PIEPER and BARGO: ACOUSTIC MEASUREMENTS OF MIGRATING MEXICAN LAMPFISH 



lire 4). Measurements were not taken <206 m due 

 to increased interference from surface scattering. 



The organisms which were responsible for pro- 

 ducing the two, shallower, 12 kHz scattering 

 layers are not known since no trawls were taken 

 from the depths of these two layers. Before the 

 migratory period, however, two trawls were com- 

 pleted from depths which might correspond to the 

 distributions of the scattering organisms. Trawls 

 25657 (1350-1432 h; 292-302 m) and 25658 (1514- 

 1547 h; 267-268 m) were completed before the scat- 

 tering layers became evident on either echo 

 sounder. They were also from shallower depths 

 than the first appearance of the 102 kHz scattering 

 layer (1638; 315-325 m). Data from these trawls 

 consisted of small numbers of Argyropelecus sla- 

 deni, Cyclothone signata, Protomyctophum croc- 

 keri, and Vinciguerria lucetia (Table 3). Of these 

 four species, only C. signata is known not to be a 

 vertical migrator (Rainwater 1975; Pearcy et al. 

 1977). Swim bladder resonance calculations for 

 the other three species are shown in Table 4. 



DISCUSSION 



Lanternfishes (Family Myctophidae) have been 

 implicated as the most important scatterers of 

 scattering layers recorded at frequencies around 

 12 kHz, especially since many of these fishes have 

 air-filled swim bladders of such a size as to be 

 resonant from 1 to 30 kHz (Hersey and Backus 

 1962). Triphoturus mexicanus is a known vertical 

 migrator off southern California (Paxton 1967) 

 and its distribution has been previously correlated 

 with scattering layers which showed diel migra- 

 tions. Barham (1966) noted that adult T. 

 mexicanus were associated with a 12 kHz scatter- 

 ing layer in the California Current, and Holton 

 (1969) correlated collections of 8-10 cm long T. 

 mexicanus with a strong scattering layer in the 

 Gulf of California. 



This paper reports on a scattering layer re- 

 corded at both 12 kHz (Figure 1) and 102 kHz 

 (Figure 2). Triphoturus mexicanus dominated the 

 net collection from this scattering layer (Table 1). 



1.0 3.0 30 4.0 



Rate of oscent of scattering loyer [m/minJ 



FIGURE 4. — Variations in the calculated target strengths for 

 Triphoturus mexicanus at 102 kHz as a function of the rate of 

 ascent of the 102 kHz scattering layer. 



Kleckner and Gibbs^ suggested that lanternfishes 

 probably regulate the gas in their swim bladders 

 during migration to maintain constant gas vol- 

 ume. Assuming that calculations of swim bladder 

 resonance can be approximated by using equa- 

 tions based on a free bubble in water (Hawkins 

 1977; Love 1978), these fish would show 12 kHz 

 resonance only between 28 and 43 m (Table 4). In 

 addition, a frequency of 102 kHz is too high for 

 possible resonance effects. We suggest that the 

 deepest 12 kHz layer and the 102 kHz layer were 

 due to a large number of T. mexicanus rather than 

 a few fishes scattering the sound at resonant fre- 

 quencies. 



Volume scattering strengths (Figure 2, Table 2) 

 and target strengths (Table 2) were calculated at 

 102 kHz for T. mexicanus. Target strength values 

 decreased as the layer migrated upwards from a 



«Kleckner, R. C, and R. H. Gibbs, Jr. 1972. Swimbladder 

 structure of Mediterranean midwater fishes and a method of 

 comparing swimbladder data with acoustic profiles, Mediter- 

 ranean Biological Studies Final Report to the U.S. Office of 

 Naval Research 1(41:230-281. 



TABLE 3.— Fishes collected from trawls 25657 and 25658, San Clemente basin, southern California, 26 January 1977. 



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