LORD ET AL.: ACOUSTIC ASSESSMENT OF MIGRATING SALMON 



however, since it was more convenient to operate 

 the buoys continuously for several hours whereas 

 the seine vessel required only 2 h for a set after 

 which it proceeded to the next station. Occasion- 

 ally the buoys were operated at a station which 

 had been fished by the seine on the same day but 

 not at the same time. Also, even at the same sta- 

 tion, it was not feasible to set the seine directly 

 around the buoys so it cannot be said that the two 

 gears sampled precisely the same water. This is 

 of some significance in any gear comparison 

 since there was considerable set-to-set variation 

 in purse seine hauls made at the same station. 



Buoy launch and recovery presented no diffi- 

 culty in any weather conditions in which buoy 

 operation was attempted. Buoy operation is usu- 

 ally limited by the presence of heavy breaking 

 seas with whitecaps in which case the entrained 

 air causes ambiguous echo counts. In the Adak 

 area the limiting weather conditions for opera- 

 tion of either the purse seine or the acoustic buoys 

 depend strongly on the wind direction. Generally 

 the purse seine can be operated in winds up to a 

 maximum of about 20 knots. The acoustic buoys 

 have been operated in higher winds with no seri- 

 ous difficulty in launch or recovery. However, the 

 aforementioned problem of entrained air usually 

 limits buoy operation to winds of less than 25 

 knots. The buoys, however, can be operated con- 

 tinuously for longer periods of time since, once 

 deployed, no further human activity is required 

 except to monitor the digital printout. 



The buoys operate synchronously so that the 

 data for each acoustic pulse may be radioed to the 

 tending vessel as soon as it is obtained. The echo 

 count data are in digital form in which all of the 

 data from each acoustic pulse is coded into a 

 single 60-bit word for telemetry to the shipboard 

 receiver. Each of these 60-bit words contains: 

 a) buoy identification number, b) the number of 

 echo counts up to a maximum of seven, and c) the 

 range from the transducer to each of the targets. 

 The data system requires that the indicated 

 number of targets agrees with the number of 

 ranges actually recorded and that the target 

 ranges must form a nondecreasing sequence. This 

 redundancy permits the detection and rejection of 

 spurious or noise contaminated data. The binary 

 coded 60-bit words are formatted to be compatible 

 with the CDC -6400 computer^ used for the data 



reduction. The tape reading and data processing 

 can be accomplished using only FORTRAN and 

 certain FORTRAN callable subroutines thus 

 avoiding the necessity of machine language pro- 

 gramming. 



The range discrimination of the acoustic sys- 

 tem is 25 cm, i.e., two fish separated in range 

 from the transducer by 25 cm or more will be 

 detected as individual fish. Six binary bits are 

 allowed for each of the seven possible ranges. 

 This presently corresponds to a range resolution 

 of 1 m, i.e., more than one fish may be detected 

 and recorded in a single 1-m range increment if 

 they are physically separated in range by at least 

 25 cm. Target coincidence is a possibility, particu- 

 larly if the fish are dense or tend to school. This 

 has not been a problem in high-seas use since the 

 average number of echoes per pulse has been of 

 the order of one. 



Figure 5 shows typical depth distribution his- 

 tograms corrected for the effect of a conical sam- 

 pling volume. The most striking feature is the 

 shallow depth at which most of the fish are found, 

 usually 5 m or less. This had been anticipated and 

 illustrates the need for an upward-looking device. 



There is the possibility of ambiguity in the in- 

 terpretation of echoes originating very near the 

 surface. Indeed this usually proves to be the 

 limiting condition in the operation of the buoys. 

 This situation manifests itself by the consistent 

 presence of targets in two or more successive 

 range increments below the surface. More detail 



DENSITY OF FiSH (ARBITRARY UNITS) 

 01 0.2 03 04 Ol 0.2 03 04 05 06 Q7 



2- 



I 6 



t- 

 Ol 

 bi 



O 



8- 



12- 



DATE 7/11/74 

 STATION 2 

 BUOY NO 4 



6- 



DATE 7/17/74 

 STATION I 

 BUOY NO I 



12-1 



^Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



Figure 5. — Typical depth distribution histogram. 



109 



