sion factor based previously on only one data 

 point. 



Eight horizontal school area to biomass conver- 

 sion factors calculated from sets directed by the 

 Alaska have a range from 10.14 to 30.22 kg/m^ 

 with a mean value of 18.42 kg/m^. The Alaska 

 participated in the experiment during the last 

 2 wk when only large schools were available in 

 shallow water. 



AUTOMATED HYDROACOUSTIC 



DATA ACQUISITION AND 



PROCESSING SYSTEM 



In an effort to reduce observer subjectivity in 

 the collection of large amounts of sonar data 

 necessary for the isolation of sampling errors and 

 biases, a decision was made to develop the capa- 

 bility to automatically count and measure the 

 horizontal dimensions of sonar targets. Peak echo 

 amplitude was also to be measured with the in- 

 tention of eventually relating it to school compac- 

 tion and depth. 



A digital PDP8/I computer with an additional 

 16k memory, an analog-to-digital converter and a 

 teletype terminal were acquired on loan from the 

 Naval Undersea Center at San Diego. Using this 

 gear, a project was undertaken which would allow 

 us to do automatically what we were doing man- 

 ually but with the additional benefits of real-time 

 target strength calculation and rapid raw data 

 processing. 



The raw data used for hand target collection is 

 in the form of a paper record containing a field of 

 parallel lines, each line being an incremental dis- 

 tance along the survey track. If the amplitude of 

 the signal is sampled during the recording of one 

 of these lines, at a sample rate of 750 samples/s 

 (velocity of sound/two-way path length), the 

 result is a record of the instantaneous echo 

 amplitude at 1-m increments along a line per- 

 pendicular to the survey track. 



When several of these lines have been recorded, 

 the result is a data field which is a numerical 

 counterpart of the paper record. Once the word 

 "target" is defined numerically, the number of 

 targets in this field can be counted. 



The numerical definitions used for this purpose 

 are: 



Threshold (THS) = some signal amplitude 

 greater than the average reverberation or 

 noise level. 



292 



FISHERY BULLETIN: VOL. 74, NO. 2 



Target line = at least five consecutive samples 

 greater than THS, preceded and followed by 

 five samples below THS. 



Target block = two target lines which have at 

 least five coincident and consecutive samples 

 greater than THS. 



Target = a target block + A^ additional coinci- 

 dent target lines, bounded by noise (signal 

 less than THS). 



The threshold, for the initial program was a 

 predetermined constant. The five sample target 

 line is selected on the assumption that a 5-m 

 target may be the smallest significant unit. The 

 two line target block is selected since random or 

 asynchronous noise greater than THS can cause a 

 target line, but will rarely cause at least five 

 coincident samples on consecutive lines. Three 

 consecutive lines of data are stored in the mem- 

 ory of the PDP8/I computer. As each new line of 

 data is stored it is tested for the presence of target 

 lines. When a target line is found, the amplitude 

 of the samples is compared and the value of the 

 peak amplitude is stored in the first data point 

 location. 



The newest data line is then compared with the 

 previous one and any occurrence of a target block 

 is recorded in the block register. The previous 

 data line is compared with the oldest data line 

 and, with the information in the target block reg- 

 ister, the following decisions are made: 



1. Is the target block the beginning of a new 

 target? If so, assign it a number and record 

 its initial range, final range, and peak 

 amplitude in the temporary target storage 

 register. 



2. Is the target block the entire target? If so, 

 store its information in the final target stor- 

 age field with the current time and the ship's 

 speed. 



3. Is the target block part of a previous target? 

 If so, update the temporary storage infor- 

 mation. 



4. Is the target block the end of a previous 

 target? If so, update the temporary informa- 

 tion and store in final storage. 



Additional logic decisions are required if two or 

 more previously recorded individual targets later 

 merge to form a single target, or if the inverse 

 should occur. 



There are four analog data input lines to the 



