FISHERY BULLETIN: VOL. 74, NO. 2 



Table 2. — A comparison of the variance in detected target densities within the cleisses 

 of bottom topography (zone) and between the zones. Probabihty <0.5 that there is an 

 other than random relationship between the four classes of bottom topography and 

 detected school occurrence rates (target densities). 



yens), an analysis of the variance would suggest 

 that there is no variance between the zones that 

 could not be explained by the existing variability 

 within the zones (Table 2). 



HORIZONTAL SCHOOL AREA TO 

 BIOMASS CONVERSION FACTORS 



Fish Trap Experiment 



The first effort toward determining a horizontal 

 school area to biomass conversion factor was con- 



9.1m 



-70m 



NETTING 



l/2" STRETCH 



104mm MONOFILAMENT 



6 



-BAMBOO 



(JLEAD (0 9 kg) 



Figure 5. — Diagram of an acoustically transparent trap for 

 ensonifying a group of fish of known size and weight. 



ducted in 1970 and briefly described in the dis- 

 cussion following the presentation of Smith's 

 ( 1970) paper and transcribed in the publication of 

 that paper. 



An acoustically transparent trap (Figure 5) 

 was constructed and live northern anchovy en- 

 closed. Two groups of fish were ensonified and 

 their horizontal area measured. A 354-kg group 

 yielded a target strength within the range fre- 

 quently encountered while a 2,017-kg group's 

 target strength was well above that observed in 

 nature for schooling fish. 



Ensonification of additional weight groups was 

 not possible due to the presence of predators and 

 attempts at visual observation of the fish aggre- 

 gation using a manned submersible eventually 

 destroyed the trap. A value of 31 kg of fish 

 biomass/m^ was derived from the 354-kg group 

 and judged to be our best estimate (Table 3). Mais 

 (pers. commun.) reports from his experience 



Table 3. — Computation of a horizontal school area to bio- 

 mass conversion factor from data gathered during the fish 

 trap experiment (February 1970). 



'The fish are schooled in an ellipse with a major radius of 2.90 m and 

 a minor radius of 1.25 m (surface area 11.39 m^). 

 ^Metric tons per square meter. 



290 



