NORTHERN ANCHOVY SCHOOL SHAPES AS RELATED TO 

 PROBLEMS IN SCHOOL SIZE ESTIMATION 



James L. Squire, Jr.' 



ABSTRACT 



Horizontal fish school profiles of the northern anchovy, Engraulis mordax, taken from day aerial 

 photographs and video tapes of school bioluminescence at night were examined to determine the 

 percentage of school area within a circular field of view and the school length and width ratios. Schools 

 observed during the day had an average length to width ratio of 2.09:1, at night the ratio was 2.53:1. 

 The percent coverage of the school's area in relation to a circle drawn tangent about the school averaged 

 42. 19c during the day and 29. 2^^ during the night. The effect of school shape on estimation of individual 

 school area as observed with a side-looking sonar was determined. School width measurements, similar 

 to that obtained by the sonar, were used to determine school area and indicated a possible average 

 overestimate of the actual school area of 1.72:1. The relation of school length and width to the error was 

 determined, indicating the gfreater the length to width ratio the greater the error. 



Profiles offish schools as viewed and photographed 

 in the horizontal plane from an airborne platform 

 have been published by numerous authors. 

 Radakov (1972), in his review offish schooling, 

 described the characteristic horizontal shapes of 

 fish schools in nature as being very diverse and 

 extremely changeable. He stated that a spherical 

 shape of a school is the rarest of all and also that a 

 school's shape, size, or density is a result of the 

 interaction between the fish and the physical and 

 biological environment. 



School shape and behavior in nature have been 

 studied with such techniques as aerial observa- 

 tion, hydroacoustic measurements, and underwa- 

 ter observation. Each of these methods has limita- 

 tions. Underwater visual observations are subject 

 to restrictions due to illumination and restricted 

 visibility. Aerial observation is limited in the day 

 by water transparency, illumination, and reflec- 

 tance from the water surface resulting from wind 

 and wave action. Visual observation of school 

 shape at night, as outlined by bioluminescent or- 

 ganisms, is limited to the moon's dark cycle or to 

 periods of no moon, and is affected by water trans- 

 parency and the density of bioluminescent or- 

 ganisms present in the water. Both day and night 

 observations are limited by the school's proximity 

 to the surface in relation to the factors affecting 

 water visibility. 



Hydroacoustic observations using lower fre- 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 

 92038. 



quency sounders of the type used commonly in 

 sonar fish surveys give imprecise images of fish 

 schools in the form of echograms that must be 

 interpreted. Greater resolution of fish school 

 shapes, but with limited range, can be obtained 

 with ultrasonic scanning equipment (Voglis and 

 Cook 1966). 



All of these observation techniques may alter 

 the environment and in many cases may result in 

 modification of fish school behavior. Fish school 

 behavior is affected when in close proximity to 

 ships, submersibles, and divers, and aircraft 

 (noise, shadow) could possibly modify the school, 

 though this is not documented. 



Surveys and research studies using variations 

 of these three observation techniques are cur- 

 rently in use for direct biomass estimation offish 

 populations by observation of individual schools, 

 school groups, and the internal structure of the 

 school. 



Hydroacoustic research on schooling fish is cur- 

 rently being conducted by the Southwest Fisheries 

 Center (Smith 1970; Hewitt et al. 1976). Coastal, 

 hydroacoustic surveys are conducted by the State 

 of California (Mais 1974) to determine a relative 

 abundance estimate of the northern anchovy, En- 

 graulis mordax. These surveys are conducted dur- 

 ing the daylight hours, as comparative tests indi- 

 cate an increased probability of detection during 

 this period (Smith 1970). 



Aerial observations by commercial fish spotters, 

 in the form of school counts and estimates of total 

 tonnage, are being used by the Southwest 



Manuscript accepted August 1977. 

 FISHERY BULLETIN: VOL. 76. NO. 2. 1978. 



443 



