641 
Sampling juvenile skipjack tuna, 
Katsuwonus pelamis, and other tunas, 
Thunnus spp., using midwater trawls 
in the tropical western Pacific 
Toshiyuki Tanabe 
Tohoku National Fisheries Research Institute 
3-27-5, Shinhama, Shiogama, Miyagi, 985-0001 Japan 
E-mail address: katsuwo@myg.affrc.goJp 
Kodo Niu 
Kasumi Senior High School 
40-1, Yada, Kasumi, Kinosaki, Hyogo, 669-6563 Japan 
Skipjack tuna, Katsuwonus pelamis, 
a highly migratory species, is one 
of the most important stocks for 
commercial fisheries in the western 
Pacific. The spawning ground of 
this species extends throughout 
equatorial and subtropical waters 
but larvae are more abundant in 
lower than higher latitudes (Nishi- 
kawa et al., 1978). Young and pre- 
adult skipjack tuna migrate great 
distances seasonally between tropi- 
cal and temperate waters, yet little 
information is available for juvenile 
stages of this species. 
Exploitation of skipjack tuna in 
the western Pacific has increased 
in recent years; an annual catch of 
940,000 metric tons was taken in 
1993 (FAO, 1995). Stock assess- 
ment and management of the spe- 
cies require detailed biological in- 
formation on each life stage but, 
largely because of sampling difficul- 
ties, data on juveniles of skipjack 
tuna and other tunas are relatively 
limited compared with those on 
adults. Juveniles swim well and 
thus escape many gear: ring nets, 
beam trawls, or small pelagic 
trawls (see Methot, 1986). Other 
gear and light traps will not pro- 
vide adequate spatial or temporal 
coverage (Thorrold, 1993). 
Consequently development of 
appropriate sampling gear has 
been an important part of our study 
of the early life history of skipjack 
tuna and other tunas. Since 1992, 
we have worked to develop a new 
method for capturing juvenile skip- 
jack tuna and tuna spp., namely a 
midwater trawl net with a large 
mouth opening, capable of high- 
speed towing. This appeared to be 
an excellent gear for sampling ju- 
veniles. In a trial survey, we cap- 
tured large numbers of juvenile 
skipjack tuna and other tuna by 
using the midwater trawl net in 
offshore waters of the tropical west- 
ern Pacific. This study is based on 
1992-94 research cruises and con- 
firms the effectiveness of the new 
midwater trawling gear. We de- 
scribe the specifications of this net 
and its effectiveness for collecting 
juvenile skipjack tuna and other 
tuna. 
Materials and methods 
The midwater trawl net Tansyu has 
a large mouth opening and was de- 
signed to be towed at high speed. 
It was developed in cooperation 
with Tohoku National Fisheries 
Research Institute and Nichimo Co. 
Ltd., Shimonoseki, Yamaguchi. Tar- 
gets for the Tansyu included skip- 
jack tuna and other tuna, ranging 
in size from 10 to 200 mm standard 
body length (SL). The goal was to 
capture sufficient numbers to char- 
acterize distribution and relative 
abundance. The Tansyu design was 
based on the midwater trawl net 
Yoko-2, which has been used for 
sampling sardine by Seikai Na- 
tional Fisheries Research Institute, 
Nagasaki (Takeshita et al., 1988). 
The total length of the net was 7 1.6 m 
and that of the headrope and foot- 
rope was 38.6 m (see Fig. 1). The 
net was composed of 8 wing pan- 
els, 12 body panels, and 6 codend 
panels; all were made of 380 D fine 
polyethylene ropes. Twine diameter 
was between 1.91 mm (P-30) and 
5.06 mm (P-210). The stretched 
mesh size was 1000 mm at the 
wings and the first panels of the 
body, diminishing successively to 57 
mm in the seventh panels of the 
body. The dimension of the fishing 
circle was 144 x 1000 mm (where 
144 = no. of meshes at mouth of net 
and 1000 m = mesh size at first 
panel of net). A codend of 60- mm 
mesh size was attached to the end 
of the body. An inner net of 30-mm 
and 8-mm mesh size was put inside 
of the codend to collect samples. At- 
tached to the headrope were 74 
floats of 300-mm diameter. A steel 
chain of 28-mm diameter was at- 
tached to the footrope. Buoyancy of 
the floats and weight of the chain 
were 762.4 kg and 660.4 kg, respec- 
tively. The bridles to the bottom, 
center, and upper wings were 18 
mm in diameter and 100 m long. 
The expected mouth opening was 
approximately 20 m wide, 18 m 
high, at a towing speed of 4.5 knots. 
The net was designed for a maxi- 
mum towing speed of 5 knots. The 
Tansyu was designed to be used 
with 1.7 m x 2.8 m otter doors. 
Manuscript accepted 10 November 1997. 
Fishery Bulletin 96:641-646 ( 1998). 
