904 
Fishery Bulletin 96(4), 1998 
of the 1990 catch. This was due partly to unfavor- 
able oceanic conditions. 
Growth rates of sardine larvae estimated in this 
study were 0.79-0.85 mm/d to a size when they im- 
migrated to the fishing grounds as 1990 April- and 
1991 March- and April-hatched cohorts, but later 
declined to 0.6-0. 7 mm/d (Fig. 8). This decline in 
growth was similar to the asymptotic growth of lar- 
val S. melanostictus in the shirasu fishing grounds 
in western Japan (Watanabe and Kuroki, 1997). Fish- 
ermen for shirasu locate a concentrated larval shoal 
by echo sounder (Mitani, 1987) and catch large num- 
bers of larvae in relatively narrow fishing grounds 
(Fig. 1). In mid-April 1990, 100 t (ca. one billion in 
number) per day of sardine larvae were caught in 
the fishing ground in our study. 1 Turbidity is a fac- 
tor that helps to retain concentrated larval shoals in 
the coastal fishing grounds (Funakoshi, 1988). Uotani 
et al. (1993) demonstrated experimentally that E. 
japonicus larvae showed a strong positive taxis to 
turbidity and tended to stay in the turbid water. 
Reduction of larval growth, after reaching the size 
when immigration to the shirasu fishing grounds 
occurred, is likely to be related to concentrations of 
large numbers of larval sardines in the narrow fish- 
ing grounds as a result of intrusions of offshore wa- 
ters (Muranaka, 1984). 
20 
10 
April 11 20 - 
N= 34 _,J 
HdfM n - 
April 15 | 
N = 35 JJ 
20 
10 
<1) 
| | | | | | I rr^ T T 1 II 1 1 1 1 I 1 1 1 1 1 1 Ur 
5 15 25 
April 16 a 20 - 
N = 35 J J 10 
. iLEVi o 
5 15 25 
April 23 || 
N=37 JL| 
Percental 
o o o c 
1 i i i i 1 i i i n i TT f 1 1 1 1 II i i i i i u 
5 15 25 
April 27 g| 20 - 
Twp- 
i i i ™ i fill i i i i i i I i 'T ~ r i U 
5 15 25 
May 7 M 
*" JL. 
i i i i i i i i i i i i i i i i i 
5 15 25 5 15 25 
20 
10 
May 7 
N=35 
drtlft || 
5 15 25 
20 - May 14 
10 - N=61 
0 1 i ■ i i i i i 
5 15 25 
Total length (mm) 
Figure 5 
Frequency distributions of TL of sardine larvae caught in 1990 (left) and 1991 (right). 
Sampling date and sample size (n) are shown in each panel. 
The modal size of sardine 
larvae caught on April 27 
and May 7 (April-hatched 
cohort) in 1990 was excep- 
tionally smaller ( 15-16 mm 
TL) than those in the other 
larval groups (Fig. 5). The 
mesh aperture of the net of 
the shirasu fishery is 2.1 
mm at the codend. The di- 
agonal of the mesh is 3.0 
mm, which is much larger 
than the body depth of 15- 
16 mm sardine larvae (about 
1.3 mm). A large proportion 
of larvae smaller than 15 mm 
could, therefore, be extruded 
from the mesh at the codend 
(Smith and Richardson, 
1970). Nevertheless, many 
larvae smaller than 15 mm 
1 1990. Marine Resources Re- 
search Center of Aichi Fisheries 
Research Institute, 2-1 Toyoura, 
Toyohama, Minami-Chita, Chita- 
gun, Aichi 47034, Japan. Unpubl. 
data 
