FISHERY BULLETIN; VOL. 74, NO. 2 



AUTOMATED SONAR SURVEY 



An automated sonar survey of the Los Angeles 

 Bight was accompUshed during the last 2 wk of 

 the charter boat cruise. A 721-nautical-mile track 

 (Figure 8) was transected two times providing a 

 3.4% areal sample of the ll,500-mile2 Bight. 

 Each track (1.7% sample) was processed as a 

 separate survey. 



Appendix Table 3 lists target counts on tracks 1 

 and 2 by target size and mid-range. Target size 

 refers to the maximum dimension normal to the 

 ship's track and is calculated from the difference 

 between the leading and trailing edges of the 

 echo envelope corrected for the pulse length ( 15 m 

 at 10 ms pulse length). The first mode, common to 

 both tracks at a target diameter of 30 m, is consis- 

 tent with earlier data collected by NMFS (approx- 

 imately 4,500 targets) and CF&G (approximately 

 23,000 targets). A second mode occurring at a 

 school diameter of 250 m is also common to both 

 tracks. This mode has not been seen before or 

 during any season in any year since sonar ac- 

 tivities were initiated off southern California. An 

 explanation for the mode, other than the reflec- 

 tion of an optimum school size, is that it may be a 

 bottom reverberation mode particular to the ob- 

 servation window used on the survey. 



Bottom reverberation, as logged by the system, 

 was collected for 2 h over water depths of ap- 

 proximately 100 m during the cruise. Distribu- 

 tions of target size, midrange, and target strength 

 are shown in Appendix Table 4. Notable are two 



32°30' 



size modes at 50 and 225 m, an optimum mid- 

 range of 450 m, and an average target strength of 

 + 5dB. 



Targets contributing to the 250-m size class 

 mode have a midrange mode of approximately 

 450 m for both tracks 1 and 2. Average target 

 strength was -h? dB for the subsample. This in- 

 formation reinforces the theory that the 250-m 

 size class mode is caused by false targets caused 

 in turn by bottom reverberation. Changes in the 

 sonar system operating parameters (i.e., the en- 

 largement of the observation window and the ad- 

 dition of a time gain circuit) are assumed to be 

 responsible for the variation in system response. 

 These changes were made to facilitate the fish 

 biomass work and will not be in effect during the 

 sonar surveys to be conducted on a series of 

 California Cooperative Oceanic Fisheries Inves- 

 tigations cruises beginning in November 1974. 

 Operating procedures will be the same as used for 

 the initial field of testing of the automated hydro- 

 acoustic data acquisition and processing system. 



Since those targets which begin or end beyond 

 the observation band are not counted, an edge 

 bias exists which is a function of the target size 

 and the extent of the observation window. Fre- 

 quencies within target size class intervals were 

 corrected for edge bias by the following formula: 



N'^ = N, 



500 



500 



I20°30' I20°00 II9°30 IIS^OO 1I8°30 II8°00' II7°30' II7°00' 



Figure 8. — Los Angeles Bight including a 721-mile sonar sur- 

 vey track transected twice, 17-26 September 1974. 



where Nd= frequency of observation within a 

 given size class 

 N'ci= frequency corrected for edge bias 

 d = mean class diameter. 



The largest school size corrected for edge bias 

 was 160 m (target size distributions fi^om previ- 

 ous cruises, CF&G and NMFS, indicate that 160 

 m includes the 99th percentile). Table 4 lists ob- 

 served frequencies, edge corrected frequencies, 

 and horizontal school area contributions for size 

 classes up to a maximum mean class diameter of 

 160 m. 



The total detected school area was 2.6 x 10^ m^ 

 for track 1 and 1.4 x 10^ m^ for track 2. Integrat- 

 ing over the entire survey area by simple propor- 

 tion, assuming no stratification, and using a con- 

 version factor of 15.16 kg/m^, biomass estimates 

 of pelagic schooling fish in the Los Angeles Bight 

 were calculated at 2.30 x 10^ metric tons and 

 1.23 x 10^ metric tons for tracks 1 and 2, respec- 



294 



