FISHERY BULLETIN: VOL. 76, NO. 2 



Fisheries Center to calculate indices of apparent 

 abundance for several coastal species, including 

 the northern anchovy (Squire 1972). To aid in the 

 detection and quantification of pilchard, Sar- 

 dinops ocellata, shoal occurrence off South Africa, 

 Cram (1974) used an airborne, low-light-level, 

 electron image intensifier to view the ocean's sur- 

 face, detecting the bioluminescence offish schools. 

 During these night aerial surveys the school's 

 horizontal surface area was interposed on the in- 

 strument's circular field of view, and running es- 

 timates of the percentage of coverage were made. 

 These percentage estimates were then used in the 

 computation of biomass estimates. 



The intensifier used by the Sea Fisheries of 

 South Africa has been used by the author off the 

 southern California coast on an experimental 

 basis. Due to the highly variable school shapes 

 encountered, making estimates of the percentage 

 of school coverage in the circular field of view are 

 difficult. Experience indicated that examination 

 of aerial color photographs and night low-level 

 video tapes of anchovy school shapes for determi- 

 nation of the percentage of school coverage within 

 a circle would be useful, particularly if in the fu- 

 ture, surveys were to be conducted at night using 

 this method for the development of biomass esti- 

 mates for the northern anchovy and other near- 

 surface schooling pelagic species. 



In addition, an analysis of anchovy horizontal 

 school shapes may assist hydroacoustic research- 

 ers in determining error parameters for computa- 

 tion of sonar biomass estimates. Hydroacoustic 

 surveys currently conducted for the northern an- 

 chovy use both side- and vertical-looking sonar to 

 detect and measure fish schools and school groups 

 during the day along a predetermined survey 

 track line. The acoustic "beam" used in these sur- 

 veys varies according to the unit and is of ±5° to 

 10°. When detecting the school, the side-looking 

 sonar measures the maximum dimension in one 

 aspect of the school, either normal to or parallel to 

 the ship's track. For the purpose of calculating 

 horizontal area, in contrast to the aircraft's verti- 

 cal view of the actual horizontal school area, the 

 echogram school width is assumed to be elliptical 

 (Smith 1970). Preliminary attempts at biomass 

 estimation from sonar surveys have used the sim- 

 ple assumption that a series of estimates of the 

 width of an elliptical school from random aspects 

 will result in an unbiased estimate of school hori- 

 zontal area. In a side-looking sonar the school 

 width is measured and provides two points of ref- 



erence with the orientation of these points about 

 the school's profile being unknown. If an ellipse is 

 fitted randomly between these two points, the re- 

 sulting average area will equal a circle, a condi- 

 tion that was not observed in aerial photographs of 

 anchovy schools. 



METHODS 



To examine the shape of northern anchovy 

 schools as observed during day and night and to 

 determine what percentage the school occupies of 

 a circle tangent to two points along the school's 

 edge and containing the school inside the circle, 

 a circle was drawn about school profiles ob- 

 tained from a series of 20 day oblique aerial color 

 photographs (from the photographic files of the 

 Southwest Fisheries Center) and of 20 night 

 photographs of fish school bioluminescence. The 

 bioluminescent anchovy school shapes were 

 photogi'aphed from a television monitor as it pro- 

 jected video tapes recorded from an airborne low- 

 light-level television camera used during anchovy 

 resource surveys off northwestern Mexico. The 

 night photographs were made available through 

 the courtesy of Zapata, Inc.^ (Zapata Fisheries), 

 Houston, Tex. The night surveys using low-light- 

 level television were conducted at elevations of up 

 to 1,828 m (6,000 ft) and this survey technique is 

 effective because the northern anchovy commonly 

 migrates to the near-surface area during hours of 

 darkness (Squire 1972). 



The actual area of the schools observed in the 

 photographs are unknown due to lack of data on 

 the aircraft's altitude, camera angle, and camera 

 geometry; however, all were taken from angles 

 approaching vertical. However, all area calcula- 

 tions are expressed in percentages of a circle 

 drawn tangent about the school's edge. 



The day school profiles were further analyzed to 

 determine what the school area would be if the 

 width measurement were considered to be equal to 

 the school's diameter and what the area would be 

 if viewed systematically from six points 30° arc) 

 about an arc of 180° around the school (based on 

 school width or diameter as determined similar to 

 the measurements made from a hydroacoustic 

 sounder). These area data calculated from the six 

 points of observation to determine school width 

 were then compared to the actual school area. 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



444 



