FISHERV BULLETIN: \0L, 69. NO. 3 



30- 



i« 



20- 



O 



10- 



0- 



o-o DAY 

 •— • NIGHT 



2.0 

 2'.4 



« I ' " ' I I 



4.0 6.0 



I I 



4.4 6.4 



II'' 

 8.0 



8A 



STANDARD LENGTH (mm) 



Figure 3. — Percent length frequencies of bigeye tuna 

 larvae captured during the day (broken line, 271 speci- 

 mens) and night (solid line, 84 specimens). 



Analyses of our data show that yellowfin tuna 

 larvae were more successfully captured in day 

 tows than at night, even though greater net 

 avoidance during the day was indicated. Had 

 net avoidance been the major factor in day- 

 night differences in abundance, more larvae 

 should have been captured at night. Apparently 

 —since the opposite is indicated — yellowfin tuna 

 larvae migrate to the surface in the day and net 

 avoidance is of minor imi)ortance, in terms of 

 numbers collected. Ueyanagi (19(51) suggested 

 that istiophorid larvae behave similarly; other 

 workers (Wade, 1951 ; Strasburg, 1960; Klawe, 



1963; Ueyanagi, 1969) found no decisive evi- 

 dence to show that yellowfin tuna larvae perform 

 a vertical diel migration to the surface. 



Our study indicated that bigeye tuna larvae 

 — like those of yellowfin tuna — migrate verti- 

 cally to the surface in the day, but the proba- 

 bilities were not as significant (P = 0.02 com- 

 pared with P <0.01 for yellowfin tuna). Net 

 avoidance was negligible for bigeye tuna larvae. 

 Ueyanagi (1969) reported a greater larval ocur- 

 rence of bigeye tuna at the surface during the 

 day than at night. 



Our evidence showed that skipjack tuna 

 larvae migrate vertically to the surface at night 

 and that net avoidance was apparently negligi- 

 ble. A vertical migration to the surface at night 

 also was suggested by Wade (1951) and Stras- 

 burg (1960). Ueyanagi (1969) reported a 

 scarcity at the surface during the day, but in- 

 creased abundance at night. 



&^ 



20- 



O 



«. ^o-\ 



o 



>- 



0- 



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

 2.0 4.0 6.0 8.0 



I I I I 



2.4 4.4 6.4 8.4 



STANDARD LENGTH (mm) 



T— r 



FlGURE 4. — Percent length frequencies of skipjack 

 larvae captured during the day (broken line, 22 ; 

 mens) and night (solid line, 197 specimens). 



558 



