yards. The maximum sonar range varied considerably 

 among schools, owing to differences in fish size and the 

 height of the waves. Ma.\imum range (880 yards) was 

 attained not on a school of fish, but on i)orpoises. 



The sonar has high resolution and a high information rate. 

 It can rapidly search a complete underwater hemisphere. 

 The high resolution makes it possible to determine accurately 

 the position of a fish relative to the ship. The high informa- 

 tion rate makes it possible to note the fish's position as 

 often as every 30 seconds. Consequently, the biologists can 

 obtain data on the swimming speed and course of a fish 

 over short periods of time. For example, an unidentified 

 subsurface target was found to be swimming at 2 knots at a 

 depth of 175 feet (fig. 29). It was tracked as it surfaced, 

 turned, and swam back to the ship. There it was identified 

 as a whitetip shark about 5 feet long. 



Much of the early work with the sonar was concerned 

 with known targets. These were either dead fish suspended 

 at a known range or skipjack tuna schools that had been 

 sighted from the bridge. Occasionally schools could be seen 

 at the surface that could not be incked up on .sonar. The 

 cause seems to be that signals vary according to the size 

 of the fish and also the sea state. Target strengths were 

 much lower for smaller fish and in high seas, where more 

 sound-reflecting air bubbles were mixed in the water. 



The sonar in effect makes the whole upper layer of the 

 sea transparent. During the cruises there might be no signs 



of fish schools at the surface, but the sonar would locate 

 numerous schools below the surface. Most of the fish were 

 in the upper 30 feet, but a second concentration was at about 

 150 feet, and a third at about 400 feet. Once a skipjack tuna 

 school that had been identified at the surface was tracked 

 to a depth of 120 feet. Again, a mixed group of skipjack 

 and yellowfin tunas was tracked to 420 feet. 



Efforts to stay with targets were fairly successful. Once 

 contact was maintained with a school of large skipjack tuna 

 for almost an hour, and for 3 hours with a group of mixed 

 skipjack and yellowfin tunas. 



As yet, species identification by sonar alone has not been 

 possible, although a painstaking review of the data may offer 

 clues. Porpoises can be distinguished from fishes, but the 

 individual species of fish cannot be determined from the 

 sonar scope. The identification of porpoises (as a group, not 

 species) is particularly easy, of course, for these animals 

 emit sounds that are detectable on the sonar. The fish 

 apparently do not, although this "passive" phase of sonar — 

 listening for fish sound.s — remains to be investigated more 

 thoroughly. Previous research by Robert T. B. Iversen at 

 the BCF Laboratory in Honolulu has shown that yellowfin 

 tuna can hear (they can be trained to respond to underwater 

 sounds) and that they themselves emit rudimentary sounds, 

 whose biological purpose is as yet unknown and which have 

 not been heard at sea. 



FIGURE 29. How fish behavior underwater is studied. Here a torget 

 later identified as a whitetip shork was spotted ot a depth of 175 feet 

 and almost directly oheod of the ship. The shark swam oway from the 

 ship for severol hundred yards, then rose to the surface and returned. 

 The octuol track shows that he followed on irregular course on his 

 trip away from the ship and his return. Distances are in feet. 



43 



