ACOUSTIC MEASUREMENTS OF A MIGRATING LAYER OF 



THE MEXICAN LAMPFISH, TRIPHOTURUS MEXICANUS, 



AT 102 KILOHERTZ 



R. E. PlEPER AND B. G. BaRGO' 



ABSTRACT 



Biological sampling in a migrating scattering layer recorded at 102 kilohertz resulted in collections 

 which consisted primarily of juvenile Triphoturus mexicanus. The scattering from this layer was 

 quantified. Volume scattering strengths and corresponding target strengths were determined. The 

 rate of migration and the target strength of T. mexicanus changed as the layer approached the surface. 

 Target strengths at 102 kilohertz ranged from -60.6 decibels at 284 m to -71.3 decibels at 206 m. 



Initial investigations of the deep scattering layer 

 (DSL) emphasized mihtary and hydrographic ap- 

 plications and used relatively low-frequency echo 

 sounders (20-80 kHz). Later, research was di- 

 rected at the biological organisms responsible for 

 sound scattering in the DSL but was still confined 

 largely to low-frequency studies. Fishes and 

 physonectid siphonophores have been identified as 

 the major scatterers in this frequency range 

 (Barham 1963). In addition to determining the 

 sources of scattering, oceanographers, fishery 

 biologists, and commercial fishermen have been 

 using acoustics to locate and quantify fish schools 

 and shoals. Quantitative studies require the mea- 

 surement of volume scattering strengths from the 

 water column and knowing (or measuring) the 

 ability of the fishes to scatter sound (acoustic cross 

 section or target strength). Recent compilations of 

 work in these areas can be found in Farquhar 

 ( 1970) and Andersen and Zahuranec ( 1977 ). 



The development of high-frequency echo sound- 

 ers ( >50 kHz) during the past 10 yr has progressed 

 to the point where research at frequencies up to 3 .0 

 MHz is now practical (Holliday and Pieper^). 

 Working with high frequencies has several advan- 

 tages over low frequencies. As the frequency is 

 increased, shorter pulses can be used and the reso- 

 lution is increased. In addition, smaller organisms 

 become better sound scatterers as the frequency is 

 increased. At 102 kHz, for example, shoals of 



'Institute for Marine and Coastal Studies, University of 

 Southern California. Los Angeles, CA 90007. 



^Holliday, D V , and R. E. Pieper. 1978. Volume scattering 

 strengths and zooplankton distributions at acoustic frequencies 

 between 0.5 and 3 MHz. Program of the 96th Meeting of the 

 Acoustical Society of America. Honolulu, Hawaii. 25 p. 



euphausiid shrimp can be detected and quantified 

 at ranges up to 300 m (Bary and Pieper 1970; 

 Pieper 1979). 



The present paper reports on two migrating 

 scattering layers recorded only at 12 kHz and a 

 deeper, third layer recorded at both 12 kHz and 

 102 kHz. Large numbers of a single size class of 

 juvenile Mexican lampfish, Triphoturus mexi- 

 canus (Gilbert 1890), were collected from the 

 deepest scattering layer. Volume scattering 

 strengths of this layer were measured at 102 kHz 

 and corresponding target strengths of T. 

 mexicanus were calculated. Although no directed 

 sampling was completed in the two, shallower, 12 

 kHz scattering layers, the possible scatterers re- 

 sponsible for these layers are indicated. We dis- 

 cuss the advantage of using acoustic frequencies 

 above swim bladder resonance for biomass studies 

 and recommend the increased usage of high- 

 frequency acoustics for biological studies in the 

 sea. 



METHODS 



Three 12 kHz scattering layers were observed 

 migrating towards the surface near sunset on an 

 acoustic survey at the northwest end of the San 

 Clemente basin off southern California on 25 and 

 26 January 1977. The deepest of these 12 kHz 

 layers was recorded as a strong scattering layer on 

 a 102 kHz echo sounder being used to study 

 euphausiid distributions (Pieper 1979). Quantita- 

 tive acoustic measurements at 102 kHz and 

 biological sampling were completed in this scat- 

 tering layer on 26 January. Salinity and tempera- 

 ture profiles were taken immediately after the tow 



Manuscript accepted Mav 1 979 



nSHERY BULLETIN: VOL 77, NO 4.1980 



935 



