| W. Marshal 



Equipment and 



Techniques for 



Monitoring the Vertical Distribution 



of Fish in Shallow Water 



ABSTRACT 



A recording echo sounder was modified to scan multiple custom 

 transducers to record the depth distribution of fishes. The sys- 

 tem has been used in tanks as well as the Columbia River and 

 eliminates the objections that other sampling methods influence 

 depth distribution. 



Bioassay tests conducted at the NMFS Prescott 

 Facility in 1972 revealed that salmonids tested in 

 8-ft deep wooden tanks survive the stress of gas 

 supersaturated water more readily than fish tested 

 in shallower tanks (Blahm, 1972). The hydrostatic 

 pressure of the water at depths effectively prevents 

 gas bubbles from forming in the organs of the ex- 

 posed fish. Therefore, few mortalities occur. 



This paper describes an acoustic counting sys- 

 tem developed to provide information on the vertical 

 distribution of Columbia River fishes. Because of 

 the river's turbidity and various practical problems 

 involved with employing fishing gear, an acoustic 

 system was considered a desirable alternative to 

 possible optical or direct capture methods. The 

 acoustic system was initially developed to record 

 the vertical distribution of fish in the Prescott Lab- 

 oratory's 8-ft test tanks. Later, equipment was 

 developed to monitor the vertical distribution of 

 fish occurring along river beaches near Prescott. 



ECHOSOUNDER LIMITATIONS 



Depth sounding equipment has been used suc- 

 cessfully for locating fish for both sport and com- 

 mercial fishing. However, the use of conventional 

 recording depth sounders to indicate the depth of 

 fish in shallow water (0 to 10 ft) is difficult because 

 of mechanical limitations. The minimum full chart 

 depth displayed by a single-stylus sounder is typi- 

 cally 100 ft. If 5-in. wide chart paper is used, fish at 

 depths to 10 ft will be displayed in a 0.5-in. wide 

 section of the chart (Fig. 1). If there are many fish 

 recorded in this 0.5-in. space, it is difficult to dif- 

 ferentiate individual fish. To spread this 0- to 10-ft 



1/2 IN 



5 IN 





 20 

 40 

 60 

 80 

 100 



FIG. 1 A typical moving-stylus depth sounder echogram when 

 the transducer is used inside a 10-ft deep test tank. 



depth display to the full 5 in., the stylus must move 

 across the paper at a speed 10 times faster than 

 before. Construction of a mechanical train to move 

 the stylus this fast is practically impossible. 



The Benmar DR-680*' sounder eliminates prob- 

 lems associated with a moving-stylus recorder by 

 using 320 individual non-moving stylii. A 400 kHz 

 carrier frequency provides high resolution of small 

 targets; individually detected fish are precisely 

 recorded (Fig. 2). Custom-made wide beam trans- 

 ducers were obtained from Webster Transducers 

 and provided a detection cone with a base diameter 

 of 3 ft in an 8-ft deep by 6-ft diameter test tank 

 (Fig. 3). 



PRELIMINARY TESTS 



In May 1973, a series of 24-hr tests was con- 

 ducted with juvenile coho salmon in the 8-ft deep 



Marshall: Marine Fish and Shellfish Division, National Marine 

 Fisheries Service, Seattle, Washington. 



"Trade names referred to in this publication do not imply en- 

 dorsement of commercial products by National Marine 

 Fisheries Service. 



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