HEWITT ET AL.: DEVELOPMENT AND USE OF SONAR MAPPING 



a representative anchovy school compaction 

 around 50 fish/m^ or a distance of two body 

 lengths between fish. Using a single fish weight 

 of 18 g and an average school thickness of 12 m 

 (Mais 1974), one obtains a horizontal school area 

 to biomass conversion factor of 8.4 kg/m^. 



Charter Boat Experiment 



A second experiment was designed and exe- 

 cuted in late summer 1974, to relate measured 

 school size, calculated target strength, and school 

 compaction. Purse seine boats were chartered to 

 make directed sets on fish schools first ensonified 

 by the acoustic system aboard the David Starr 

 Jordan.^ Target strength and school size were 

 calculated from the observation. The fishing boat 

 supplied information on the tonnage caught and 

 the portion of the school taken. Using these data, 

 a biomass conversion factor was calculated for 

 each school by dividing the total estimated school 

 tonnage by a circular area based on the difference 

 between its near and far ranges. 



Fifty-two sets were judged to be the minimum 

 sample size necessary to distinguish between two 

 estimates of the portion of detectable pelagic 

 aggregations that are schools of northern an- 

 chovy. Squire (1972), using data from 6 yr of ob- 

 servations from several commercial air spot- 

 ters, reported that at least 50% of the surface 

 schools off southern California can be expected to 

 be anchovy. Mais (pers. commun.) estimates that 

 90% of the schools sampled by mid-water trawl 

 are anchovy. 



Seventy-six sets were made landing 1,901 short 

 tons of anchovy; 63 were directed by the David 

 Starr Jordan and 13 directed by the State of 

 California's RV Alaska. Forty-nine positive data 

 points were tabulated from the David Starr Jor- 

 dan's work and eight from the Alaska. 



Average target size was 119 m (as measured by 

 the difference between the near and far ranges on 

 a line perpendicular to the ship's head) with a 

 range from 31 to 305 m. Average peak target 

 strength was +5.18 dB (as calculated from peak 



amplitude and range dependent losses) with a 

 range from -9 to +18 dB and a SD of 5.63 dB. 



Practical considerations forced us to expend a 

 larger portion of effort on schools of larger than 

 average size and target strength. This circum- 

 stance accounts for the fourfold increase in 

 median target size and a 15-dB increase in mean 

 target strength over a sonar-generated data base 

 reported earlier. In addition, this sample was cho- 

 sen from a detected school population whose 

 acoustic dimensions were, in general, larger than 

 that experienced on previous cruises. 



To facilitate the direction of sets, the observa- 

 tion window was increased fi:"om 250 to 500 m 

 wide and moved 100 m closer to the vessel. A 

 time-varied gain increase was also accomplished 

 in the receiver previous to signal display on an 

 oscilloscope. Either or both of these changes to 

 the sonar system configuration could produce cir- 

 cumstances under which similar data distribu- 

 tions would appear to be different. Point scat- 

 ters encountered when plotting target size versus 

 target strength, target strength versus horizontal 

 school area to biomass conversion factor, and 

 target size versus horizontal school area to 

 biomass conversion factor are too wide to detect a 

 relationship between these school parameters. 



A distribution of horizontal school area to 

 biomass conversion factors is presented in Figure 

 6. The distribution is skewed right with an 

 arithmetic mean of 15.16 kg/m^. While no rela- 

 tionship is as yet demonstrated between indi- 

 vidual target strengths and horizontal school 

 area to biomass conversion factors, the data have 

 contributed to a refinement of a general conver- 



®Contracts were let for a total of 104 sets assuming 50% suc- 

 cess rate for positive sets and a permit was secured from CF&G 

 to land 2,500 tons of anchovy during the experiment. A charter 

 agreement was written establishing criteria for the successful 

 bidders as minimum tonnage bid with the proceeds from any 

 excess tonnage, not to exceed the permit, to be given to the 

 State. In addition, each boat was guaranteed a fixed fee over 

 and above the proceeds from the landed fish. 



'0.125 0.25 0.5 I 2 4 8 16 32 64 128 



HORIZONTAL SCHOOL AREA TO BIOMASS CONVERSION FACTOR (kg/m^) 



Figure 6. — Distribution of horizontal school area to biomass 

 conversion factors obtained from the charter boat experiment. 



291 



