Table l. — Characteristics of depth and thickness of pelagic fish 

 schools studied during three seasons. 



to a common volume before the correlations were 

 calculated. The average temperature gradient 

 was calculated for each depth interval. Correla- 

 tion coefficients of -0.97 (.V = 9, 17 schools), -0.95 

 (N = 17, 121 schools), and -0.91 (N = 13, 221 

 schools) were computed between the thermal gra- 

 dients and the school depth distribution for De- 

 cember, May, and September, respectively. The 

 measurements of temperature gradient, and to 

 some degree the fish distributions, are derived 

 from continuous measures and tend to be serially 

 correlated. Without a measure of the degree of this 

 correlation confidence limits cannot be expressed 

 for the correlation coefficients indicated. Hence, 

 these correlation coefficients are provided as de- 

 scriptors of this specific data set rather than for 

 predictive purposes. For these locations, times, 

 school groups, and the 10 m vertical resolution, a 

 linear equation based on the thermal gradient de- 

 scribed the school vertical distribution with <&% 

 unexplained variation in December, 10% in May, 

 and 177f in September. 



We suggest that the observed correlation in all 

 three measurement sets is evidence of a causative 

 factor rather than chance. In December, the most 

 probable school location was at a depth whose 

 temperature was 16°-17° C. Temperatures at the 

 corresponding depths in May and in September 

 were 14°-15° C and 18°-20° C. Within this range of 

 temperatures ( 16-20° C) we did not find any sea- 

 sonal pattern indicating a preference for a particu- 

 lar water temperature. We think the correlation of 

 the thermal gradient with school mean depth and 

 the relatively thin character of the schools is more 

 likely evidence of a thermally associated thin 

 layering of some part of the fishes' food supply 

 rather than a direct result of the temperature 

 profile. 



An analysis of the thickness measurements in- 

 dicates a tendency toward thin schools in each 

 survey (Figure 2d, i, ni. To the extent that the data 



base collected is characteristic of the entire school 

 group, the most probable thickness of a school 

 selected randomly from the three groups studied 

 was consistently near 4 m. In May, one-half of the 

 schools were thinner than 3.4 m. As will be discus- 

 sed later, the schools in May exhibited sound-scat- 

 tering characteristics consistent with a dominant 

 population of anchovy larvae. The school horizon- 

 tal dimensions did not differ substantially from 

 those measured previously for adult fish in other 

 seasons (Hewitt et al. 1976; Larsen see footnote 1; 

 Holliday^). 



Over one-third of the fish schools observed were 

 at depths <20 m. More than ll'^i of the schools 

 were above 10 ni. An echo sounder operated in the 

 conventional manner would have properly rep- 

 resented the distribution offish school depths only 

 in December, when the thermocline was relatively 

 deep. A direct comparison of the bottom bounce 

 procedure and conventional sounding was con- 

 ducted in September when an 18 kHz hull-mount- 

 ed sounder was used simultaneously with the bot- 

 tom bounce instrumentation. The bottom bounce 

 data distribution ( Figure 2 1 is seen to differ in both 

 shape and sample size from the comparable con- 

 ventional sounder data ( Figure 3 ). The differences 

 in shape are principally due to undersampling of 

 the top 20 m of the water column by the conven- 

 tional system. The large difference in numbers of 

 schools observed simultaneously by the two 

 methods is due to a combination of the factors 

 mentioned in the introduction plus the smaller 

 sampling volume of the conventional system at 

 shallow depths. 



Broadband acoustic signatures of the targets 

 within the school group were obtained either dur- 

 ing the bottom bounce measurements or within a 

 few hours of the bottom bounce data acquisition. 

 The broadband signatures of the targets varied, 

 but were largely consistent with several sizes of 

 northern anchovy. The acoustic signatures, sup- 

 porting biological data, and ancillary environ- 

 mental data are detailed elsewhere (Holliday see 

 footnote 2 ). The acoustic resonances observed dur- 

 ing December were consistent with adult anchovy 

 (0.2 to 0.7 ml swim bladder volume) and a small 

 percentage of unknown targets not containing 

 gas-filled swim bladders. In May, the schools were 

 dominated by acoustic scattering consistent with 

 25-30 mm anchovy larvae and a few juvenile an- 



^HoUiday, D. V. 1978. MORDAX UlII IV Tracor Doc. No, 

 T-78-SD-002/3;4:-U, San Diego, Calif., 2260 p. 



492 



