RICHARDS AND SIMMONS: DISTRIBUTION OF TUNA LARVAE 



30H 





20- 



o 

 o 



i: 10 



0- 



2.0 

 24 



' I ' 

 40 



6.0 



4.4 6.4 

 STANDARD 



' " I ' 

 8.0 



I 

 8/4 



LENGTH 



T-r-r-rA/T-r 



10.0 22.0 



104 

 (mm ) 



224 



Our night tows caught little tunny larvae 

 more successfully than day tows, but differences 

 were not as pronounced as they were for skip- 

 jack tuna larvae (P = 0.03 compared with 

 P <0.01 for skipjack tuna larvae). Since a 

 greater ability to dodge the net during the day 

 was indicated, day-night differences could have 

 been caused by migration to the surface at night, 

 net avoidance, or a combination of both. Among 

 larvae of yellowfin tuna, bigeye tuna, skipjack 

 tuna, and Auxis, net avoidance was negligible 

 or ineffective in detecting day-night differences 

 in abundance. The higher frequency of night 

 captures of little tunny larvae, therefore, was 

 probably caused primarily by vertical migration 

 to the surface at night. Vertical migration to 

 the surface at night also was suggested for the 

 closely related Eidhynnus yaito (=E. affinis) 

 by Wade (1951). 



Figure 5. — Percent length frequencies little tunny larvae 

 captured during the day (broken line, 134 specimens) and 

 night (solid line, 72 specimens). 



^20- 



O 





10- 



o 



oc 0- 



o-o 



o-o DAY 

 NIGHT 



I I I I I I ' ' I ' ' 

 2.0 4.0 6.0 



I I I 



2.4 4.4 6.4 



I I I I I I I I I I I I I I I I I I I I I I ''' I ' 



8.0 10.0 12.0 14.0 16.0 18.0 20.0 



I I I I I I I 



8.4 104 124 14.4 16.4 18.4 > 



STANDARD LENGTH (mm) 



Figure 6. — Percent length frequencies of Auxis larvae captured during the day (broken line, 1,636 specimens) 



and night (solid line, 1,082 specimens). 



559 



