RICHARDS AND SIMMONS: DISTRIBUTION OF TUNA LARVAE 



Table 2. — Probabilities of equal catches of larvae in day and night 



plankton tows. 



Species 



Number of 

 day tows 



Number of 

 night tows 



Total number of standard- 

 ized larvae per number of 

 successful tows 



Day 



Night 



Probability 

 of equal 

 catches 



easily. We also reasoned that large larvae, be- 

 ing better swimmers than smaller larvae, should 

 have been captured less frequently in day col- 

 lections than in night collections. Thus, if net 

 avoidance was demonstrable, the lengths of 

 larvae caught during the day should have been 

 smaller. 



Percent length frequencies of each species of 

 larvae collected in the day and night were plotted 

 from the following data: Yellowfin tuna, 1,009 

 day-caught larvae, 340 night-caught larvae (Fig- 

 ure 2); bigeye tuna, 271 day, 84 night (Figure 

 3) ; skipjack tuna, 22 day, 197 night (Figure 4) ; 

 little tunny, 134 day, 72 night (Figure 5) ; and 

 Auxis, 1,636 day, 1,082 night (Figure 6). 



The length frequencies of night-caught larvae 

 tended to be skewed more toward the larger 

 sizes than did the day-caught larvae. The bi- 

 modal frequency of skipjack tuna captured dur- 

 ing the day could have been due to the small 

 sample size. The Mann-Whitney U test (Siegel, 

 19.56) was applied to the frequencies to deter- 

 mine if night-caught larvae were significantly 

 larger than day-caught larvae. Probabilities of 

 less than 0.01 that larvae were the same length 

 were found for yellowfin tuna, little tunny, and 

 Auxis; bigeye and skipjack tunas had probabil- 

 ities of 0.06 and 0.22, respectively. It should be 

 noted also that the largest larvae of every spe- 

 cies but bigeye tuna were captured at night. 

 We tentatively conclude that there was greater 

 net avoidance during the day for yellowfin tuna, 

 little tunny, and Auxis, but little net avoidance 

 for bigeye and skipjack tunas. 



A differential vertical migration on the basis 

 of size also should be considered as a possible 

 explanation for the capture of larger larvae at 

 night. Certain evidence causes us to reject this 

 possibility, however. Ueyanagi (1969) found 



30- 



»^ 



20- 



O 



O 



^ 10 



0- 



o-o DAY 

 •— • NIGHT 



2.0 

 2A 



I I I I I I I I I I I I I I I I I 

 4.0 6.0 8.0 10.0 



4.4 6.4 8.4 10.4 

 STANDARD LENGTH (mm) 



Figure 2. — Percent length frequencies of yellowfin tuna 

 larvae captured during the day (broken line, 1,009 

 specimens) and night (solid line, 340 specimens). 



that the size composition of tuna larvae taken in 

 night surface tows resembled the size composi- 

 tion of those taken during both day and night 

 at depth. Smaller larvae were more numerous 

 in catches made at the surface during the day. 

 The implication is that net avoidance of larger 

 larvae is greater at the surface during the day, 

 and there is no indication of a vertical migration 

 of the two size groups in opposition to one an- 

 other. 



557 



