards the surface. The first two scattering layers, (a) and (b), to 

 migrate (starting around 1720 and 1745, respectively) were only recorded at 12 kHz. The third scattering layer (c) (starting around 

 1750 1 was also recorded on a 102 kHz echo sounder. The echogram also shows scattering from single fish or small fish schools between 50 

 m and the surface. The interference indicated is from the ship's echo sounder. 



937 



