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Fishery Bulletin 107(4) 
Size selection 
The patterns of prey-size selection by C. argus in 
Hawaii are of particular interest in the context of 
questions regarding the high mortality of coral reef fish 
recruits (here defined as postsettlement individuals 
less than several months of age). During recruitment 
pulses, large numbers of fish that have completed their 
planktonic phase settle on reefs (Walsh, 1987). Mortal- 
ity of these recruits in the first 100 days after arrival 
can exceed 99% (Werner and Gilliam, 1984; Doherty 
et al., 2004). Although predation is thought to be an 
important cause of this mortality (Beets, 1997; Webster, 
2002), few studies have conclusively shown the impli- 
cation of specific predators (Connell, 1998; Beukers- 
Stewart and Jones, 2004). In this study, the majority of 
aulostomids, holocentrids, and monacanthids consumed 
by C. argus were recruits, as indicated by comparison 
of sizes in the diet (TL as small as 9.3, 5.2, 3.8 cm, 
respectively) with published size ranges for different 
life history stages of these families (Leis and Carson- 
Ewart, 2000; Randall, 2007). This finding indicates 
that the large-body predator C. argus contributes to 
early mortality of reef fishes. 
Interestingly, the smallest consumed sizes of the prey 
families above were rarely or never observed in WHAP 
underwater visual surveys. This inability to account for 
recruits may in part be due to low detectability of these 
small and potentially cryptic individuals. However, the 
surveys were designed to account for fishes closely asso- 
ciated with the bottom, new recruits, and fishes hiding 
in cracks (Tissot et al., 2004). It therefore appears more 
likely that high mortality of recruits between their 
arrival on the reef and the occurrence of the surveys 
(4-6 surveys per site in 2003, i.e., every 2-3 months) 
explains this pattern. Considering the spatial and tem- 
poral heterogeneity of recruitment (Dufour et al., 1996), 
predation on recruits may also account for the divergent 
dietary importance of small holocentrids and monacan- 
thids between islands in this study. 
In contrast to the focus on recruits for the families 
above, for several families of smaller-body individuals 
(e.g., Acanthuridae, Chatodontidae), differences between 
mean size in the diet and in the wild were small. In 
these cases, comparison with published size ranges 
(Leis and Carson-Ewart, 2000; Randall, 2007; Claisse 
et al., 2009) showed that consumed individuals repre- 
sented in large parts juveniles and small adults, and not 
recruits. This result indicates that ecological effects of 
C. argus predation differ between prey families. In par- 
ticular, although predation-induced mortality in several 
families of larger-body individuals would be limited to 
recruits (adults finding escape from predation through 
their large size), in several families of smaller-body in- 
dividuals, it can also affect juveniles and adults. From a 
predator perspective, C. argus nutrition was then based 
on 1) accessible sizes of reef fishes such as Acanthuri- 
dae, Chaetodontidae, or Scaridae, present year-round 
on the reef, and 2) recruitment pulses providing access 
to small individuals of different families, including the 
Aulostomidae and Monacanthidae, of which adults es- 
cape predation because of their large size. 
Methodological considerations 
Despite the large sample size of the present study com- 
pared to that of many previous grouper feeding studies, 
cumulative prey curves indicated that sample size was 
too small to fully characterize dietary breadth at the 
species level, and dietary breadth of Oahu C. argus at 
the family level. This stresses the importance of assess- 
ing sample size sufficiency in grouper studies with ana- 
lytical tools such as cumulative prey curves (Ferry and 
Cailliet, 1996). The strong correlation of the number of 
identified taxa with the sample size of C. argus studies 
in the literature impressively confirms the importance 
of this issue. Secondly, several authors have pointed out 
that daytime visual surveys may not accurately reflect 
the importance of nocturnal taxa (e.g., Ackerman and 
Belwood, 2000). The 90-fold higher abundance of apogo- 
nids observed in nighttime surveys compared to that 
observed in daytime surveys at the same sites in this 
study underscores this potential limitation of daytime 
surveys. Finally, in this study, feeding on recruit-sized 
individuals of some prey fishes of which adults were only 
rarely observed on Kona reefs was able to explain the 
high preference for these species. This demonstrates that 
inclusion of ecological information to interpret electivity 
values is as important as the choice of a suitable index 
of electivity (Lechowicz, 1982). 
Conclusions 
The dominant position of C. argus in the guild of 
large piscivores in Kona shows that this species has 
become an important component of MHI reef ecosystems 
since its introduction and raises the question of how 
this species affects native fishes. The differences in the 
strength and mechanism of effects of predation among 
prey families in this study indicate that C. argus has 
the potential to affect the composition of reef fish as- 
semblages. However, structuring effects of predation on 
fish assemblages are also important in systems without 
introduced predators (Hixon, 1991). In addition, native 
predators in the MHI have sharply declined owing to 
overfishing over the past century — a decline that is re- 
flected in low predator densities in the MHI compared 
to the remote northwestern Hawaiian Islands (protected 
from fishing) (Friedlander and DeMartini, 2002). This 
raises the question of whether C. argus is only fulfilling 
the ecological role previously played by native predators. 
The results presented here do not suffice to answer 
these questions. However, by identifying the prey taxa 
and sizes constituting C. argus diet, they open the way 
partly (the missing part being data on C. argus daily 
ration) for a quantitative estimate of consumption by 
this species (Bromley, 1994), which could further ad- 
vance our understanding of its effects on native fishes 
in Hawaii. 
