LORD ET AL.: ACOUSTIC ASSESSMENT OF MIGRATING SALMON 



RADIO LINK 



BUOY WITH 

 TRANSMITTER 



22.1-m SURFACE 

 SEARCH DIAMETER 



TRANSDUCER 



ventral, head, dorsal, and tail aspects. Inspection 

 of this figure shows that the target strength de- 

 creases rapidly for head or tail views but is fairly 

 constant at -30 dB over ±30° when viewed from 

 the dorsal or ventral aspect. This severe depen- 

 dence of target strength on aspect angle was the 

 limiting factor in the choice of transducer beam 

 width. For the high-seas system, a 28° conical 

 beam is used. This gives an adequate sample vol- 

 ume and minimizes target-size fluctuations to a 

 manageable level. A time-variable-gain (TVG) 

 receiver, adjusted so that its output for a particu- 

 lar target is independent of target range, is used 

 to limit signal dynamic range at the detector. 

 This technique, and proper adjustment of abso- 

 lute sensitivity, keeps the search volume rela- 

 tively constant over a fairly wide range of target 

 strength (±15 dB). 



Estimates of the fish density in the Adak area 

 indicate that the average count per sample will 

 be less than one fish. Schooling habits of these 

 salmon also reveal that only rarely will more 

 than a few salmon be included in any one sample. 



RECEIVING 

 ANTENNA 



SHIPBOARD RECEIVER 

 a DIGITAL RECORDER 



Figure l. — Schematic illustration of prototype buoy used at 

 Adak in 1974. 



COAJtIAl. 

 CABLE — 



TRANSDUCER 



UNDERWATER 

 SYSTEM 



TRANSMITTER 



TRANSMITTER- 

 CONTROL 

 (REP RATE PULSE 

 LENGTH, ate) 



FISH 

 DETECTOR 



TELEMETRY 

 CONTROL a 



TRANSMITTER 



MASTER TIMING 



AND DATA 



PROCESSING 



LOGIC 



SERIAL 



DATA 



STORAGE 



PREAMPLIFIER 

 AND TVG 

 AMPLIFIER 



SURFACE 



DETECTOR 



BUOY ELECTRONICS 



/N 



RADIO 

 RECEIVER 



DATA 



DETECTOR ft 



SYNCHRON IZER 



BUFFER 

 STORAGE 



TIMING ft 



CONTROL 

 LOGIC 



Digital 



TAPE 



RECORDER 



SHIPBOARD SYSTEM 



Figure 2. — Simplified block diagram of the prototype high- 

 seas system. 



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