FISHERY BULLETIN: VOL. 74, NO. 2 



whereby the number of sound velocity 

 profiles taken in an area-time stratum 

 would be limited to the number of samples 

 necessary to reduce the standard error to a 

 uniform value for all strata. A probability of 

 detection diagram could then be constructed 

 from the ray trace analyses and target 

 counts corrected by range. We have not so 

 far considered these effects in our area of 

 operation, however, the implication of 

 undersampling should be investigated when 

 designing a serious stock assessment survey 

 using sonar. 



4. Diurnal and seasonal variations in school 

 sizes can be expected. In order to properly 

 evaluate their affect on a stock assessment 

 scheme the period and amplitude of these 

 variations must be measured. The collection 

 of a data base sufficient in size to detail 

 these changes, as well as geographic dis- 

 tribution patterns by season, was the pri- 

 mary motivation in designing an automated 

 data collection system. 



5. While it appears that influences of bottom 

 topography on school distribution may be 

 neglected, there is no reason to expect areal 

 distributions to be uniform. In fact, there is 

 evidence from aerial reconnaissance, sonar 

 transects obtained at long ranges (2,500 m), 

 and fishermen that fish schools may be dis- 

 tributed in a highly contagious fashion simi- 

 lar to the distributions of fish eggs and lar- 

 vae. In our opinion, this is a most important 

 consideration in arriving at an optimum 

 survey design. Smith^° and MacCall^^ have 

 approached the problem by direct measure- 

 ment and simulation modeling and suggest 

 a transect spacing of 15 miles as adequate to 

 reconstruct groups of anchovy schools off 

 southern California. 



e 

 in 



6. Holliday (1972, 1974) demonstrated th 

 feasibility of sizing individual fish within 

 schools and provided information which 

 would aid in species identification. A de- 



'"Smith, P. E. 1975. Precision of sonar mapping for pelagic 

 fish assessment in the Cahfomia Current area. SWFC Adminis- 

 trative Report No. LJ-75-60. Southwest Fisheries Center, 

 NMFS, NOAA, La Jolla, CA 92038. 



'^MacCall, A. 1975. Anchovy population survey simulation. 

 Contribution No. 4, CalCOFI Anchovy Workshop, July 1975. 

 Document on hand at the Southwest Fisheries Center, NMFS, 

 NOAA, La Jolla, CA 92038. 



velopment of these techniques as practical 

 additions to a sonar survey system would 

 reduce a presently loosely quantified factor, 

 i.e., the percent of detected schools which 

 can be expected to be the target species of a 

 survey. 



With regard to school target strength: 



1. The target strength of an individual fish is 

 an essential element in interpreting the 

 measured target strength of a school. At the 

 frequencies commonly used for sonar map- 

 ping we can expect interference of energy 

 reflected fi'om the various scattering parts of 

 a fish. This makes the target strength of a 

 fish strongly aspect dependent. Unfortu- 

 nately there is presently no method of 

 acoustically determining the aspect of indi- 

 viduals in a school and hence their effective 

 target strength. As such, the maximum dor- 

 sal or side aspect target strength is gener- 

 ally an overestimate and the use of these 

 values in interpreting school target 

 strengths results in an underestimate of the 

 number of individual scatterers. 



2. We may also expect multiple scattering, 

 shadowing, and attenuation within a school. 

 These effects may tend to reduce or enhance 

 the target strength of a school and cannot be 

 evaluated until we know the effective con- 

 tribution of the fish taken as individual 

 scatterers. Love (1971) stated that the 

 quantification of a fish school using its 

 target strength is possible because the 

 target strength of a school depends on the 

 average size, number, distribution, and as- 

 pect of the individuals in the school. If the 

 effects of the distribution offish in space and 

 their aspect can be removed, we may as- 

 sume an average size and estimate their 

 numbers. 



3. We have assumed spherical spreading losses 

 which may only be expected in a three-di- 

 mensional homogeneous fluid. In fact, the 

 upper mixed layer, in which we operate our 

 sonar, is characteristically bounded by den- 

 sity discontinuities which reflect and re- 

 fract sound waves. The actual path of 

 transmitted and target-reflected sound 

 waves may not be direct as implied in the 

 use of spherical transmission losses. 



296 



