121 
NOAA 
National Marine 
Fisheries Service 
Fishery Bulletin 
str established 1881 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Development of swimming speed and schooling 
behavior of juvenile white rockfish {Sebastes 
client ) in relation to ambient light intensity 
Hikaru Nakano 1 
Kofi Hirakawa 2 
Jun Shoji (contact author ) 1 
Email address for contact author: jshoji@hiroshima-u.ac.jp 
Abstract— To clarify the ontogenetic 
changes in antipredator behavior 
of juvenile white rockfish ( Sebastes 
cheni), effects of light intensity (0.1, 
1, 10, 100, 1000, and 10,000 lx) and 
body size (23.4, 30.3, 41.1, 49.4 and 
58.6 mm in total length [TL] ) on 
swimming (cruising [swimming] 
speed [CS] and burst [swimming] 
speed [BS]) and schooling behavior 
(nearest neighbor distance [NND] 
and separation angle [SA]) were ex- 
amined under laboratory conditions. 
The CS was higher under higher 
light intensities for all length class- 
es. The BS increased with increas- 
es in fish body size at the highest 
light intensity although there was 
no significant effect of body size at 
the lowest light intensity. The NND 
was largest for the smallest fish-size 
class at all light intensities. The ef- 
fects of light intensity and fish body 
size on SA were not significant. 
Schooling behavior of the juvenile 
white rockfish was determined to 
begin to develop at body sizes >30 
mm TL. In this study, we found the 
ability of juvenile white rockfish to 
avoid predation through schooling 
behavior to be minimal in compari- 
son with other species because the 
instinct appears to not be well devel- 
oped during the early postsettlement 
period (20-30 mm TL). 
Manuscript submitted 7 January 2014. 
Manuscript accepted 21 January 2015. 
Fish. Bull. 113:121-128 (2015). 
doi: 10.7755/FB. 113.2.2 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
1 Takehara Fisheries Research Laboratory 
Hiroshima University 
5-8-1 Minato-machi 
Takehara, Hiroshima 725-0024, Japan 
2 Hiroshima Prefectural Sea Farming Center 
185-12 Takasaki 
Takehara, Hiroshima 729-2313, Japan 
Vegetated habitats, such as seagrass 
and macroalgae beds, are recognized 
as critical habitat for a variety of 
fish species because they serve as 
feeding grounds and refuges from 
predation for early life stages of fish- 
es (Heck and Orth, 2006). However, 
observations of higher abundances of 
piscivorous fishes during night than 
during the day in recent field sur- 
veys have indicated that larval and 
juvenile fishes are exposed to higher 
rates of mortality (owing to preda- 
tion at night in vegetated habitats) 
than rates of mortality owing to pre- 
dation during the day in these habi- 
tats (Hindell et al., 2000; Guest et 
al., 2003). In addition, stomach con- 
tent analyses of piscivorous fishes, 
coupled with tethering experiments 
in seagrass beds in the Seto Inland 
Sea off southwestern Japan, have 
revealed that the predation rate on 
juvenile white rockfish ( Sebastes 
cheni) by piscivorous fishes was sig- 
nificantly higher at night than dur- 
ing the day (Kinoshita et ah, 2012). 
Throughout this article, we refer to 
Sebastes cheni as “white rockfish” to 
reduce confusion with other common 
names applied to this and other spe- 
cies of Sebastes. For example, S. che- 
ni has been called “black rockfish” in 
other publications, but that common 
name is used in the United States 
and Canada for a different species, 
S. melanops. Other common names 
for S. cheni are “rockfish,” “Japanese 
black rockfish,” “white seapearch” 
and “shiro-mebaru.” 
The recent observations noted 
in the previous paragraph suggest 
that a re-evaluation is needed for a 
common understanding that aquatic 
macrophytes have a function as a 
refuge from predation for juvenile 
fishes (e.g., Petr, 2000) — a notion that 
has been established mainly on the 
basis of observations during the day. 
Comprehensive analysis of predator- 
prey interactions during both day 
and night, with a focus on possible 
diurnal changes in both predator and 
prey behaviors, would help us better 
understand the survival mechanisms 
of early life stages of fishes in these 
habitats. 
Rockfishes ( Sebastes spp.) are dis- 
tributed widely in coastal waters of 
the western North Pacific and are 
commercially and recreationally im- 
portant species (Love et al., 1991; 
