NOTE Tanabe and Niu: Sampling juvenile Katsuwonus pelamis and Thunnus spp 
643 
than that for daytime (Table 2). Other 
dominant species collected at nighttime 
were myctophids, cephalopods, and 
euphausids; those taken in daylight were 
engraulids and acanthurids. 
Size classes of skipjack tuna and other 
tunas were widely represented; life 
stages from postlarva to early young were 
apparent. Where SL could be measured, 
specimens represented 92.2% of skipjack 
tuna, 93.5% of other tunas. Skipjack tuna 
SL ranged from 7.1 mm to 171.6 mm 
(Fig. 3); other tunas ranged from 8.0 mm 
to 139.8 mm (Fig. 4). The mean SL was 
25.7 mm for skipjack tuna, 27.4 mm for 
other tunas, with modes at 10-20 mm 
and 20-30 mm, respectively. Juveniles 
were abundant and were the dominant 
life stage of both species in our samples. 
The composition of larvae, juvenile, and 
young was 1.6%, 97.6%, and 0.8% for 
skipjack tuna, 0.7%, 98.6%, and 0.7% for 
other tunas. Specimens of both taxa col- 
lected at night tended to be larger than 
those collected during daylight. For skip- 
jack tuna, the size of specimens during 
daytime ranged from 7.1 mm to 81.8 mm 
(average 23.8 mm); at nighttime lengths 
ranged from 8.1 mm to 171.6 mm (aver- 
age 27.7 mm). Tunas captured in daytime 
ranged from 8.0 mm to 54.7 mm (aver- 
age 22.6 mm), at night from 11.0 mm to 
139.8 mm (average 31.7 mm). There was 
a significant difference in the average SL 
of both species between daytime and 
nighttime (Cochran-Cox t-test, P<0.05). 
Young skipjack tuna and other tunas 
were collected only at night and the num- 
ber of specimens per tow was less than 
six. Juveniles were as just as abundant 
at day as at night. 
Figure 2 
The sampling area. Stations (•) on the cruise tracks ( — ) indicate loca- 
tions where the midwater trawls were deployed. 
0 20 40 60 30 100 120 140 160 180 
Standard length (mm) 
Figure 3 
Length -frequency distribution of K. pelamis collected by the midwater trawl 
net in 1992-94 trial surveys. 
Discussion 
The midwater trawl net Tansyu made it possible to 
collect large numbers of juvenile skipjack tuna and 
other tunas during both day and night; this was not 
possible with the sampling gears previously avail- 
able. The results of our survey demonstrate that the 
Tansyu was effective in collecting juveniles of skip- 
jack tuna and other tunas. 
In previous studies, various sampling gears were 
used for sampling larvae of skipjack tuna and other 
tunas (Table 3), the results of which demonstrate that 
Table 1 
Results of collection of juveniles and young K. pelamis and 
Thunnus spp. by the midwater trawl net in 1992-94. Inds. = 
individuals. 
Occurrence i 
Total catch 
Catch per tow 2 
Species % 
(inds.) 
(inds.) 
K. pelamis 49.8 
3218 
9.8 
Thunnus spp. 27.5 
1074 
3.3 
1 Number of tows in which juveniles were caught/total tows x 100. 
2 Total catch of juveniles/total tows. 
