Nakano et al.: Development of swimming speed and schooling behavior in juvenile Sebastes cheni 
127 
body size. These results indicate that the effect of ju- 
venile total length on the BS (a speed that allows for 
prompt reply to predator’s attack) is less during night 
than during day. Small white rockfish juveniles (21.3 
mm TL) in captivity feed during day and are not active 
during night (Shoji et al., 2011). Larger white rockfish 
juveniles begin to feed during night at a body size >80 
mm TL (at 1 year old) when their major prey shifts 
from crustaceans to fish (Kinoshita et al., 2014). The 
low BS observed under low light conditions can be at- 
tributed to their low activity during night. 
Juveniles of the white rockfish form loosely aligned 
aggregations and maintain their body angle in incon- 
sistent directions. The ability of white rockfish juve- 
niles to avoid predation through the use of schooling 
behavior would be minimal because the ability of ju- 
venile white rockfish to school is not well developed 
during the early postsettlement period (about 20-30 
mm TL). Therefore, juveniles are considered most vul- 
nerable to predation during the early postsettlement 
period. School formation develops in juveniles with in- 
creasing body length, indicating that vulnerability to 
predation may decrease with growth. The size-selective 
predation (lower predation rate in larger white rockfish 
juveniles) observed in natural habitats during night 
(Kinoshita et al., 2014) could be explained by the devel- 
opment of anti predator behavior by juveniles through 
improvement in their schooling behavior as indicated 
by the decrease in NND at >30.3 mm TL. In contrast, 
improvement of swimming performance cannot explain 
the size-selective predation because BSs were not sig- 
nificantly higher for fish in the larger size classes at 
the lowest light intensity. 
Acknowledgments 
We express our thanks to S. Iwasaki, Y. Chiba, Y. Ogino, 
other laboratory staff members, and students for their 
assistance in our experiments. This study was support- 
ed partly by the Global Environment Research Fund 
(El 102) of the Ministry of the Environment, Japan. 
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