ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on July 21, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(6):1272-1281; doi:10.1093/icesjms/fsm111
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Quantitative diet assessment of wobbegong sharks (genus Orectolobus) in New South Wales, Australia
1 Marine Mammal Research Group, Graduate School of the Environment, Macquarie University, Sydney, NSW 2109, Australia
2 NSW Department of Primary Industries, Port Stephens Fisheries Centre, Taylors Beach Road, Taylors Beach, NSW 2316, Australia
Correspondence to C. Huveneers: tel: +61 2 9850 7980; fax: +61 2 9850 7972; e-mail: charlie.huveneers{at}gse.mq.edu.au
Huveneers, C., Otway, N. M., Gibbs, S. E., and Harcourt, R. G. 2007. Quantitative diet assessment of wobbegong sharks (genus Orectolobus) in New South Wales, Australia. – ICES Journal of Marine Science, 64: 1272–1281.The diets of three species of wobbegong (Orectolobus ornatus, O. maculatus, and O. halei) in New South Wales, Australia, were investigated using stomach contents from specimens caught by commercial fishers. Some 80% of wobbegongs caught by commercial setline, and 60% caught by trap or scuba diving, had empty stomachs, most likely due to regurgitation. Wobbegongs were frequently hooked in the stomach (80–90% of the catch), potentially contributing to the greater proportion of empty stomachs compared with other species of shark. The diet of all three species was primarily osteichthyans, but with some cephalopods and chondrichthyans. Interspecific differences in the diets were related to total length of the shark. Octopuses were more frequent in the diet of O. ornatus (dwarf ornate wobbegong) than other wobbegong species, possibly through the smaller adult size facilitating capture of octopuses in small holes/crevices. Orectolobus halei fed more frequently on pelagic species and chondrichthyans, possibly because of their greater mobility. Wobbegongs feed at a high trophic level, and their removal from their ecosystem may impact lower trophic levels.
Keywords: commercial fishery, diet, New South Wales, Orectolobus, wobbegongs
Received 30 October 2006; accepted 7 June 2007; advance access publication 21 July 2007.
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Introduction |
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Sharks are among the top predators in the marine environment (Cortés, 1999) and have an important role in energy exchange between trophic levels (TLs) (Cortés and Gruber, 1990; Wetherbee et al., 1990). As such, any removal of top predators from coastal ecosystems has the potential to cause trophic cascades that may result in alterations to the abundance of lower trophic species (Jennings and Kaiser, 1998; Myers et al., 2007). However, there is an absence of quantitative information on the diet of sharks in many ecosystems, making assessment of how they contribute to marine trophic structure difficult (Cortés, 1999).
Wobbegong sharks are abundant predators, commonly found at coastal rocky reefs off New South Wales (NSW), Australia (Last and Stevens 1994; Compagno, 2001). They are commercially targeted in NSW waters by fishers within the Ocean Trap and Line Fishery, and are sold as boneless fillets or "flake". The commercial catch has declined from ca. 150 t in 1990/1991 to ca. 70 t in 1999/2000, a decrease of more than 50% in a decade (Pease and Grinberg, 1995; NSW DPI, unpublished data). The extent to which this decline can be attributed to fishing is unclear, and whether the trend is consistent across species is unknown, raising concern for the status of wobbegong shark populations in the region. As a result, spotted and ornate wobbegongs are classified by the IUCN as "Vulnerable" in NSW (Cavanagh et al., 2003). Quantification of wobbegong diet is critical to understanding the potential effects of their removal (via a commercial fishery) on the coastal marine ecosystem. Improved understanding of the role of wobbegongs in marine ecosystems will enhance fishery management.
Wobbegongs are demersal sharks that are usually seen resting on the substratum (Whitley, 1940; Stead, 1963; Munro, 1967; Coleman, 1980; Last and Stevens, 1994; Compagno, 2001). However, they have also been observed ambushing or actively chasing prey (Whitley, 1940; Last and Stevens, 1994; Compagno, 2001). At present, it is unclear whether feeding follows a diel pattern (Huveneers et al., 2006). Although descriptive studies with limited sample sizes suggest that wobbegongs feed on a variety of prey (Cochrane, 1992; Chidlow, 2003), a quantitative assessment of their diet will provide a critical first step in understanding trophic interactions and possible ecosystem effects of the wobbegong fishery.
Here, we investigated the diets of the three wobbegong species (Orectolobus ornatus, O. maculatus, and O. halei) occurring in coastal waters off NSW using stomach contents collected from animals caught commercially or as bycatch, or collected by hand. Diet components were examined and the relative importance of each prey item was quantified. Interspecies variation, prey diversity, and dietary overlap were also investigated to assess resource partitioning between the three species.
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Material and methods |
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Collection of specimens and stomach processing
Wobbegongs were obtained using setlines, lobster traps, and by scuba diving between June 2003 and May 2006. Most stomachs were collected aboard commercial fishing vessels in the Ocean Trap and Line Fishery targeting wobbegongs with demersal setlines at four locations in NSW (Nambucca Heads, Port Stephens, Newcastle, and Sydney) (Figure 1). All three species were collected throughout the year and at all locations (except in Sydney, where O. ornatus is not found). Sample size was supplemented by wobbegongs taken as bycatch in lobster traps, and collected while scuba diving. Once captured, wobbegongs were identified to species, the sex was determined, and total length was measured to the nearest millimetre. The position of the embedded hook (i.e. in the mouth, oesophagus, or cardiac stomach) was recorded for each wobbegong caught by the setline fishery before excising the stomach. Wobbegong foreguts (anterior of oesophagus to pyloric sphincter) were removed, placed in labelled bags, and stored at –20°C for later analysis in the laboratory.
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In the laboratory, stomach contents were thawed and washed with water. Identification was based on intact and remaining hard items, including cephalopod beaks, fish otoliths, and internal and external skeletal material, combined with general shape and anatomical features of the prey. Recognizable prey items were identified to the lowest taxon possible, using reference collections of fish otoliths (78 Australian species) and cephalopod beaks (
20 species) held at the South Australian Museum, and reference guides (Smale et al., 1995; Lu and Ickeringill, 2002). Contents identified as bait via prominent hook marks and/or knife cuts were excluded from the analysis. The number of empty stomachs together with the number of stomachs containing only bait was recorded and expressed as a percentage of the total number examined. Prey items were assigned to a prey category (demersal, predominantly demersal, or pelagic) according to Stevens and Wiley (1986), using standard taxonomic literature and Kuiter (2000). The wet mass of each food item was determined on an electronic balance to the nearest 0.01 g when prey items were small and on a spring balance (600 ± 5 g or 2.5 ± 0.02 kg) when prey was large.
Analysis
The contributions of different prey items to each species' diet were determined by the numerical importance (%N; Hyslop, 1980), frequency of occurrence (%F; Hynes, 1950; Hyslop, 1980), and mass (%M; Pillay, 1952; Hyslop, 1980). The index of relative importance (IRI; Pinkas et al., 1971), which incorporates the other three indices, was also calculated, and expressed as a percentage (%IRI; Cortés, 1997). A three-dimensional graphic representation, where each point on the graph represents the numerical importance, percentage importance by mass, and percentage occurrence, was also used as an alternative to summary tables (Costello, 1990; Cortés, 1997). Only prey families with at least one of the quantifying parameters >10% were represented in the graph to avoid a cluster of minor prey items close to the origin of the axis.
All quantitative analyses were undertaken using %IRI at a family level, and excluding data for contents that were not identifiable to family level (Simpfendorfer et al., 2001). To determine whether the sample size was sufficient accurately to describe the diet of wobbegongs, the cumulative number of prey species was plotted against the number of stomachs examined. The order in which stomachs were analysed was randomized ten times (Ferry et al., 1997). Cumulative curves were considered asymptotic if at least ten previous values of the total number of prey were in the range of the asymptotic number of prey ± 0.5. The number of stomachs at which the number of prey items reached an asymptotic value identified the minimum sample size required to describe the diet adequately (Cailliet et al., 1986; Cortés, 1997).
Low sample size resulting from the large proportion of empty stomachs prevented investigation of ontogenetic and sexual variation in the diet of the wobbegong species. Dietary overlap between species was calculated using Horn's (1966) index of overlap (R0). Values of 0–0.29, 0.3–0.59, or >0.6 indicate low, medium, or high overlap, respectively (Langton, 1982). Diet diversity (breadth) of each species was also calculated using the combined index (CI; Cortés et al., 1996), calculated by taking the average of the Levin's index (B) and the Shannon–Weiner index (H') standardized on a scale of 0–1 (Krebs, 1999). Similarity among species was also examined with cluster analysis in Primer v5.2.9 (Clarke and Gorely, 2001), using the techniques described in Clarke and Warwick (2001). Analysis of diet differences between species was undertaken following Platell and Potter (2001), and White et al. (2004). Diet data within each species were randomly allocated into groups of four or five, and mean values were determined. Mean IRI values were then square-root transformed and a similarity matrix produced, using the Bray–Curtis similarity coefficient. An MDS ordination plot was obtained from the resulting similarity matrix. One-way analyses of similarities (ANOSIM) were used to test for significant differences among the diets of the three species. Similarity percentages (SIMPER) were used to determine the dietary categories that typified particular species and/or contributed most to the dissimilarities between species (Clarke, 1993). Multivariate dispersion (MVDISP) was used to determine the degree of dispersion of the diet samples on ordination plots (Somerfield and Clarke, 1997). Results obtained from the multivariate analyses were compared with the dietary overlap and breadth indices.
Finally, the TL for each species of wobbegong was calculated following Cortés (1999):
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Results |
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A total of 641 wobbegongs (285 O. ornatus, 155 O. maculatus, and 201 O. halei) was examined for diet (Table 1). Wobbegongs with empty stomachs or bait only were common (ca. 80% in wobbegongs caught on setlines). Wobbegongs caught in traps or while scuba diving had a lesser proportion of empty stomachs (ca. 60%). Wobbegongs were mainly hooked in the cardiac stomach (75%). In a few cases (n = 23), the hook had perforated the stomach wall and damaged the liver or vertebral column. In all, 313 prey items were found in 144 stomachs, and ca. 50% of these contained a single prey item only.
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Cumulative diversity and quantitative description of wobbegong diets
The cumulative curve for wobbegongs (species combined) reached an asymptote after
130 stomachs had been examined (Figure 2a). Although the cumulative curves for each species described a general asymptotic relationship, they did not reach asymptotic stabilization (Figure 2b–d), suggesting that the stomachs sampled may not be entirely representative of each species' diet.
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Other items in the stomachs of wobbegongs included stones in single specimens of O. maculatus and O. halei, and in two O. ornatus, algal fragments in 14 O. ornatus, 3 O. maculatus, and 3 O. halei; and molluscan shells in single specimens of O. ornatus and O. halei. These items may have represented incidental ingestion or the stomach contents of prey, and were not included in the overall analysis.
Orectolobus ornatus
Bony fish (ten families) were the dominant food of O. ornatus in terms of %N, %M, %F, and %IRI, followed by cephalopods (two families) (Table 2, Figure 3a). The rankings of prey items in the bony fish category differed according to the method of quantification. The IRI indicated that unidentified bony fish were most prominent, followed by sparids, with Pagrus auratus (snapper) particularly important. When quantified by mass, Gymnothorax prasinus (green moray) was the most prominent prey item, followed by P. auratus (snapper), Girella tricuspidata (blackfish), and unidentified bony fish. Within the bony fish, G. prasinus, P. auratus, and G. tricuspidata contributed to a large proportion of the total mass of the stomach contents sampled, whereas unidentified items contributed more to abundance than to mass. Other bony fish found as prey and ranked in accordance with the IRI included species from the families Batrachoididae, Carangidae, Platycephalidae, Monacanthidae, Mugilidae, Pempheridae, and Berycidae. Octopuses dominated the cephalopod category, contributing more numerically than by mass to the diet, and were followed by cuttlefish. Most prey taxa were demersal, only the Carangidae being pelagic.
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Orectolobus maculatus
Bony fish (14 families) also dominated the food of O. maculatus in terms of %N, %M, %F, and %IRI, and were followed by cephalopods (various species of octopus), then chondrichthyans (Table 2, Figure 3b). The rankings of prey items in the bony fish category also differed by method of quantification. The IRI indicated that unidentified bony fish dominated, followed by P. auratus (snapper) and Scomber australasicus (slimy mackerel). When quantified by mass, sparids were the most prominent prey, followed by unidentified bony fish, Muraenesox bagio (pike eel), and kyphosids. Unidentified items contributed more numerically than by mass to the diet, whereas P. auratus, M. bagio, and Scorpis spp. all contributed more gravimetrically than numerically. Other bony fish found as prey of O. maculatus and ranked in accordance with the IRI included species from the families Carangidae, Sciaenidae, Berycidae, Dinolestidae, Moridae, Labridae, Serranidae, Mugilidae, Monacanthidae, and Diodonti-dae. Within the chondrichthyan group, prey items from the order Heterodontiformes, and the families Rhinobatidae and Triakidae were found in stomach contents. Prey taxa were mostly demersal, only two prey groups (Carangidae and Scombridae) being identified as pelagic.
Orectolobus halei
Bony fish (11 families) again dominated the food of O. halei in terms of %N, %M, %F, and %IRI, and contributed more numerically to the diet than by mass (Table 2, Figure 3c). However, chondrichthyans and cephalopods were consumed by O. halei in small numbers, but made up a large proportion of the total mass of the stomach contents. In contrast to the situation for O. maculatus, chondrichthyans (three families) were the second most dominant prey group in the diet of O. halei on the basis of %N, %M, %F, and %IRI, followed by cephalopods (Octopus spp.). Within the bony fish category and when quantified by mass, carangids were the most important prey group, followed by sciaenids, labrids, and kyphosids. Trachurus novaezelandiae and S. australasicus were important numerical contributors to the diet of O. halei, but only made up a small proportion of the total mass of prey items examined. A few Achoerodus viridis, Argyrosomus japonicus, and unidentified kyphosids were found, but they constituted a large proportion of the stomach contents by mass. Other bony fish found as prey and ranked in accordance with the IRI included species from the families Clupeidae, Berycidae, Sparidae, Arripidae, Uranoscopidae, and Diodontidae. Of chondrichthyans, Heterodontus portusjacksoni, O. ornatus, and species of the family Rhinobatidae were found in the stomachs. Although the diet of O. halei had a broader diversity of pelagic prey than O. ornatus and O. maculatus, the prey composition of O. halei was similar to that of the other two orectolobids, and was mostly demersal.
Species comparison
Diet breadth of O. maculatus and O. ornatus was similar and relatively low (ca. 0.3), and that of O. halei was even smaller (0.18). Overall, the diets of the three species differed significantly (ANOSIM: R-statistic = 0.184, p < 0.01). The diets of O. ornatus and O. halei overlapped only slightly (0.29 Horn's index) and were significantly different (ANOSIM: R-statistic = 0.336, p < 0.01). However, neither the diets of O. ornatus and O. maculatus nor those of O. maculatus and O. halei differed significantly (ANOSIM: R-statistics = 0.077 and 0.01, respectively, and p > 0.05 in both cases). These relationships were supported by Horn's index, suggesting strong overlap of the diets of O. ornatus and O. maculatus (0.87), and a medium overlap of the diets of O. maculatus and O. halei (0.46). Octopuses accounted for most of the difference between diet compositions of the three species of wobbegong, with carangids and sparids also contributing (SIMPER). Samples from all three species were largely scattered, with dispersion values of 0.87, 1.07, and 1.29 for O. ornatus, O. maculatus, and O. halei, respectively. The MDS plot had a high stress level, indicating a poor fit between actual distance measures and distance in the ordination, so the plot was difficult to interpret and did not show any major trends.
TL of wobbegong sharks
All three species of wobbegong fed on secondary consumers, at TL > 3. Trophic levels of prey categories were 3.2, 3.24, and 3.65 for cephalopods, osteichthyans, and chondrichthyans, respectively. The trophic levels of the three species were 4.23, 4.24, and 4.25 for O. ornatus, O. maculatus, and O. halei, respectively, making them all tertiary consumers.
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Discussion |
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Although the proportion of empty stomachs was high, the data obtained contribute to better understanding of the diet and feeding habits of wobbegongs and their role in the trophic ecology of NSW coastal rocky reefs. The use of samples collected by commercial fishers imposes an overlying pattern of collection times and areas. Results may also have been influenced by temporal or spatial variation in prey availability. However, the main fish fauna does not differ across the NSW state, and wobbegong prey species are found throughout the sampling locations (Yearsley et al., 1999; Hutchins and Swainston, 2001). Sampling was throughout the year, with all species caught at each location, except at Sydney where O. ornatus is not found. Therefore, spatial and temporal biases attributable to prey availability are expected to be minimal.
The total combined percentage of empty stomachs was 80% when wobbegongs were caught on setlines, and 50–65% when caught in traps or while scuba diving. These values are high when compared with those of other species of shark (Wetherbee et al., 1990; Simpfendorfer, 1998; Joyce et al., 2002; Morato et al., 2003). Nevertheless, they were consistent with other wobbegong diet studies (Cochrane, 1992; Chidlow, 2003), which found that 60–70% of stomachs examined were empty. The use of setlines may explain the high percentage of empty stomachs (Cortés, 1997). Bait is likely to be more attractive to hungry sharks rather than to those with full stomachs, because fish that feed to satiation have a reduced response to the odour of bait (Lokkeborg et al., 1995). The greater percentage of stomachs with prey in wobbegongs caught in traps and while scuba diving support this. However, Chidlow (2003) showed that 70% of wobbegongs caught in gillnets had empty stomachs. Moreover, many wobbegongs were captured with full stomachs and bait, indicating that wobbegongs with full stomachs may still be attracted to, and feed on, bait. One O. halei was found with 19 bony fish in its stomach, but still took the bait. Regurgitation of the stomach contents was often observed when wobbegongs were brought to the surface and onto the boat, and may have been an attempt to dislodge the hook (CH, pers. obs.). Regurgitation of stomach contents is likely to explain the high percentage of empty stomachs recorded (Wetherbee and Cortés, 2004), but infrequent feeding or short periods of feeding followed by periods of rapid digestion cannot be discounted.
Overall, 80–90% of the wobbegongs examined were hooked in the cardiac stomach or anterior oesophagus. Wobbegongs feed in a similar manner to angel sharks (Squatina australis), but take prey in front of the shark (Compagno, 2001). The short broad mouth and large broad pharynx produces suction, and prey items are usually swallowed whole. This behaviour may explain why hooks were embedded more frequently in the cardiac stomach than in the mouth. Although most commercial fishers in NSW use J-shaped hooks to catch wobbegongs, research elsewhere (e.g. Cooke and Suski, 2004) has shown that the use of circle hooks can decrease the proportion of stomach-hooked animals and, on occasions, cause a simultaneous reduction in overall catch. The environmental impact statement for the NSW Ocean Trap and Line Fishery (NSW DPI, 2006) has recommended a minimum legal size limit of 130 cm total length for all species of wobbegong. Consequently, future research should examine the effects that J-shaped and circle hooks have on catches and post-release mortality of individuals less than this recommended size. It is imperative that such studies be done before the introduction of size limits, because the results of this study (i.e. the stomach- or oesophagus-hooking of 80–90% of all wobbegongs) suggest that a minimum size limit will result in unquantified, fishing-related mortality. Introducing such management action does not adhere to the principles of ecologically sustainable development, nor would it be in line with the objectives of the National Plan of Action for the Conservation and Management of Sharks in Australian waters (Shark Advisory Group and Lack, 2004).
The diets of wobbegongs in NSW waters were dominated by bony fish, with cephalopods and chondrichthyans also important. Most of the prey species were demersal, closely associated with reef ecosystems, and consistent with the habitats of wobbegongs and previous descriptions of their diet (e.g. Last and Stevens, 1994; Compagno, 2001; Chidlow, 2003). Only a few prey items were classified as pelagic. Midwater schooling fish such as T. novaezelandiae and S. australasicus form large schools close to the seabed during periods of low water temperature. The greatest proportions of T. novaezelandiae and S. australasicus were in stomachs of O. halei caught off Sydney in July, when water temperature is low (i.e. <16°C), suggesting that O. halei might have fed on these prey species when they were close to the seabed.
In Western Australia (WA), Chidlow (2003) found that bony fish were the dominant prey of wobbegongs, with occurrences of 60% and 66.7% in O. ornatus and O. maculatus, respectively. The present study found a large proportion of bony fish, about 95%IRI in all three species, similar to the results of earlier studies in northern NSW (Cochrane, 1992). Trachurus novaezelandiae and S. australasicus were numerically important prey of O. halei, with multiples of both prey species found in single stomachs, e.g. up to ten T. novaezelandiae and nine S. australasicus in the stomach of one large O. halei (1800 mm TL), possibly inflating the IRI. However, the relative importance of carangids and scombrids was diminished when %F was considered, because few wobbegongs consumed pelagic species. Prey items with large mass and low frequency, such as P. auratus in O. ornatus or A. japonicus in O. halei, were only found in a few wobbegongs, so the IRI was enhanced by the prey's gravimetric importance.
Previous reports have suggested that the diet of wobbegongs includes other sharks and possibly cannibalism for O. maculatus (Coleman, 1980; Compagno, 2001), but we did not find evidence of cannibalism. Cochrane (1992) failed to find any evidence of chondrichthyan prey in his study of the diet of wobbegongs, but Chidlow (2003) and we, in this study, did find sharks and rays in wobbegong stomachs. The presence of O. ornatus in the stomach of an O. halei and several observations of feeding behaviour (R. Brislane, Nambucca Heads, pers. obs.; P. Hitchins, South West Rocks Dive Centre, pers. obs.; CH, pers. obs.) confirm that O. ornatus is regularly consumed by O. halei.
Cephalopods were noted as prey of wobbegongs in this study and also by Cochrane (1992) and Chidlow (2003). However, the IRI for cephalopods in NSW (0.5–7.8%) was much lower than in WA (28%; Chidlow, 2003). The lesser proportion of cephalopods in wobbegongs from NSW than in WA was offset by the increased proportions of bony fish and chondrichthyans. Octopuses were clearly the dominant cephalopod prey in NSW and WA, reflecting their presence in nearshore rocky habitats and the demersal feeding behaviour of wobbegongs.
We found no crustaceans in wobbegong stomachs, agreeing with the findings of Chidlow (2003), but contrasting with those of Cochrane (1992), who reported that crustaceans were taken at a %F of 6.6 (though derived from a single crustacean found in the stomach of one individual). Other authors (e.g. Whitley, 1940; Stead, 1963; Last and Stevens, 1994; Compagno, 2001) have reported crustaceans in the diets of wobbegongs. Crustaceans may feature in the diets of neonates and/or juvenile wobbegongs, reflecting possible ontogenetic changes in diet. Small O. ornatus (<600 mm total length), and O. maculatus and O. halei (each <1100 mm total length) could not be sampled in the present study, but perhaps should be the focus of future research.
Overlap indices and the ANOSIM suggested that the diet of O. halei was statistically different from that of O. ornatus. However, there were no differences between O. ornatus and O. maculatus, or between O. maculatus and O. halei. Interspecific differences in dentition were not evident (Huveneers, 2006) and cannot explain the dissimilarities. The differences in the diets of O. halei and O. ornatus were mostly attributable to the larger number of pelagic prey items such as the carangid T. novaezelandiae and the scombrid S. australasicus, and low prevalence of octopus in the diet of O. halei. The greater importance of octopus in O. ornatus than in O. maculatus or O. halei may be due to the smaller adult size of O. ornatus (Compagno, 2001; Huveneers, 2006; Huveneers et al., 2007) facilitating the capture of octopuses in small holes/crevices not accessible to the larger O. maculatus and O. halei. Larger sharks may feed on larger prey, and may be more efficient at capturing faster prey (Wetherbee et al., 1990; Simpfendorfer et al., 2001), perhaps explaining why pelagic prey and chondrichthyans occurred in the diet of O. halei. A change in diet with total length is usually reported within chondrichthyan species as ontogenetic variation (Lowe et al., 1996; Ebert, 2002) reducing intraspecific competition.
The TL we calculated for wobbegongs (4.24) is similar to that of an earlier study (4.3; Chidlow, 2003), and is the highest of the Orectolobiformes (average 3.6, maximum 4.1; Cortés, 1999). However, the estimated TL for Orectolobiformes from Cortés (1999) did not include the Orectolobidae. The high TL of wobbegongs indicates that they are top predators, although in part this is attributable to the chondrichthyans (TL 3.65) found in the diet of O. maculatus and O. halei and to cephalopods in the diet of O. ornatus. Cortés (1999) did not find elasmobranchs in the diet of any Orectolobiformes, explaining their lower average TL of 3.6.
Given the high TL of wobbegongs [higher than seabirds and similar to some marine mammals (Cortés, 1999)], the removal of these top predators may potentially have top-down effects on their prey and other lower level consumers. Several authors have asserted that trophic cascades are rare in large, diverse ecosystems buffered by multiple trophic links and spatial heterogeneity (Strong, 1992). Therefore, there are debates whether sharks exert significant top-down effects (Stevens et al., 2000; Kitchell et al., 2002). However, the removal of large, predatory fish, including sharks, has led to species declines and changes in community structure through competitive (Fogarty and Murawski, 1998) and predatory release (Baum et al., 2003; Shepherd and Myers, 2005; Ward and Myers, 2005; Myers et al., 2007). Ecosystem models exist for southeastern Australia (Goldsworthy et al., 2003), but they do not include sharks, hindering assessment of the effects of wobbegong removal on animals at lower TLs. The development of an ecosystem model that includes chondrichthyans and identifies the effects of their removal in NSW waters is clearly needed.
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Acknowledgements |
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We thank Reala Brislane, Jason Moyce, Ian Puckeridge, Mark Phelps, and Shannon Fantham for assistance on board their fishing vessels, and several interns and volunteers for help with sampling. We also gratefully acknowledge the analytical assistance of Terry Walker, and the reviews by Simon Allen, Joe Bizzaro, and two anonymous referees, all of which helped us improve the manuscript. CH was supported by an international Macquarie University Research Scholarship. Financial support was provided by the Graduate School of the Environment, NSW Department of Primary Industries, and the Australian Geographic Society. The study was undertaken under the NSW DPI permit number PO03/0057 and a Macquarie University Ethics Committee approval number 2003/011.
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References |
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