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Fishery Bulletin 113(2) 
Figure 2 
Cruising [swimming] speed (CS; measured in millimeters per 
second) of juvenile white rockfish ( Sebastes cheni ) at each 
size class (total length in millimeters) under different light 
conditions (from 0.1 to 10,000 lx). Error bars indicate stan- 
dard deviations and letters indicate significant difference in 
CS among fish at different light conditions within the same 
size class (Kruskal-Wallis test with Steel-Dwass’s test as 
post-hoc test: PcO.001). 
P<0.05), but no size effect was detected under the lowest 
light intensity of 0.1 lx (Kruskal-Wallis test: P>0.05). 
The BS under the lowest light intensity was significantly 
lower than BS under the highest light intensity for fish 
in all size classes tested (Wilcoxon test: P<0.05 for fish 
23.4-49.4 mm TL and P<0.01 for fish 58.6 mm TL; Fig. 5). 
Discussion 
Cruising [swimming] speed 
Changes in CS and adaptation to low-light conditions 
have been reported in larvae and juveniles of a vari- 
ety of fish species (Blaxter, 1968; Batty et al., 1990; 
Miyazaki et al., 2000). In general, CS increases with 
increases in body size of larval and juvenile fishes. De- 
velopment of their sensory organs enables larger ju- 
venile fishes to be more active under dark conditions. 
The results from a comparison of CS among groups of 
fish of different body sizes in different light conditions 
in this study indicated that the CS of white rockfish 
juveniles <41.1 mm TL was lower under low light con- 
ditions than under greater light intensities. The CS of 
fish >41.1 mm TL increased in relation to speeds of 
smaller fish at all light intensities. 
Mean CS was slowest for juvenile white rockfish 
under intermediate light intensity in the 2 largest 
size classes: 49.4 mm TL under 10 lx and 58.6 mm 
TL under 1 lx. In a previous laboratory experiment, 
white trevally ( Pseudocaranx dentex ) juveniles were 
reported to have swum along the tank wall under to- 
tal darkness at a CS faster than their CS at a higher 
light intensity (0.1 lx; Miyazaki et al., 2000). In our 
study, similar swimming behavior was observed in ju- 
venile white rockfish at 49.4 and 58.6 mm TL under 
0.1 lx (H. Nakano, personal observ.). Swimming may 
help white rockfish juveniles maintain stable NND 
under lowest light intensity (0.1 lx in this study) by 
providing motion cues to the lateral-line sensors of 
the fish: by contrast, they likely are able to maintain 
NND under intermediate light conditions (e.g., 1-10 
lx) by using their vision. In general, juvenile fishes de- 
velop swimming and schooling behaviors under dark 
conditions with the formation of their lateral lines, a 
change that enables them to keep favorable distance 
from other fishes without strong visual cues (Masuda, 
2006; Miyazaki et al., 2000). Development of lateral 
lines at 30-40 mm TL (Suzuki and Aida, 1999) may 
relate to the increase in CS observed in our study for 
white rockfish juveniles under the low light conditions 
(0.1-1 lx in this study). 
