17 



shrimp trawl. Various trawl modifications 

 suggested from these results have been in- 

 corporated into a fourth try -net, which will 

 be test -fished in Kasitsna Bay. 



Central Pacific Fisheries Investigations 



"CROMWELL" STUDIES SONAR 

 TECHNIQUES TO TRACK TUNA 



BCF'S Townsend Cromwell cruised off 

 the leeward coasts of Oahu and Hawaii during 

 August to conduct sonar studies (Cruise 26). 

 It had 5 major missions: 



1 . To develop techniques for tracking tuna 

 schools with continuous transmitting, frequen- 

 cy-modulated (CTFM) sonar, with emphasis 

 on large skipjack. 



Sonar returns from three dead skipjack', 

 supported singly at the surface by airborne 

 balloons, showed echo strength was affected 

 by changes in the fish's dorsoventral orien- 

 tation. Returns were strongest when the fish 

 were in the normal swimming position; when 

 they were canted, returns were considerably 

 weaker. 



In 50 hours of searching for tuna schools, 

 the sonar contacted 3 schools of small skipjack 

 tuna (less than 8 pounds), one school of 10- 

 20 pound skipjack, and one school of 10-20 

 pound skipjack, together with yellowfin tuna 

 of about 50 pounds. As in the past, contact 

 with the small skipjack tuna was very short. 

 The longest contact was 4.5 minutes. The 

 returns from the school of larger skipjack 

 tuna were intermittently masked by returns 

 from a very choppy sea, so that contact was 

 quickly lost. The school of skipjack and yel- 

 lowfin tunas was tracked for 18 minutes and 

 up to a distance of 410 meters. 



2. To examine relation between depth of 

 targets located by sonar and temperature 

 profile of the environment. 



Stations planned over various parts of the 

 counterclockwise eddy west of Hawaii were 

 abandoned because examination of tempera- 

 ture profiles indicated that it was directly 

 downstream of Alenuihaha Channel. In their 

 stead, three other stations were selected 

 (track chart): one over a bank that sloped 

 from 500 to 1,500 fathoms in a somewhat ex- 



LEGEND : 



AREAS OF SONAR OPERATION 

 STD STATION 

 LONGLINE STATION 



Track chart Townsend Cromwell Cruise 26. 



posed area, another over a similar bank but 

 in a well protected area, and a third in an ex- 

 posed area with the bottom at 2,300 fathoms. 

 A sonar search pattern of 24 -hour duration 

 was executed three times at each station. 

 STD casts were made at the end of each 24- 

 hour search. 



A total of 879 targets was located: of these 

 44 were identified as porpoises, 1 as marlin, 

 and 1 as a school of skipjack tuna. Rates of 

 target contacts during night search were 2.5 

 per hour for surface targets and 0.5 per hour 

 for non-surface targets; during day search, 

 3.6 and 1.7 per hour, respectively. Surface 

 targets, 77 percent of contacts, occurred most 

 frequently between the hours of 0800 and 1200. 

 The depth distribution of the non-surface tar- 

 gets appeared to be centered at, or just above, 

 the salinity maximum. 



