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Fishery Bulletin 96(3), 1998 
Figure 1 
A schematic diagram of the midwater trawl net Tansyu that was devel- 
oped to collect juvenile skipjack tuna and other tunas. The numerals on 
the left side indicate the length of each panel. The numerals within the 
diagram indicate twine size and mesh size of the net. 
A scale model of the Tansyu was con- 
structed and tested in a tank at Nichimo 
Co. Ltd. to test its design and performance. 
Research cruises were carried out 
from late October to early December, 
1992-94. The survey covered an area 
from offshore of Palau past the Calorine 
Islands in the tropical western Pacific 
(Fig. 2). A 400-ton class stern trawler 
was chartered by the Fisheries Agency, 
Ministry of Agriculture, Forestry, and 
Fisheries, Tokyo, to conduct the mid- 
water trawl trials. 
Midwater trawling was typically con- 
ducted four times daily; standard du- 
ration of towing was approximately one 
hour. Towing time was defined as the 
duration from beginning of the net tow 
to commencement of warp hauling. Ten 
strata were occupied from near surface 
to 200 m, in 20-m increments of water 
depth; separate tows in each stratum 
were carried out. Depth of the net was 
established by noting the warp length, 
and towing speed was usually set 4 to 5 
knots against the currents. A net-depth 
recorder (Furuno FNR-200) was used to 
monitor continuously the mouth open- 
ing and depth of the net throughout the 
trawling operation. 
As soon as the net was brought on 
deck, the collection from the codend was 
weighed as whole wet weight; skipjack 
tuna and other tunas were sorted from 
the collection. Samples were fixed in 
10% buffered formalin and preserved in 
80% ethanol. All skipjack tuna speci- 
mens were identified after the cruise 
following the methods of Matsumoto et 
al. (1984). The species of tunas were 
identified as Thunnus spp. and samples of skipjack 
tuna and other tunas were measured individually to 
the nearest 0.1 mm SL with calipers. 
In this report, the early life stages of skipjack tuna 
were defined as follows: larva: from hatching to less 
than 10 mm SL; juvenile: from 10 mm SL to less than 
100 mm SL; and young: from 100 mm SL to less than 
300 mm SL. 
Results 
The new trawl was deployed a total of 327 tows 
throughout the three years of the study. The target 
taxa, skipjack tuna and other tunas, were captured 
in 163 tows; skipjack tuna and other tunas co-oc- 
curred in 66 tows. Over four thousand skipjack tuna 
and other tunas were collected (Table 1). The inci- 
dence of juveniles collected remained high in all 
years. The maximum number of specimens collected 
in a single tow was over 100 juveniles of both taxa. 
The diel distribution of sampling effort was 41.8% 
during daytime, 58.2% at nighttime (Table 2). For 
skipjack tuna, the mean daytime occurrence was 
about 5% higher than that at nighttime, and the 
mean daylight catch per tow was about four indi- 
viduals greater than that at nighttime; a similar 
pattern was observed for other tuna, but the differ- 
ences were not statistically significant. The whole 
wet weight for nighttime usually tended to be larger 
