NOTES 



THICKNESS AND DEPTH DISTRIBUTIONS 



OF SOME EPIPELAGIC FISH SCHOOLS 



OFF SOUTHERN CALIFORNIA 



Many schooling fish species such as northern an- 

 chovy, Engraiilis morda.x; jack mackerel, Trach- 

 iiriis symmetrictis; and Pacific mackerel. Scomber 

 japonicus, are adept at avoidance of surface ves- 

 sels, even those moving at relatively high speeds. 

 Evasion behavior has complicated measurement 

 of the vertical extent, thickness, and distribution 

 in depth of such fish schools using standard echo 

 sounding techniques. In addition, hull-mounted 

 echo sounders are usually 3 to 4 m below the sur- 

 face, are blanked for the duration of the transmit- 

 ted pulses and are relatively ineffective for 

 another 5 to 10 m due to high surface and volume 

 reverberations immediately following the pulse 

 transmission. The combination of evasion be- 

 havior and transducer mounting and operation 

 often results in poor sampling of the upper 10 to 20 

 m of the water column by hull-mounted echo 

 sounders. 



Commercial fishermen routinely use air spot- 

 ters to guide them to school groups (Squire 1972). 

 Fishermen often set their gear visually in water so 

 rich with plankton that visibility is severely re- 

 stricted. An awareness of these practices and of 

 the implication that many of the fish landed com- 

 mercially are caught at relatively shallow depths 

 emphasizes the need for a good tool for studying 

 shallow schools. Determination of fish size from 

 swim bladder resonance data requires accurate 

 measurement of the depth and thickness of 

 schools, including those in the upper 20 m (HoUi- 

 day 1977). 



When operating an echo sounder in shallow wa- 

 ter, multiple "bottom" echo traces often appear. 

 The second "bottom" in these traces is an image of 

 the sea surface as reflected by the sea floor. With 

 appropriate attention to signal processing it is 

 possible to make measurements on subsurface 

 targets detected via sound which has been reflect- 

 ed from the seabed. Under these conditions, a 

 school of fish near the surface will appear just 

 above the second "bottom." The procedure used to 

 obtain the data presented in this paper is a varia- 

 tion on this observation. 



Materials and Methods 



Measurements of the mean depth and thickness 

 of schools or aggregations of marine organisms 

 were made at three locations in the California 

 Current near the southern California coast. Each 

 location was occupied during a different season, 

 the first in December 1976, the second in May 

 1977, and the last durmg September 1977. The 

 December and May work was done near Santa 

 Catalina Island and the September data were 

 taken about 15 mi southwest of Oceanside, Calif 

 In December, only 17 schools were studied, be- 

 cause the location of the ship at that time did not 

 coincide with the presence of a large school group. 

 All measurements were made during daylight 

 hours. The schools studied were previously de- 

 tected on the 30 kHz sonar in its normal side-look- 

 ing mode. In May, 121 schools were studied and in 

 September measurements were made on 221 

 targets. 



Our bottom bounce system was implemented 

 using the sonar aboard the NOAA ship David 

 Starr Jordan. The procedure involved a 30 kHz 

 sonar, steerable in the vertical and horizontal 

 planes, with a capability for depression to 90°, i.e., 

 vertical as in the standard echo sounding mode. 

 Over a flat bottom, the sonar was normally oper- 

 ated at a depression angle of 80° to 85°, depending 

 on water depth. The horizontal steering was to 

 either port or starboard, that is, normal to the 

 ship's track. This allowed sampling of a path 

 whose width varied with water depth on the 

 selected side of the ship and parallel to the ship's 

 track (Figure 1). As an example, for a flat bottom, a 

 depression angle of 80° and a water depth of 500 m, 

 a path was studied with an inner edge which inter- 

 sected the surface 70 m from the ship's track, and 

 an outer edge which extended to 275 m. These 

 limits were derived from the sonar's 12° beam 

 width, defined as the -6 dB point on the two way 

 (transmit and receive) beam pattern. 



The data reported in this note were acquired 

 using 1 ms CW pulse waveforms. It was deter- 

 mined that reliable measurements could be made 

 in water depths up to 500 m with bottom slopes of 

 up to 1.4° using a source level of 216 db//l/xPa at 1 

 m. Bottom characteristics were thought to be 



FISHERY BULLETIN VOL 77. NO 2. 1979 



489 



