HEWITT ET AL.: DEVELOPMENT AND USE OF SONAR MAPPING 



From the data presented so far (Figures 3, 4) we 

 may assume the most probable target strength 

 for all schools to be -9 dB. Further, assuming 

 that the "typical" school has a vertical dimension 

 of 12 m and that the measured target strength is 

 the summation of scattering strength of the indi- 

 vidual fish ensonified with no effects from multi- 

 ple scattering or attenuation, we may use Equa- 

 tion (6) to estimate q: 



TSj 



Spacing 



Bottom Topography 



Fixed transect surveys require that the dis- 

 tribution of schools be independent of fixed geo- 

 graphic locales whose scale is smaller than tran- 

 sect spacing. 



A cruise in March-April 1974, was designed to 

 test a postulated relationship between the oc- 

 currence of pelagic fish schools and bottom topog- 

 raphy. The area chosen was the Los Angeles 

 Bight and for the purposes of the experiment was 

 defined as that body of water bounded by the 

 southern California coast from Pt. Arguello to the 

 U.S. -Mexican border and seaward by a line ex- 

 tending south from Pt. Arguello to a point west of 

 San Miguel Island, thence southeast along an ex- 

 tension of the Santa Rosa-Cortez Ridge to a point 

 north of the east end of Cortez Bank, thence east 

 to the intersection of the shoreline and the 

 U.S. -Mexican border. The survey area, excluding 

 island masses, contains approximately 11.5 x 10^ 

 square nautical miles of sea surface area. 



The "Bight" was further divided into four 

 classes of bottom topography and transects de- 

 signed to distribute survey effort within these 

 zones as described below. The method used was to 

 delineate and compute the combined areas of the 

 first three categories and then assign the remain- 

 ing area to the fourth general zone. 



Combined seas and swells in excess of 7 

 feet prohibited sonar operations on 1 day out of 

 12 and somewhat altered the distribution of sur- 

 vey effort. A detailed breakdown of zones and ac- 

 tual survey effort is listed in Appendix Table 1. 



Daylight sonar tracking was accomplished dur- 

 ing two time periods separated by 2 wk: 25-29 

 March, 1 April, and 15-19 April 1974. No differ- 

 ence in schooling behavior was detected between 

 the two periods and results are presented for the 

 total cruise time in Appendix Table 2. If an area 

 was surveyed and no targets were detected, a "0" 

 under "No. targets obs." so indicates; if an area 

 was not surveyed during one or both time periods 

 then no numbers are recorded in the appropriate 

 columns. "Linear nautical miles surveyed" is the 

 distance traversed while sonar tracking over the 

 designated area. The observation window (250 m 

 wide beginning at 200 m from the ship, and 90° to 

 starboard from the ship's track) is multiplied by 

 the linear distance traversed and divided into the 

 number of targets observed to obtain target den- 

 sity, expressed in units of targets per square 

 nautical mile. 



The geographic names of various topographic 

 features are commonly accepted and can be lo- 

 cated on National Ocean Survey bathymetric 

 maps (numbers 1205N-15, 1206N-16, 1306N-19, 

 and 1306N-20) with the exception of the following 

 features informally named for the sake of con- 

 venience: Coronado Bank (lying immediately to 

 the east of Coronado Escarpment), San Diego Es- 

 carpment (along the west side of the San Diego 

 trough), Cortez Escarpment (east-northeast of 

 Cortez Bank), San Clemente Bank (a relatively 

 deep bank northeast of San Clemente Island), 

 Santa Rosa North and South Bank, San Nicolas 

 Escarpment (southeast of San Nicholas Island), 

 Santa Cruz Bank (south-southeast of Santa Rosa 

 Island), Santa Barbara Escarpment (west of 

 Santa Barbara Island at the southeast end of 

 Santa Cruz Basin), Santa Barbara Bank (north of 

 Santa Barbara Island), and Santa Monica Es- 

 carpment (along the southwest side of Santa 

 Monica Basin). 



The data fail to support the notion that the oc- 

 currence of pelagic fish schools can be related to 

 bottom topography over which they are detected. 

 Mean target densities (number of targets ob- 

 served per square nautical mile) were calculated 

 for the four classes of bottom topography and al- 

 though these densities range from 2.98 (banks 

 and seamounts) to 8.23 (escarpments and can- 



289 



