FISHERY BULLETIN: VOL 77. NO I 



with a submersible salinity, temperature, and 

 depth (STD) recorder. 



Acoustic Measurements 



A Ross Laboratories^ 102 kHz echo sounder with 

 its transducer housed in an Endeco V-fin was used 

 in conjunction with a 12 kHz hull mounted Edo 

 transducer triggered by an Edo model 444 trans- 

 ceiver and model 551 recorder. Information from 

 the 12 kHz sounder was recorded only as qualita- 

 tive echograms. Acoustic data from the 102 kHz 

 echo sounder was recorded qualitatively as echo- 

 grams, and quantitatively over specific 20 m 

 (26.87 ms) intervals where the scattering was ob- 

 served. The returned signal for this interval was 

 electronically squared and integrated for each 

 pulse, and the value displayed on a chart recorder. 

 The average scattering level (RL ) over this 20 m 

 interval was then calculated and the volume scat- 

 tering strength iSj was determined by the follow- 

 ing (Urick 1975): 



the layer. The samples were preserved in 10% 

 buffered Formalin in the field and transferred to 

 70% ethanol in the laboratory. All fishes were 

 identified to species and their standard length 

 measured. 



The density of fishes collected was calculated by 

 dividing the number of animals caught by the 

 product of ship speed (meters per minute) times 

 the length of the tow (minutes) times the mouth 

 area (square meters). The mouth area of the net 

 has been calculated to be 2.36 m^ assuming a 45° 

 fishing angle (Davies and Barham 1969). 



Swim bladders were measured from 12 T. 

 mexicanus which represented the range of sizes of 

 this species collected by the trawl in the 102 kHz 

 scattering layer (trawl 25660''). Swim bladder 

 measurements were also taken from four 

 Protomyctophum crocken, five Argyropelecus sla- 

 deni, and five Vinciguerria h/cetia collected from 

 two earlier trawls (trawls 25657 and 25658). The 

 volume of the swim bladder was calculated by 

 using the formula for a prolate spheroid (Capen^). 



S,{dB/m^)=RL-SL -h 401ogr -H 2or - 10 log V 



Calculations 



where RL = average received (scattering) 

 level 

 SL = source level 

 r = mean range of the 20 m interval 

 a = absoprtion loss per m^ 

 V = volume insonified = 

 (cT/2)(^r2) 

 where c = speed of sound 

 r = pulse length 

 M' = solid angle of the 

 ideal two-way 

 beam pattern. 



Volume scattering strengths were determined 

 at various times (and depths) as the layer mi- 

 grated toward the surface. 



Biological Sampling 



Biological samples were collected with a 6-ft 

 modified Tucker trawl with an acoustically con- 

 trolled opening-closing sequence and a continuous 

 depth readout on a Giffl recorder. While sampling 

 in the migrating scattering layer, the net depth 

 was regulated to keep pace with the movement of 



The density of fishes was then used to calculate 

 their average target strength (TS ) by applying the 

 formula: 



TS = S„ (fishes) - 10 log (fishes m-') 



where S;, (fishes) 



10 log[log 1 0.1 S, (total) 

 - log 1 0.1 S,. (plankton)]. 



The S,, value for plankton was calculated from an 

 average of the two integration values recorded 

 after the fish scattering layer had migrated out of 

 the integration window. 



The depth where the swim bladder would reso- 

 nate at 12 kHz was calculated for the range of 

 swim bladder sizes observed assuming constant 

 swim bladder volume with depth. These calcula- 

 tions were determined by solving for z (depth) in 

 the following simplified formula for resonance of 

 an air bubble in water (Clay and Medwin 1977; 

 equation 6.3.10): 



/po = (3.25 X 10«/a)(l +0.l2)^ 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service. NOAA. 



'All trawl numbers mentioned refer to ship's station numbers 

 (Ve/ero/V.University of Southern California I. 



^Capen, R. L. 1967. Swimbladder morphology of some 

 mesopelagic fishes in relation to sound scattering. U.S. Navy 

 Electronics Laboratory, San Diego, Calif, Rep 1447. 2.5 p. 



936 



