472 
Fishery Bulletin 107(4) 
not opportunistic. This finding is surprising because 
groupers are often considered generalist, opportunis- 
tic predators on account of their wide dietary breadth 
(Parrish, 1987). However, the only other study in which 
the relative importance of prey fishes in the wild was 
compared with that in grouper diet (Beukers-Stewart 
and Jones, 2004) showed a strong preference for clupeids 
and the avoidance of pomacentrids in the grouper spe- 
cies Cephalopholis boenak and C. cyanostigma in Aus- 
tralia, and it was concluded that this selective feeding 
behavior contrasted with the perception of opportunistic 
predation. The agreement between Beukers-Stewart 
and Jones’s and our findings shows that grouper pre- 
dation may diverge from opportunistic predation more 
commonly than previously thought. 
The concurrent avoidance of pomacentrids by C. argus 
in Hawaii and Australia indicates a low vulnerability 
of this family to grouper predation. An explanation may 
lie in the close reef association and resulting proximity 
to refuges of many pomacentrids (Beukers-Stewart and 
Jones, 2004). Incidentally, such a connection between 
refuges and low vulnerability to predation may help to 
explain the recent observation by Jones et al. (2004) 
that coral-associated fishes (including pomacentrids) 
decline with loss of coral cover during bleaching epi- 
sodes. At the same time, the few pomacentrids in the 
diet of C. argus in Hawaii belonged to the reef-associ- 
ated genus Stegastes, whereas Chromis spp., which are 
abundant in mid-water, were completely absent. Such 
low vulnerability of mid-water pomacentrids may be 
related to the close reef association of C. argus , which 
rarely ventures into the open water column (Hobson, 
1974). Low clupeid abundance on Kona reefs in 2003 
presumably explains the absence of this family in the 
diet of C. argus. 
The terms “preference” and “avoidance” as related 
to electivity indices do not necessarily solely reflect 
active choice (e.g., one that is based on nutritional 
value) but also depend on the vulnerability of prey to 
capture (Scharf et al., 1998). In the present study, this 
differentiation was useful in interpreting electivity pat- 
terns. In particular, vulnerability provided a straight- 
forward explanation for the contrast between a strong 
preference for the nocturnal priacanthids and holocen- 
trids and a strong avoidance of the equally nocturnal 
apogonids: whereas the two preferred taxa hide under 
ledges or in caves during daytime, which are commonly 
frequented by C. argus (Randall, 2007), the apogonids 
hide in small reef crevices inaccessible to a large-body 
predator. Similarly, the complete lack of cryptic fami- 
lies (e.g., Blennidae, Gobidae, Scorpaenidae) and 
the rareness of planktonic taxa in the diet of C. 
argus may be due to their low vulnerability, if 
one considers the low visibility of and proximity 
to cover for cryptic taxa and the usually loose 
reef association for planktonic taxa. 
The patterns observed for nocturnal taxa, as 
well as the low importance of crustaceans and 
large importance of diurnally active fishes in C. 
argus diet, indicate that feeding of this predator 
in Hawaii was diurnal or crepuscular. This find- 
ing confirms that of Hobson (1974) that C. argus 
is a diurnal feeder in Hawaii and is consistent 
with limited nocturnal movement of C. argus in 
Hawaii (A. Meyer, personal commun. 1 ). In con- 
trast, C. argus in Madagascar feeds during day 
and night (Harmelin-Vivien and Bouchon, 1976). 
The rhythm of feeding activity of C. argus thus 
appears to be variable among regions. 
Regarding diurnally active prey taxa, differ- 
ential vulnerability may offer an explanation 
for divergent electivity among the abundant 
families Pomacentridae, Labridae, and Acan- 
thuridae, and among moderately abundant fami- 
lies such as the Chaetodontidae and Scaridae. 
However, it cannot reasonably account for the 
strong preference for the rare aulostomids, holo- 
centrids, and monacanthids. This latter pattern 
appears to be an artifact of recruitment pulses 
and is further discussed in the context of size 
selection below. 
1 Meyer, A. 2009. Hawaii Division of Aquatic Resources, 
1039 Sand Island Parkway, Honolulu, HI 96821. 
1.0 
0.5 
UJ 0.0 
-0.5 - 
- 1.0 
"Preference" 
n 
"Avoidance" 
T" 
T" 
T~ 
T" 
~r~ 
~r~ 
-r~ 
-r~ 
jf 
# # # # ^ 
0 / </ 0 / c? $ / # 
6 ° . 
a # 
.o' 
■ 6°- - 8 ? if 
-v 
G 
Family 
>° 
-.o' 
Figure 4 
Ivlev’s electivity index E t for prey fishes present in the diet of 
Cephalopholis argus in Kona (Kuhliidae excluded because of a 
lack of observations in reef fish surveys). Positive values of 
indicate higher relative importance (“preference”) in the diet 
than in the environment, and negative values of E t indicate 
lower relative importance (“avoidance”) in the diet than in 
the environment. Abundances of nocturnally active species 
(*), underestimated by daytime surveys, were adjusted on the 
basis of abundances observed in nighttime surveys (see Meth- 
ods section). 
