Dierking et al.: Diet composition and prey selection of Cephalopholis argus in Hawaii 
471 
Family 
Figure 3 
Densities of fish families in typical Cephalopholis argus habitat 
along the Kona coast (families contributing <1% to total fish 
abundance are summarized under “All others”), based on repli- 
cated daytime underwater visual surveys at 23 sites along this 
coast in 2003 (see Fig. 1). The inlay shows densities of families 
in the guild of large piscivores (mean mass >50 g) along the 
Kona coast. Notes: *=at least in part nocturnally active; impor- 
tance therefore underestimated by daytime surveys. #=roving 
predator; abundance difficult to assess with the belt transects 
in this study. 
reef fish in this area. Densities of Cephalopholis 
argus observed in Kona (0.70 ind./lOO m 2 ) were 
lower than those in the Red Sea at shallow 
depths (1.32 ind./lOO m 2 ), but much higher than 
in the Red Sea at depths >10 m, where other 
species of the genus Cephalopholis outcompete 
C. argus (Shpigel and Fishelson, 1989b). In 
addition, the density of C. argus was higher 
than that of the ecologically important grou- 
per Plectropomus leopardus in Australia (0.53 
ind/100 m 2 ) (St. John, 1999). In this context, 
knowledge of the feeding patterns of C. argus 
in Hawaii is particularly relevant because it 
provides the basis for the assessment of effects 
of C. argus on native species. 
Diet composition was characterized by the 
large dietary importance of fishes (97.7% by 
%IRI), wide dietary breadth (4 crustacean and 
16 fish families present in the diet), and a near- 
ly exclusive focus on benthic reef-associated 
fishes. These patterns are typical for groupers, 
which are usually ambush predators that hunt 
close to the reef and prey on a wide range of 
different fishes, as well as crustaceans and, 
in some cases, cephalopods (Parrish, 1987). In 
C. argus, a strong focus on fish prey was also 
previously observed in locations outside the 
MHI (e.g., 77.5% by %N in the Society Islands 
[Randall and Brock, I960]; 95.7% by %M in 
Madagascar [Harmelin-Vivien and Bouchon, 
1976]; 92% by %N in the South Pacific [Ran- 
dall, 1980]; 95% by %N in the Red Sea [Shpigel 
and Fishelson, 1989a]). Although C. argus is a 
non-native species in Hawaii, its feeding ecol- 
ogy thus appears to reflect the feeding ecology 
of the species in native habitats. 
Half of all fish families that inhabit reefs in Kona 
(32 families in 2003) were present in the diet of C. 
argus — a finding that demonstrates that groupers may 
prey on a large proportion of the fish taxa present in 
their habitat. The dietary breadth (fishes and crusta- 
ceans combined) observed in our study was much wider 
than those previously reported for C. argus in Hawaii 
(1 family observed in 10 samples [Hobson, 1974]), in the 
South Pacific (5 families in 39 samples [Randall, 1980], 
and in the Red Sea (8 families in 78 samples [Shpigel 
and Fishelson, 1989a]). In contrast, the dietary breadth 
equaled the breadth reported for the Society Islands (21 
families in 280 samples [Randall and Brock, I960]). 
Considering the small samples sizes of most previous 
studies, and the association of dietary breadth with 
sample size (see Results section on cumulative prey 
curves), differences were likely related to sample size 
and not to divergent feeding ecology. This interpretation 
is supported by the strong correlation of the number of 
identified taxa and sample sizes of previous studies (see 
added data points in Figure 2). 
The similar vacuity rate, stomach fullness, impor- 
tance of fish prey, and dietary breadth observed in the 
Kona and Oahu samples indicated that feeding ecology 
was consistent between those islands. The small differ- 
ences in dietary importance observed for some families 
could be related to chance variation, in particular if oc- 
currence of a family in the diet is infrequent. Although 
the lack of a reef fish survey program comparable to 
the WHAP program for Kona does not allow a direct 
comparison between Oahu and Kona reef fish assem- 
blages, divergent dietary importance could also reflect 
variations in fish assemblages, for example, variations 
due to differences in reef habitat or fishing pressure. In 
particular, Williams et al. (2008) demonstrated that the 
abundance of fishery target species in the MHI is nega- 
tively correlated with local human population density, 
which is noteworthy in this context because population 
density on Oahu is more than 30-fold higher than on 
Hawaii Island. In contrast, the large differences ob- 
served for small individuals of the Holocentridae and 
Monacanthidae are probably related to local recruit- 
ment pulses, which is further discussed in the section 
on size selection below. 
Prey selection 
The wide range of electivity values for prey fishes in the 
diet of C. argus in this study indicates that feeding was 
