Figure 3. — Data measured with a conventional echo sounder operating at 18 kHz. The fish school depth distribution (al, cumulative 

 distribution (b). and thermal profile (cl are associated with the scale on the left. The distribution of school thickness (d) and cumulative 

 distnbution le) are associated with the scale on the right. 



chovy, 60-90 mm long. The Septerpber broadband 

 acoustic work revealed sound-scattering charac- 

 teristic of a mix of juvenile and adult anchovy with 

 a minor component of jack mackerel. These obser- 

 vations are consistent with known abundances of 

 pelagic fish in the area (Squire 1972). In December 

 and September, the presence of anchovy, jack 

 mackerel, and squid was confirmed by trawl re- 

 sults from depths near the seasonal thermocline. 

 The trawls were conducted during the night fol- 

 lowing each of the acoustic measurement periods. 

 The trawl results from May were largely negative 

 with respect to adult schooling fish. Qualitatively, 

 increased water clarity in the area off the north 

 end of Santa Catalina Island during May may 

 have permitted greater trawl avoidance than was 

 the case closer to shore at the December and Sep- 

 tember stations. 



Only one of many possible variations of the bot- 

 tom bounce technique has been discussed. Before 

 attempting an adaptation of the procedure to a 

 particular problem, one should consider, as a 

 minimum, the impact of the following. One of the 

 first parameters at a designer's disposal in acous- 

 tics is the frequency to be used. The reflectivity of 

 the target, the bottom, and the surface are all 

 dependent on frequency as is the absorption of the 

 sound in the water column. Ambient noise, ship's 

 self-noise, and reverberation levels are also 

 strongly frequency dependent. Pulse length, pulse 

 type, and a variety of signal processing techniques 

 are also at the designer's disposal and must be 

 determined. 



Some important parameters are not available to 

 the designer for selection. These include bottom 

 characteristics (scattering, absorption, and pene- 

 tration), bottom slope, and surface conditions such 

 as roughness and the presence of entrained bub- 

 bles. These characteristics of the intended operat- 

 ing area must be considered as well as maximum 

 and minimum water depths before designing a 

 bottom bounce system for a particular application. 

 While it is readily apparent that there will be a 

 maximum effective operating depth for any given 

 system design in a particular physical environ- 

 ment, it is also important to realize that shallow 

 reflection angles at the bottom and surface may 

 limit shallow water operation. Another important 

 shallow water limitation is the persistence of bot- 

 tom reverberation for ranges at which one desires 

 to make measurements on schools. 



Conclusions 



The thickness and vertical distributions of shal- 

 low schools of many fish cannot be accurately mea- 

 sured by conventional echo sounding techniques. 

 Consequently, a new approach which makes use of 

 the bottom bounce acoustic propagation path has 

 been developed and used for measurements on 

 northern anchovy schools off southern California. 

 These measurements, at three locations in three 

 different seasons, revealed that the schools oc- 

 cupied a depth zone only a few meters thick. Good 

 agreement was found between occurrences of the 

 mean depths of the schools and the seasonal ther- 



493 



