PHOTOGRAPHIC METHOD FOR MEASURING 



SPACING AND DENSITY WITHIN 



PELAGIC FISH SCHOOLS AT SEA 



Few measurements exist of the spacing and den- 

 sity of fish within schools in the sea (Radakov 

 1973) although these characters have been well- 

 studied in the laboratory (Breder 1954; Keen- 

 leyside 1955; Dambach 1963; Williams 1964; John 

 1964; Cullen et al. 1965; Hunter 1966; van Olst 

 and Hunter 1970; Symons 1971). The density and 

 spacing of fish within schools under natural 

 conditions must be known if realistic fish 

 abundance estimates are to be made from sonar 

 survey data (Hewitt et al. 1976). This note de- 

 scribes a camera system that photographed fish 

 schools at sea and a method used for estimating 

 the density and interfish spacing from the 

 photographs. 



The camera system 1 consisted of an anodized 

 aluminum casing which housed a spring-driven 

 advance 35-mm camera, strobe light, and electri- 

 cal components. The system was made watertight 

 by creating a vacuum which sealed the acrylic 

 lenses to the casing. Attached to the casing were a 

 depth release with expendable chain ballast, 

 floats, and a signal flag (Figure 1). 



Upon immersion, the camera assumed an 

 upright position, closing a mercury switch and 

 starting an electric timer which activated the 

 camera shutter and strobe light simultaneously. 

 The system took 14 photographs per drop at set 

 intervals of 24 or 48 s while sinking at a rate of 10 



'Designed by Daniel M. Brown, Scripps Institution of 

 Oceanography (SIO) from an idea of John D. Isaacs, SIO. 

 Blueprints are available at the Marine Sciences Development 

 Shop, SIO. 



FLAG-FLOAT UNIT 



CAMERA HOUSING 



--•-PLASTIC FLAGS 

 — CHEMICAL GLOW LIGHT 



10' ALUMINUM POLE 



-ALUMINUM TRAWL 

 FLOATS 



-STAINLESS STEEL 

 PIPE 



-20' -3/8" 



POLYPROPYLENE ROPE 



-3/8" STAINLESS STEEL 

 -CAMERA SHACKLE 



— VACUUM VALVE 

 -STROBE 



-24" x I" NYLON WEBBING 



DEPTH RELEASE 



SOLUBLE RELEASE 



-I" THICK NYLON WEBBING 



BALLAST 



29 LINKS - 1/2" ANCHOR 

 CHAIN (.3276 lb/ link ) 



FIGURE 1. — (A) The Isaacs-Brown free vehicle drop camera. 

 (B) A lateral view of the upper camera housing. Once the 

 camera was upright, the mercury switch closed and the electric 

 timer discharged every 24 or 48 s which caused the solenoid to 

 contract bringing the depressor arm down on the shutter re- 

 lease. The strobe light fired simultaneously and the film was 

 advanced automatically. (C) The wiring diagram for the cam- 

 era system. 



B 



FOAM PADDING 



SHUTTER 

 RELEASE" 



STROBE 

 LEAD 



ACRYLIC 

 LENS 



LATERAL VIEW OF UPPER 

 CAMERA HOUSING 



MERCURY SWITCH 

 MICRO-SWITCH 

 AS 408 A-l 



~~UjuuuU 



dormeyer 



B24-755 A-l 



10 2 

 °9 o 3 ' 



12 V 



II PIN SOCKET 

 MICROTRONICS 

 DIGILAY 275-IA 



-o-f^J SWITCH 



I ALCO DPDT 

 MST 205 



2500 MFD 

 25V dc 



H'l'k 



22 5 V 



22 5 V 



H 



DROP CAMERA WIRING 



230 



