DEVELOPMENT AND USE OF SONAR MAPPING FOR PELAGIC 

 STOCK ASSESSMENT IN THE CALIFORNIA CURRENT AREA^ 



Roger P. Hewitt, Paul E. Smith, and John C. Brown^ 



ABSTRACT 



A method for pelagic fish stock assessment is presented which utilizes a fixed sonar beam for mapping 

 fish schools. Samples of the two major acoustic properties of fish schools are presented, i.e., acousti- 

 cally derived horizontal dimensions (representative of school volume) and target strengths (which 

 may be representative of school compaction). Sampling biases and sources of sampling variability in 

 the measurement of these properties are discussed. The results of two experiments, conducted to 

 determine the weight of a fish school as a function of its acoustic characteristics, are presented. In the 

 first experiment, an acoustically transparent trap was used to recreate an aggregation offish and in 

 the second, commercial fishing boats were chartered to capture whole schools. An automated sonar 

 data acquisition and processing system is described and test results presented. The results of paired 

 automated surveys of the Los Angeles (southern California) Bight are presented and discussed. The 

 paper reports development of the sonar-fish school mapping method first documented by P. E. Smith 

 in 1970. 



Field investigations, conducted in cooperation with the Navy and the California Department of 

 Fish and Game, indicate a median school size of 30 m diameter, a mean fish density of 15 kg offish 

 biomass per square meter of horizontal school area, and a biomass estimate of 1.23 to 2.30 x 10^ 

 metric tons for pelagic schooled targets in the Los Angeles Bight. 



Fishermen have used hydroacoustic apparatus 

 for locating concentrations of fish for almost as 

 long as practical echo sounding devices have been 

 available, although quantification of the informa- 

 tion they provide has been attempted only in re- 

 cent years. Horizontal echo ranging (sonar) to 

 locate fish schools was first used off the coast of 

 California in 1946 (Smith 1947; Smith and 

 Ahlstrom 1948). The 1950 progress report of the 

 California Cooperative Sardine Research pro- 

 gram notes the use of sonar and echo sounders on 

 the RV Yellowfin for locating fish schools, and 

 cites the "considerable experimental value" of the 

 acoustic apparatus. A research sonar on the 

 RV David Starr Jordan has been used to count 

 fish schools in the eastern tropical Pacific (Mc- 

 Clendon 1968) and in the California Current 

 area (Smith 1970). For recent reviews of the 

 use of echo sounders and sonars for fishery re- 

 search, consult Forbes and Nakken (1972) and 

 Cushing (1973). 



The work presented here is a method for quan- 

 tifying sonar records and further using these re- 



^Conducted under a grant from the Marine Research Com- 

 mittee of the California Department of Fish and Game as part 

 of the California Cooperative Oceanic Fisheries Investigations, 

 and in cooperation with the United States Navy. 



^Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, La Jolla, CA 92038. 



Manuscript accepted October 1975. 

 FISHERY BULLETIN: VOL. 74, NO. 2, 1976. 



cords for estimating the size of pelagic fish stocks. 

 The paper is divided into four sections: 



1. The section entitled "Sources of sampling 

 variability" describes the scale and variance 

 of measured acoustic parameters of fish 

 schools, i.e., horizontal fish school dimen- 

 sions and peak target strength or echo in- 

 tensity. It further discusses major biases af- 

 fecting the measurement of these values. 



2. The estimation of fish biomass in an 

 aggregation involves the determination of a 

 conversion factor by which the detected 

 horizontal area of a fish school may be mul- 

 tiplied. Experiments to determine the 

 weight of the fish under a square meter of 

 school area are described in a section en- 

 titled "Horizontal school area to biomass 

 conversion factors." 



3. An automated data acquisition system is de- 

 scribed in the third section. 



4. The results of a paired sonar survey of the 

 Los Angeles Bight, utilizing the automated 

 system and a biomass factor determined 

 during the cruise, are presented and discus- 

 sed in the fourth section. 



This report is the second in a series describing 



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