PIEPER and BARGO: ACOUSTIC MEASUREMENTS OF MIGRATING MEXICAN LAMPFISH 



gratory rate with change in water type around 180 

 m (Figure 3) may or may not reflect the reason for 

 the observed migratory pattern. 



The scatterers responsible for the two, shal- 

 lower, 12 kHz scattering layers cannot be spe- 

 cifically determined in the present study. Of the 

 fishes collected from two previous tows (Table 3) 

 only Cyclothone signata is known not to migrate 

 into surface waters (Pearcy et al. 1977). Since Vin- 

 ciguerria lucetia has been collected at the surface 

 at night, it is probably a vertical migrator (Grey 

 1964). The information on the vertical distribu- 

 tion and migration for Argyropelecus sladeni and 

 Protomyctophum crockeri indicates that vertical 

 migration is unlikely, but the data on these two 

 species are sparse and incomplete. Argyropelecus 

 sladeni has been collected both day and night at 

 depths from to 2,000 m (Baird 1971; Rainwater 

 1975; Pearcy et al. 1977), although the center of 

 their distribution appears to be from 100 to 500 m. 

 The information on P. crockeri shows similar 

 broad distributions (Paxton 1967; Rainwater 

 1975; Pearcy et al. 1977), although Paxton stated 

 that they only reach depths of 150 m at night and 

 Wisner' stated that they are not caught above 100 

 m at night. 



Since the two, shallower, 12 kHz scattering 

 layers were not recorded on the 102 kHz echo 

 sounder, it is likely that swim bladder resonance 

 at 12 kHz from a small number of organisms was 

 responsible for the scattering. Based on swim 

 bladder measurements made at the surface and 

 assuming regulation of swim bladder volume to 

 maintain constant volume during migration, the 

 depths where 12 kHz resonance would occur were 

 calculated (Table 4) for the range of sizes of the 

 fishes collected. These calculations indicate that 

 A. sladeni and V. lucetia would show 12 kHz res- 

 onance at depths from 10 to 160 m and 49 to 182 

 m, respectively. Thus, we suggest that one or both 

 fishes could be responsible for the shallower, 12 

 kHz scattering layers. The depth range for 12 kHz 

 resonance for P. crockeri (43-80 m) indicates that 

 it was probably not the source of either of the 

 scattering layers. In addition, both shallow layers 

 reached a depth of 40-50 m during the migration 

 and P. crockeri has not been collected at depths 

 <100 m at night (Paxton 1967; Wisner see foot- 

 note 7). It is also possible, however, that the shal- 



'Wisner, R. L. 1976. The taxonomy and distribution of 

 lanternfishes (Family Myctophidae) of the eastern Pacific 

 Ocean. Navy Ocean Research and Development Activity, Bay 

 St. Louis, Miss., Rep. 3. 229 p. 



lower layers resulted from an organism or organ- 

 isms not collected by the two net tows discussed. 



The potential use of high-frequency acoustics 

 for studying the distribution, behavior, and abun- 

 dance of scattering organisms is strongly indi- 

 cated. Echo sounders operated at frequencies 

 above 30 kHz are working at frequencies above 

 swim bladder resonance and therefore, reflect the 

 biomass of scatterers more accurately. In addition, 

 they generally have narrow beam angles and 

 utilize short pulse lengths (3.5° beam angle and 

 1.0 ms pulse length in this study) which produce 

 finer resolution in the scattering patterns. Cali- 

 brated, multifrequency acoustic systems used in 

 conjunction with sophisticated net systems are 

 needed to better define distributional patterns and 

 interactions of these midwater organisms. 



SUMMARY AND CONCLUSIONS 



Triphoturus mexicanus is known to migrate 

 vertically in the water column (Paxton 1967). We 

 have shown that juvenile T. mexicanus were the 

 major sound scatterers in a migrating scattering 

 layer recorded at both 102 kHz and 12 kHz. Calcu- 

 lated target strengths for T. mexicanus at 102 kHz 

 varied from -60.6 dB at 284 m to -71.3 dB at 206 

 m. This decrease in target strength with depth was 

 probably due to a change in the orientation of the 

 fish in the water column. The lowest target 

 strength ( - 71.3 dB) occurred when the scattering 

 layer was migrating towards the surface at its 

 highest rate and, therefore, the fishes should be 

 oriented more vertically in the water column. 



Two, shallower, scattering layers were recorded 

 at 12 kHz but not 102 kHz. We suggest that these 

 two layers probably resulted from scattering 

 which occurred from fishes with swim bladders 

 which 1) resonated at 12 kHz and 2) were regu- 

 lated to maintain constant swim bladder volume 

 during migration. Vinciguerria lucetia and A. 

 sladeni are both possible scatterers of these layers 

 although A. sladeni is not known to be a vertical 

 migrator. 



The importance of using acoustics to study 

 mesopelagic organisms is indicated. Echo sound- 

 ers can be used to both qualitatively direct biologi- 

 cal sampling and quantitatively determine dis- 

 tributions and biomass. High-frequency echo 

 sounders (e.g., 102 kHz in this study) have an 

 advantage over low-frequency echo sounders. 

 Target strength measurements on the midwater 

 fishes, however, are needed to better predict the 



941 



