INTRODUCTION 



The study reported here was undertaken as part of 

 a broad, continuing investigation of the marine organisms 

 which act as sonic scatterers and thus affect the operation 

 of sonar systems. This effort was substantially aided in 

 the 1950's, when the advent of the bathyscaph TRIESTE 

 permitted direct observation of organisms residing in zones 

 of strong sonic reflection (deep scattering layers, or DSL). 

 Thus, E. G. Barham was able for the first time, in 196 2, 

 to make direct visual identification of many of the most 

 prominent organisms and to obtain some estimates of their 



1 2 



population densities in these areas, ' 



Such observations made from the TRIESTE and 

 more recently from the Cousteau Souscoupe Sous Marine 

 ("diving saucer") have indicated that siphonophores are 

 among the two or three most prominent types of organisms 

 found in association with the DSL in the San Diego Trough 

 area and also in the region of Cabo San Lucas, Baja 

 California -- two areas where the DSL is highly developed. 3 



Of the siphonophore species observed directly by 

 Barham and also captured in net hauls, by far the most 

 abundant in the vicinity of San Diego has been Nanomta bijuga 

 (Delle Chiaje) 1841, an ubiquitous form belonging to the 

 sub-order Physonectae found in tropical and temperate 

 seas in many parts of the world (fig. 1). Members of this 

 taxonomic group are characterized by the presence of a 

 gas -filled float or pneumatophore, which possibly contrib- 

 utes as much or more to acoustic reflection and reverbera- 

 tion as the gas -filled swim bladders of fishes. These 

 organisms obviously offer a valuable potential for increas- 

 ing knowledge of the dynamics of the deep scattering layer 

 (DSL) and for making predictions concerning its acoustic 

 behavior in areas where siphonophores are a major constit- 

 uent. 



'See list of references, p. 33. 



