Diet composition and prey selection of the introduced grouper species peacock hind (Cephalopholis argus) in Hawaii.
Animal feeding behavior
Predation (Biology) (Research)
Williams, Ivor D.
Walsh, William J.
|Publication:||Name: Fishery Bulletin Publisher: National Marine Fisheries Service Audience: Academic Format: Magazine/Journal Subject: Zoology and wildlife conservation Copyright: COPYRIGHT 2009 National Marine Fisheries Service ISSN: 0090-0656|
|Issue:||Date: Oct, 2009 Source Volume: 107 Source Issue: 4|
|Topic:||Event Code: 310 Science & research Canadian Subject Form: Animal feeding behaviour|
|Product:||Product Code: 0912075 Grouper NAICS Code: 114111 Finfish Fishing SIC Code: 0912 Finfish|
|Geographic:||Geographic Scope: United States Geographic Name: Hawaii Geographic Code: 1U9HI Hawaii; 1USA United States|
Abstract--The introduced grouper species peacock hind
(Cephalopholis argus), was the dominant large-body piscivore on the Main
Hawaiian Island (MHI) reefs assessed by underwater visual surveys in
this study. However, published data on C. argus feeding ecology are
scarce, and the role of this species in Hawaiian reef ecosystems is
presently not well understood. Here we provide the first comprehensive
assessment of the diet composition, prey electivity (dietary importance
of prey taxa compared to their availability on reefs), and size
selectivity (prey sizes in the diet compared to sizes on reefs) of this
important predator in the MHI. Diet consisted 97.7% of fishes and was
characterized by a wide taxonomic breadth. Surprisingly, feeding was not
opportunistic, as indicated by a strongly divergent electivity for
different prey fishes. In addition, whereas some families of large-body
species were represented in the diet exclusively by recruit-size
individuals (e.g., Aulostomidae), several families of smaller-body
species were also represented by juveniles or adults (e.g.,
Chaetodontidae). Both the strength and mechanisms of the effects of C.
argus predation are therefore likely to differ among prey families. This
study provides the basis for a quantitative estimate of prey consumption
by C. argus, which would further increase understanding of impacts of
this species on native fishes in Hawaii.
The grouper species peacock hind (Cephalopholis argus) (Serranidae: Epinephelinae), a reef fish predator that is native over most of the Indo-Pacific region, was introduced to the Main Hawaiian Islands (MHI) in 1956 as part of an introduction program of snapper and grouper species intended to enhance nearshore fisheries (Randall, 1987). Today, this non-native predator occupies a dominant position in the guild of large piscivores on many reefs in the MHI (see Results section). As impacts of piscivores on prey fishes have been demonstrated in a number of studies (e.g., Webster, 2002; Hixon and Jones, 2005), the abundance of C. argus raises the question of how, if at all, it affects native reef fishes in Hawaii. However, the only published study of C. argus feeding in the MHI to date was based on a sample of 10 specimens (Hobson, 1974), which is insufficient to elucidate feeding patterns.
Groupers are among the most common predatory reef fishes worldwide (Parrish, 1987). They play an important role in shaping reef communities (Goeden, 1982; Parrish, 1987) and are of large commercial importance in coral reef fisheries (Heemstra and Randall, 1993). However, data on the general feeding ecology of this family remain surprisingly scarce (Beukers-Stewart and Jones, 2004). In addition, although concern about declines of many grouper species worldwide due to overfishing has led to a renewed research focus on this family (Morris et al., 2000), much of the work to date has concentrated on a limited number of species, e.g., the coral trout (Plectropomus leopardus) in Australia (e.g., Kingsford, 1992; St. John, 1999), the Nassau grouper (Epinephelus striatus) in the Caribbean Sea (e.g., Eggleston et al., 1998), or the dusky grouper (Epinephelus marginatus) in the Mediterranean Sea (e.g., Renones et al., 2002).
Most studies of grouper diet have been based on analysis of stomach contents. However, because of the difficulty of obtaining large grouper samples (Beukers-Stewart and Jones, 2004), as well as the high prevalence of empty stomachs due to prey regurgitation during capture and because of the characteristics of grouper feeding ecology (Dierking and Meyer, in press), detailed descriptions of grouper diet remain rare. Notably, only one study of C. argus diet (Randall and Brock, 1960), and a few studies of other grouper species (but see Kingsford, 1992; St. John, 1999; Beukers-Stewart and Jones, 2004), have been based on more than 50 full stomachs. Interpretation of feeding patterns is further complicated by the lack of studies comparing dietary composition with prey availability in the wild (but see Beukers-Stewart and Jones, 2004).
[FIGURE 1 OMITTED]
We examined the feeding patterns of C. argus based on stomach content analysis of the largest sample of this species (n=285) available to date. In addition, we assessed the patterns in the context of the composition of the reef fish assemblage in Hawaii, which was determined by underwater visual surveys. The main goal was to describe the diet composition, prey electivity (dietary importance of a taxon compared to its availability on reefs), and size selectivity (prey sizes in the diet compared to sizes on reefs) of C. argus in Hawaii. Secondary goals were to assess the mechanisms by which this non-native predator may affect prey fishes and to provide data required for the quantitative estimation of prey consumption by this species.
Material and methods
Study organism and sampling sites
To our knowledge, the establishment of C. argus in Hawaii represents the first documented case of the successful invasion of a non-native habitat by a grouper. Today this generally piscivorous species (Parrish, 1987) is found around all of the MHI. It is particularly abundant along the western coastline of the island of Hawaii (Kona coast hereafter, following common terminology in Hawaii), which harbors some of the least disturbed reefs in the MHI and is the source of important economic revenues from diving tourism and the aquarium fish industry (Tissot et al., 2004). Despite its abundance, a fishery for C. argus never developed because it turned out to be a carrier of ciguatoxin, the agent of ciguatera fish poisoning (Dierking and Campora, 2009).
To determine the diet composition of C. argus, stomachs of all specimens were opened and any prey items were removed. The analysis of contents then followed the procedures described by Hyslop (1980). Specifically, for the two island samples and the combined overall sample, the vacuity rate (i.e., the proportion of empty stomachs) was determined. Differences in vacuity between islands were assessed with a chi-square test. For full stomachs, the number of prey items was counted, and the SL and TL of fish prey and the carapace length of crustacean prey were determined to the nearest mm, where digestion state allowed reliable measurements. For fishes, SL could be determined more often than TL because skeletons are slow to disintegrate during digestion. In these cases, TL was calculated from SL according to published SL-TL equations for the respective taxon (Froese and Pauly, 2009). The M of all prey items was recorded to the nearest mg. Mean relative stomach content M was then calculated as total M of prey items divided by the total M of C. argus specimens.
We identified each prey item to the lowest possible taxonomic level, using Randall's (1996; 2007) key for fish prey, and Hoover's (1998) key for crustacean prey. Cumulative prey curves (Ferry and Cailliet, 1996) derived from plotting the cumulative number of unique prey taxa against the cumulative number of analyzed stomachs allowed us to assess whether sample sizes were large enough to accurately characterize dietary breadth. These curves reach an asymptote if sample size is sufficient. To determine dietary importance of prey, for all identified prey types and families, the numerical importance (%N), frequency of occurrence (%0) (calculated from full stomachs), and gravimetric importance (%M), as well as the index of relative importance (IRI) and the %IRI (proportion of the IRI of a taxon to the sum of IRis of all taxa) were calculated. The IRI incorporates the individual indices in the formula
IRI = (%N + %M) x %0 (1)
and may provide a more accurate description of dietary importance than its components by canceling out their individuals biases, such as the overestimation of the importance of an abundant but small prey item by the %N (Cortes, 1996). Because %N, %M, %0, and %IRI indicated that fishes dominated the diet of C. argus, for the comparison of diets between islands and the calculation of electivity, the indices were recalculated for the fish component of the diet alone.
Composition of the reef fish assemblage
Underwater visual surveys with scuba were used to determine reef fish abundances and sizes (in 5-cm bins, i.e., 0-5 cm, 5-10 cm, etc.) at 23 sites (depth range 8.2 m-18.2 m, mean depth 11.9 m). All sites were located in the dominant reef habitat of the Kona coast, reef shelves with moderate to high finger coral (Porites compressa) cover. Each survey of a site involved four divers (two pairs), who between them surveyed four 25 x 4 m (100 [m.sup.2]) belt transects that were permanently installed at each site. Each transect count consisted of one rapid swim to count mobile and midwater species, and a slow return swim closer to the bottom to record fishes in and around the benthos. Sites were surveyed 4 to 6 times during the year 2003, generally between 0840 and 1600 hours. The detailed sampling regime was described by Tissot et al. (2004). Surveys were conducted under the direction of the West Hawaii Aquarium Project (WHAP, a collaboration of the Hawaii Division of Aquatic Resources (HDAR), the University of Hawaii at Hilo, and Washington State University), and are therefore referred to as "WHAP surveys" here.
WHAP survey counts were used to calculate mean fish densities (individuals/100 [m.sup.2]) and relative numerical importance (%N) of reef fishes in Kona in 2003 (i.e., grand mean of densities for the year 2003 at the 23 sites). In addition, relative importance in terms of biomass (%M) was determined for large (mean body M >50 g) piscivores. For this purpose, the M of individuals was estimated from their TL by using conversion equations for the respective taxa (Froese and Pauly, 2009). Finally, size-frequency distributions of reef fishes in Kona were calculated from the combined WHAP survey counts for 2003.
Abundances of nocturnally active taxa tend to be underestimated by data collected during daytime surveys (Ackerman and Belwood, 2000). For the calculation of electivity (see next section), abundances of the nocturnal apogonids, holocentrids, and priacanthids were therefore estimated by nighttime surveys ("Night WHAP") that took place in 2003 at the same sites surveyed during daytime. The ratios of nighttime to daytime abundances for these families were 90.6, 2.6, and 1.5, respectively.
To determine the taxonomic focus of predation, we used Ivlev's electivity index (Ivlev, 1961):
[E.sub.i] = ([r.sub.i] - [p.sub.i]) / ([r.sub.i] + [p.sub.i]), (2)
where [r.sub.i] = numeric importance (%N) of fish family i in the diet of C. argus; and
[p.sub.i] = %N of the same family in the reef environment.
[E.sub.i] can take values between -1 and 1. Positive values indicate "preference" (a taxon overrepresented in the diet in relation to its availability in the environment), and negative values "avoidance" (a taxon underrepresented in the diet in relation to its availability) (Lechowicz, 1982). Because of the scarcity of reef fish abundance data for Oahu for the year 2003, when stomach contents for this study were obtained, we based electivity calculations on diet composition data obtained from the Kona sample, and on reef fish abundance data for the Kona coast from WHAP surveys in 2003.
To assess the size focus of C. argus predation, length-frequency distributions of important fish families in the diet of C. argus were compared with length-frequency distributions of the same taxa in the reef environment in Kona, by using 2-sample Kolmogorov-Smirnov tests. The family Priacanthidae was excluded from the analysis despite its dietary importance because low counts in underwater surveys did not allow for meaningful comparison. Minitab 14 software (Minitab Inc., State College, PA) was used for all statistical analyses, and results were considered significant at P<0.05.
Morphometrics of C. argus
Cephalopholis argus SL ranged from 13.2 to 44.0 cm (mean: 26.9 cm), and M from 69 g to 2847 g (mean: 721 g). Morphometric relationships (M-TL, M-SL, SL-TL), which have not been reported for this species from large sample sizes, are summarized in Table 1.
The stomach vacuity rate for the overall sample of 285 analyzed stomachs was 44.2%, and the mean relative stomach content M was 0.74% of C. argus body M (empty stomachs included in the calculation). Overall, 219 prey items were recovered from 159 full stomachs. Reef fishes were the principal diet component (97.7% by %IRI). Crustaceans were the only other higher taxonomic group in the diet, but were of minor importance (2.3% by %IRI) (Table 2). Dietary breadth was wide; a total of 24 prey species (20 fish species) in 20 different prey families (16 fish families) were found in the diet. At the same time, the three most important families in the diet made up almost 60%, and the eight most important fish families close to 90% of the total diet (by %IRI). In declining order of importance, these families were the Scaridae, Acanthuridae, Holocentridae (exclusively of the subfamily Holocentrinae, the squirrelfishes), Monacanthidae, Priacanthidae, Chaetodontidae, Aulostomidae, and Cirrhitidae (Table 2).
Oahu and Kona C. argus populations did not differ significantly in either stomach vacuity rate (50.0% vs. 40.7%; chi-square test, n=285, P=0.13) or relative stomach fullness (0.73% vs. 0.76% of own body M; t-test, n=285: P=0.84). Both island populations also revealed the importance of fish prey in their diet (96.7% vs. 98.1% by %IRI) and showed a similar overall dietary breadth (14 versus 18 families). In addition, the Scaridae, Acanthuridae, and Priacanthidae were concurrently among the five most important prey families by %IRI for both islands (Table 3). Still, diet composition differed in several respects. Most importantly, the dominant prey family by %IRI in the diet of C. argus from Kona (Holocentridae) was not found in the diet of C. argus from Oahu, and vice versa, the dominant family in the diet of Oahu C. argus (Monacanthidae) was rare in the diet of C. argus from Kona. The only other family for which %IRI values differed by >5% was the Chaetodontidae, which was more important in the Kona than the Oahu diet. Balistidae and Mullidae were slightly more important in the Kona diet, and Pomacentridae, Cirrhitidae, and Synodontidae in the Oahu diet. For all remaining families, absolute %IRI values differed by <1% (Table 3).
Cumulative prey curves for family-level analyses based on the overall (i.e., Oahu and Kona samples combined) and on the Kona sample showed strong asymptotic behavior (Fig. 2, left panels), which indicates that sample size was sufficiently large to describe dietary breadth. In contrast, curves for the species-level analysis based on the overall sample and for the family-level analysis based on the Oahu sample (Fig. 2, right panels) showed only a slight approximation to an asymptote, and dietary breadth may therefore be incompletely described at these levels.
[FIGURE 2 OMITTED]
Composition of the reef fish assemblage
Figure 3 summarizes the mean densities of the 32 different families of reef fishes observed on Kona reefs during the 2003 WHAP surveys. The families Acanthuridae and Pomacentridae, with densities of 69.2 and 61.8 individuals/100 [m.sup.2], together accounted for almost 75% of the total fish assemblage in terms of %N. Seven other families contributed at more than 1% by %N, the most important ones being the Labridae (9.0%) and Chaetodontidae (4.6%). The importance of nocturnally active families may be higher than shown in Figure 3, which is based on daytime WHAP surveys alone. In particular, recalculated numerical importance for the nocturnal families Apogonidae, Holocentridae, and Priacanthidae (based on nighttime surveys showing a 90.6, 2.6, and 1.5 times higher abundance than during daytime; see Material and methods section) would be 7.5%, 6.8%, and <0.1%, respectively. The guild of large-body piscivores contributed 0.7% to total fish abundance. Within this guild, C. argus (density=0.70 individuals/100 [m.sup.2]) was the dominant taxon, contributing 56% by %N (Fig. 3) and 84% by %M--the larger value for the latter being due to a higher mean body M for C. argus compared to the other predators in the analysis.
All of the 10 most abundant species on Kona reefs were found in the stomachs of C. argus. However, although some fishes that were rare in the reef environment ("the wild") in Kona were important components of C. argus diet (e.g., Priacanthidae: %[N.sub.reef environment] <0.1%, %[N.sub.diet]= 9.2%), others such as the Pomacentridae, although highly abundant on Kona reefs (%N=31.5%), had low dietary importance (%N=1.5%) (Fig. 3, Table 2). Consequently, the electivity values of prey families ranged widely, from values of [E.sub.i] near 1 (strong preference) to -0.91 (strong avoidance). This pattern was consistent both for diurnally and nocturnally active taxa (Fig. 4). Specifically, of the nocturnal taxa, priacanthids and holocentrids were highly preferred, whereas apogonids were avoided. Diurnally active families can be divided into three broad categories based on their electivity (Fig. 4) and their abundance on reefs (Fig. 3): 1) negative electivity, large wild abundance (Pomacentridae, Labridae, Acanthuridae); 2) moderate positive electivity, moderate to large wild abundance (e.g., Chaetodontidae, Mullidae, Scaridae); and 3) large positive electivity, low wild abundance (Aulostomidae, Monacanthidae).
The mean TL of reef fishes in the diet of C. argus was 7.2 cm, and was thus significantly smaller than the mean TL of reef fishes in the wild of 9.1 cm (Kolmogorov-Smirnov test, D=0.11, P=0.034) (Fig. 5). Mean size in the diet was consistently lower than mean size in the wild for all analyzed prey families. However, specific patterns differed strongly. On one end of the spectrum, for generally small-body families, such as the Acanthuridae and Chaetodontidae, differences between consumed and wild sizes were near significant ([D.sub.Acanthuridae= = 0.32, P=0.057) or significant ([D.sub.Chaetodontidae]=0.55, P=0.026), but small in absolute terms, with size classes occurring in the diet overlapping strongly with those observed in the wild. The Scaridae (D=0.42, P=0.021) represented an intermediate case, with predation focused on the smaller size classes present in the wild, but clear overlap between size-frequency distributions in the diet and in the wild. Finally, for the generally large-body families Monacanthidae (D=0.47), Aulostomidae (D=0.82), and Holocentridae (D = 0.99), size differences between the diet and the wild were large and highly significant (P<0.01), and sizes as small as in the diet were rarely observed in the wild (Fig. 5).
The comparison of reef fish densities in this study showed that C. argus constituted more than half of the guild of large piscivores in Kona by numerical abundance, and more than 80% of this guild by biomass. Because consumption partly depends on the biomass of a predator (Cortes, 1996), it therefore appears safe to say that C. argus has become the dominant large-body predatory reef fish in this area. Densities of Cephalopholis argus observed in Kona (0.70 ind./100 [m.sup.2]) were lower than those in the Red Sea at shallow depths (1.32 ind./100 [m.sup.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 grouper Plectropomus leopardus in Australia (0.53 ind/100 [m.sup.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.
[FIGURE 3 OMITTED]
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 nearly 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, 1960]; 95.7% by %M in Madagascar [Harmelin-Vivien and Bouchon, 1976]; 92% by %N in the South Pacific [Randall, 1980]; 95% by %N in the Red Sea [Shpigel and Fishelson, 1989a]). Although C. argus is a non-native species in Hawaii, its feeding ecology 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 crustaceans 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, 1960]). 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, importance of fish prey, and dietary breadth observed in the Kona and Oahu samples indicated that feeding ecology was consistent between those islands. The small differences in dietary importance observed for some families could be related to chance variation, in particular if occurrence 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 assemblages, 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 negatively 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 observed for small individuals of the Holocentridae and Monacanthidae are probably related to local recruitment pulses, which is further discussed in the section on size selection below.
The wide range of electivity values for prey fishes in the diet of C. argus in this study indicates that feeding was not opportunistic. This finding is surprising because groupers are often considered generalist, opportunistic 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 species Cephalopholis boenak and C. cyanostigma in Australia, 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 predation 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-associated 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.
[FIGURE 4 OMITTED]
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 patterns. In particular, vulnerability provided a straightforward explanation for the contrast between a strong preference for the nocturnal priacanthids and holocentrids 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 families (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 finding 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 contrast, 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, differential vulnerability may offer an explanation for divergent electivity among the abundant families Pomacentridae, Labridae, and Acanthuridae, and among moderately abundant families such as the Chaetodontidae and Scaridae. However, it cannot reasonably account for the strong preference for the rare aulostomids, holocentrids, and monacanthids. This latter pattern appears to be an artifact of recruitment pulses and is further discussed in the context of size selection below.
[FIGURE 5 OMITTED]
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). Mortality 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 implication 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 associated 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 temporal heterogeneity of recruitment (Dufour et al., 1996), predation on recruits may also account for the divergent dietary importance of small holocentrids and monacanthids 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 represented 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 particular, 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 individuals, 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 Acanthuridae, 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 escape predation because of their large size.
Despite the large sample size of the present study compared 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 assessing sample size sufficiency in grouper studies with analytical 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 apogonids 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).
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 assemblages. 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 reflected 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 advance our understanding of its effects on native fishes in Hawaii.
We thank C. Birkeland, S. Conant, R. Kinzie, Y. Hokama, and three anonymous reviewers for helpful comments regarding this paper, and A. Meyer, T. Clark, S. Fujimoto, and R. Robertson for help with fieldwork. The Hawaii Cooperative Fishery Research Unit and the Hawaii Division of Aquatic Resources (HDAR) provided logistical support. The study benefited from long-term data series of fish abundances collected by the West Hawaii Aquarium Project and the HDAR. Research was funded by the National Oceanic and Atmospheric Administration, Center for Sponsored Coastal Ocean Science, under awards NA05NOS4261157 and NA06NOS4260200 to the University of Hawaii for the Hawaii Coral Reef Initiative. J.D. was supported by a Fulbright scholarship.
Manuscript submitted 18 December 2008.
Manuscript accepted 24 June 2009.
Fish. Bull. 107:464-476 (2009).
The views and opinions expressed or implied in this article are those of the author and do not necessarily reflect the position of the National Marine Fisheries Service, NOAA.
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(1) Meyer, A. 2009. Hawaii Division of Aquatic Resources, 1039 Sand Island Parkway. Honolulu. HI 96821.
Jan Dierking (contact author) (1,2)
Ivor D. Williams (3,4)
William J. Walsh (3)
E-mail address for contact author: email@example.com
(1) Department of Zoology University of Hawaii 2S38 The Mall Honolulu, Hawaii 96822
(2) Present address: Centre d'Oceanologie de Marseille, UMR CNRS 6540 Universite de la Mediterranee, Station Marine d'Endoume Rue de la Batterie des Lions 13007 Marseille, France
(3) Hawaii Division of Aquatic Resources, Honokohau Marina 74-380B Kealakehe Pkwy Kailua-Kona, Hawaii 96740
(4) Hawaii Cooperative Fishery Research Unit Department of Zoology University of Hawaii at Manoa Honolulu, Hawaii 96822
Table 1 Morphometic relationships between total length (TL), standard length (SL, in cm), and wet mass (M, in g) for peacock hind (Cephalopholis argus) in Hawaii. [r.sup.2] = regression fit. Relationship a b n [r.sup.2] M = a TL (b) 0.0125 3.122 110 0.98 M = a SL (b) 0.0309 3.013 110 0.97 SL = a + b TL -0.244 0.8494 304 0.99 Table 2 Diet composition of peacock hind (Cephalopholis argus) in Hawaii, based on Kona and Oahu samples combined ([n.sub.total] = 285, [n.sub.full stomachs] = 159), by number (N), occurrence (i.e., number of stomachs in which a taxon occurred) (0), and mass (M, in g). Dietary importance is indicated by percent by number (%N), percent by occurrence (%O) (calculated on the basis of full stomachs), percent by mass (%M), and percent index of relative importance (%IRI, based on the index of relative importance IRI = (%N + %M) x %O). %0 and %IRI for fishes and crustaceans are nonadded values (i.e., they correspond to these two food types, not the sum of their components). Unidentified fish and crustacean prey were excluded from family-level calculations of % indices. Prey taxon N 0 M Fish 185 144 1346.0 Acanthuridae 14 13 137.8 Acanthurus nigrofuscus 3 3 11.2 Acanthurus nigroris 2 2 89.8 Zebrasoma flavescens 2 2 18.7 unidentified Acanthuridae 7 6 18.2 Apogonidae 3 3 15.3 unidentified Apogonidae 3 3 15.3 Aulostomidae 6 6 29.4 Aulostomus chinensis 6 6 29.4 Balistidae 3 3 63.5 Xanthichthys auromarginatus 1 1 53.5 unidentified Balistidae 2 2 10.0 Chaetodontidae 6 6 78.6 Chaetodon multicinctus 1 1 16.9 Forcipiger flavissimus 3 3 21.8 Hemitaurichthys polylepis 1 1 24.8 unidentified Chaetodontidae 1 1 15.1 Cirrhitidae 4 4 37.7 Amblycirrhitus bimacula 1 1 4.6 unidentified Cirrhitidae 3 3 33.1 Holocentridae 16 16 52.6 Sargocentron punctatissimum 1 1 4.0 unidentified Holocentrinae 15 15 48.6 Kuhliidae 1 1 17.5 Kuhlia spp. 1 1 17.5 Labridae 2 2 29.5 Pseudocheilinus tetrataenia 1 1 1.7 Stethojulis balteata 1 1 27.8 Monacanthidae 15 13 52.5 Pervagor aspricaudus 2 2 19.5 unidentified Monacanthidae 13 11 33.0 Mullidae 3 3 40.3 Unidentified Mullidae 3 3 40.3 Pomacanthidae 1 1 19.8 Centropyge potteri 1 1 19.8 Pomacentridae 3 3 35.4 Stegastes marginatus 2 2 29.1 unidentified Pomacentridae 1 1 6.3 Priacanthidae 10 8 125.1 Heteropriacanthus cruentatus 9 7 122.2 unidentified Priacanthidae 1 1 2.9 Scaridae 12 12 319.0 Calotomus carolinus 1 1 72.0 Scarus psittacus 4 4 107.1 unidentified Scaridae 7 7 139.9 Synodontidae 1 1 41.6 Saurida gracilis 1 1 41.6 Unidentified fishes 85 70 250.4 Crustaceans 34 29 78.6 Grapsidae 1 1 9.0 Plagusia depressa 1 1 9.0 Hippolytidae 10 10 38.1 Saron marmoratus 10 10 38.1 Portunidae 2 2 12.0 Charybdis hawaiensis 1 1 10.6 Charybdis paucidentata 1 1 1.4 Rhynchocinetidae 1 1 0.3 unidentified Rhynchocinetidae 1 1 0.3 Unidentified crustaceans 20 18 19.2 Subtotal (unidentified fishes and crustaceans excluded) 114 109 1154.9 Total (all prey items) 219 197 1424.5 Prey taxon %N 0% Fish 84.5 90.6 Acanthuridae 12.3 8.2 Acanthurus nigrofuscus Acanthurus nigroris Zebrasoma flavescens unidentified Acanthuridae Apogonidae 2.6 1.9 unidentified Apogonidae Aulostomidae 5.3 3.8 Aulostomus chinensis Balistidae 2.6 1.9 Xanthichthys auromarginatus unidentified Balistidae Chaetodontidae 5.3 3.8 Chaetodon multicinctus Forcipiger flavissimus Hemitaurichthys polylepis unidentified Chaetodontidae Cirrhitidae 3.5 2.5 Amblycirrhitus bimacula unidentified Cirrhitidae Holocentridae 14.0 10.1 Sargocentron punctatissimum unidentified Holocentrinae Kuhliidae 0.9 0.6 Kuhlia spp. Labridae 1.8 1.3 Pseudocheilinus tetrataenia Stethojulis balteata Monacanthidae 13.2 8.2 Pervagor aspricaudus unidentified Monacanthidae Mullidae 2.6 1.9 Unidentified Mullidae Pomacanthidae 0.9 0.6 Centropyge potteri Pomacentridae 2.6 1.9 Stegastes marginatus unidentified Pomacentridae Priacanthidae 8.8 5.0 Heteropriacanthus cruentatus unidentified Priacanthidae Scaridae 10.5 7.5 Calotomus carolinus Scarus psittacus unidentified Scaridae Synodontidae 0.9 0.6 Saurida gracilis Unidentified fishes Crustaceans 15.5 18.2 Grapsidae 0.9 0.6 Plagusia depressa Hippolytidae 8.8 6.3 Saron marmoratus Portunidae 1.8 1.3 Charybdis hawaiensis Charybdis paucidentata Rhynchocinetidae 0.9 0.6 unidentified Rhynchocinetidae Unidentified crustaceans Subtotal (unidentified fishes and crustaceans excluded) 100.0 108.8 Total (all prey items) Prey taxon %M %IRI Fish 95.5 97.7 Acanthuridae 11.9 17.3 Acanthurus nigrofuscus Acanthurus nigroris Zebrasoma flavescens unidentified Acanthuridae Apogonidae 1.3 0.7 unidentified Apogonidae Aulostomidae 2.5 4.3 Aulostomus chinensis Balistidae 5.5 1.3 Xanthichthys auromarginatus unidentified Balistidae Chaetodontidae 6.8 4.0 Chaetodon multicinctus Forcipiger flavissimus Hemitaurichthys polylepis unidentified Chaetodontidae Cirrhitidae 3.3 1.5 Amblycirrhitus bimacula unidentified Cirrhitidae Holocentridae 4.6 16.3 Sargocentron punctatissimum unidentified Holocentrinae Kuhliidae 1.5 0.1 Kuhlia spp. Labridae 2.6 0.5 Pseudocheilinus tetrataenia Stethojulis balteata Monacanthidae 4.5 12.6 Pervagor aspricaudus unidentified Monacanthidae Mullidae 3.5 1.0 Unidentified Mullidae Pomacanthidae 1.7 0.1 Centropyge potteri Pomacentridae 3.1 0.9 Stegastes marginatus unidentified Pomacentridae Priacanthidae 10.8 8.6 Heteropriacanthus cruentatus unidentified Priacanthidae Scaridae 27.6 25.1 Calotomus carolinus Scarus psittacus unidentified Scaridae Synodontidae 3.6 0.2 Saurida gracilis Unidentified fishes Crustaceans 5.5 2.3 Grapsidae 0.8 0.1 Plagusia depressa Hippolytidae 3.3 6.6 Saron marmoratus Portunidae 1.0 0.3 Charybdis hawaiensis Charybdis paucidentata Rhynchocinetidae 0.0 0.0 unidentified Rhynchocinetidae Unidentified crustaceans Subtotal (unidentified fishes and crustaceans excluded) 100.0 100.0 Total (all prey items) Table 3 Composition of the fish portion of the diet of peacock hind (Cephalopholis argus) from Hawaii Island (n = 179) and Oahu (n = 106), based on fish prey identified to at least the family level. Crustaceans were excluded from the analysis because of their relatively minor dietary importance for C. argus from both islands (%IRI =1.9% and 3.3%, respectively). Dietary importance is indicated by percent by number (%N), percent by occurrence (%O) (calculated on the basis of full stomachs), percent by mass (%M), and percent index of relative importance (%IRI). Island Hawaii Family %N %O %M %IRI Acanthuridae 16.9 9.4 13.0 20.9 Apogonidae 3.1 1.9 1.2 0.6 Aulostomidae 6.2 3.8 3.1 2.6 Balistidae 4.6 2.8 9.1 2.9 Chaetodontidae 7.7 4.7 10.1 6.2 Cirrhitidae 3.1 1.9 0.9 0.6 Holocentridae 24.6 15.1 7.5 35.8 Kuhliidae 1.5 0.9 2.5 0.3 Labridae 1.5 0.9 0.2 0.1 Monacanthidae 6.2 2.8 1.6 1.6 Mullidae 4.6 2.8 5.8 2.2 Pomacanthidae -- -- -- -- Pomacentridae 1.5 0.9 0.9 0.2 Priacanthidae 9.2 5.7 9.2 7.7 Scaridae 9.2 5.7 34.8 18.4 Synodontidae -- -- -- -- Island Oahu Family %N %O %M %IRI Acanthuridae 8.6 5.7 11.9 7.1 Apogonidae 2.9 1.9 1.7 0.5 Aulostomidae 5.7 3.8 2.0 1.8 Balistidae -- -- -- -- Chaetodontidae 2.9 1.9 2.1 0.6 Cirrhitidae 5.7 3.8 7.9 3.2 Holocentridae -- -- -- -- Kuhliidae -- -- -- -- Labridae 2.9 1.9 7.0 1.1 Monacanthidae 31.4 18.9 10.3 48.6 Mullidae -- -- -- -- Pomacanthidae 2.9 1.9 5.0 0.9 Pomacentridae 5.7 3.8 7.3 3.0 Priacanthidae 11.4 3.8 15.2 6.2 Scaridae 17.1 11.3 19.1 25.3 Synodontidae 2.9 1.9 10.5 1.6
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