Size, sex and seasonal patterns in the assemblage of Carcharhiniformes in a sub-tropical bay

Abstract

Size, sex and seasonal patterns among Carcharhiniformes were examined in shallow regions of Moreton Bay, Queensland, Australia. A total of 1259 sharks were caught, comprising 13 species. The Australian sharpnose shark Rhizoprionodon taylori and the blacktip complex Carcharhinus limbatus–Carcharhinus tilstoni comprised 55% of all shark individuals. Neonates were observed for five species including the dusky shark Carcharhinus obscurus, which contrary to previous reports was relatively abundant in shallow, predominantly estuarine waters. Three contrasting patterns of occurrence were observed: smaller species were abundant and present throughout much of their ontogeny, larger species were mainly caught as neonates or juveniles and vagrant species were only caught during the warmer months. The shark assemblage differed significantly among seasons. While many species were observed during the warmer months, species diversity was lower in winter when C. obscurus comprised 43% of the catch. Overall, the results indicated that spatial and temporal distribution patterns were not synchronous for all species. The capture of small numbers of neonate C. obscurus in late autumn and winter demonstrates that parturition among Carcharhiniformes is not confined to spring and summer in sub-tropical waters.

Introduction

The shallow tropical and sub-tropical coastal waters of Australia support an abundant and diverse array of elasmobranchs (Last & Stevens, 2009; White & Sommerville, 2010). The most common sharks found in these waters belong to the family Carcharhinidae and exhibit a variety of life-history strategies (Cortés, 2004). Within this family, smaller species such as the Australian sharpnose shark Rhizoprionodon taylori (Ogilby 1915) occupy shallow regions throughout much of their life (Simpfendorfer & Milward, 1993) while larger sharks such as the common blacktip Carcharhinus limbatus (Müller & Henle 1839) may typically only occupy shallow regions during their early life history (Castro, 1996; Heupel et al., 2004) or, in the case of pregnant sharks, around the time of parturition. These shallow regions often act as nursery areas and may offer an abundance of prey or a reduction in predators which facilitate growth and may reduce natural mortality (Heupel et al., 2007).

Occupying coastal waters may have been an effective evolutionary strategy for minimizing juvenile mortality (Simpfendorfer & Milward, 1993); however, the continued success of this strategy may be compromised in some coastal regions. Rapid human population growth has led to the development of commercial fisheries and subsequent mortality of sharks (Clarke et al., 2006). Recreational fishing, habitat modification and degradation are additional pressures that have the potential to affect sharks that occupy shallow coastal waters (Knip et al., 2010; Lynch et al., 2010).

Despite the emergence of fisheries and research related to sharks, for nearly half of all reported shark species, insufficient data exist to determine their population status (Heupel & Simpfendorfer, 2010). This represents a major challenge in terms of sustainably managing fisheries and assessing the conservation status of sharks. The collection of species-specific abundance data from fisheries-dependent and independent sampling can improve knowledge on the distribution and ecology of sharks in inshore waters. When viewed in conjunction with life-history data (e.g. size and age at- maturity and fecundity), tag–recapture data, movement data derived from active- and passive-monitoring studies and photographic identification studies (Musick & Bonfil, 2004; Heupel et al., 2006; Marshall & Pierce, 2012), this information can assist in risk assessments and management strategies.

The Indo-Australasian region (Australia, Indonesia, New Caledonia, New Guinea and New Zealand) supports 427 species of chondrichthyans, including almost half of the global total of known sharks (White & Kyne, 2010). Within this region, diversity is highest in Australia (White & Kyne, 2010). In the tropical waters of northern Australia, the biology of sharks has been fairly well studied. For example, published studies have focussed on life histories (Stevens & McLoughlin, 1991; Simpfendorfer, 1992; Simpfendorfer & Milward, 1993; Harry et al., 2011a), habitat use (Knip et al., 2011), fisheries interactions (Robbins et al., 2006; Lynch et al., 2010; Harry et al., 2011b) and genetics (Ovenden et al., 2010) of Carcharhiniformes. Knowledge gaps exist, however, in the biology and ecology of sharks in sub-tropical Australian waters. Within the sub-tropical waters of Queensland, on the east coast of Australia, published studies on Carcharhiniformes are restricted to dietary and habitat studies on the Australian weasel shark Hemigaleus australiensis White, Last & Compagno 2005 (Taylor, 2008) and the slit-eye shark Loxodon macrorhinus Müller & Henle 1839 (Gutteridge et al., 2011), ontogenetic habitat use of the bull shark Carcharhinus leucas (Müller & Henle 1839) (Werry et al., 2011) and spatial and seasonal partitioning among larger sharks (Taylor et al., 2011).

This study explores the carcharhiniform assemblage in Moreton Bay, a large sub-tropical embayment. The bay is a Marine Park that supports both recreational and commercial fisheries and lies adjacent to rapidly expanding conurbations in south-east Queensland. The major aims were to identify the species composition and sex ratios of Carcharhiniformes, to determine size at birth and timing of parturition, to identify size-related patterns of occurrence and to examine the extent to which the shark assemblage varies among months and seasons. This study provides the first ecological information for numerous species in the region. Collectively, the results allow the degree of synchrony in spatial and temporal distribution patterns of Carcharhiniformes to be examined in a known region of high shark diversity.

Materials and methods

Study site and sampling

Moreton Bay is a sub-tropical bay of c. 1600 km² located 27° S, c. 400 km south of the Tropic of Capricorn (Dennison & Abal, 1999). The bay has an average depth of 15 m and contains a wide variety of habitats including mangroves, sea grass meadows, littoral and sub-littoral reefs, algal and sponge beds, fringing coral reefs and various grades of bare sandy to muddy substrata (Johnson, 1999). Two major biodiverse regions have been identified in Moreton Bay: an inshore estuarine-dominated region characterized by relatively high turbidity and low salinity and a marine-dominated region characterized by low turbidity and high salinity (Davie & Hooper, 1998).

Study sites were located in the central region of the bay and were sampled every month from May 2004 to March 2007 [for a map of the bay and location of fisheries-dependent sampling, see Taylor & Bennett (2008)]. Sharks were caught by a commercial gillnetter operating within the East Coast Inshore Finfish Fishery (ECIFFF; Roelofs, 2011) and in a fisheries-independent survey. Commercial gillnetting was confined to the inshore estuarine region, mostly within Waterloo Bay (27° 24′ S; 153° 12′ E), with a bottom-set gillnet typically deployed 1 h before dawn and fished for 2 h in shallow water (≤2 m deep) over sandy substrata. Three different monofilament gillnets were used throughout the study, although only one net was used during each fishing event. Net length varied between 700 and 800 m, with a 2 m drop, and mesh sizes of 7·6, 8·9 and 15·2 cm. The 7·6 and 8·9 cm mesh size gillnets were used throughout all four seasons while the 15·2 cm mesh size gillnet was used in the austral spring (September to November), summer (December to February) and autumn (March to May) but not in winter (June to August). Sharks were identified and measured on land shortly before they were processed for sale at a commercial seafood marketing store.

The fisheries-independent survey was conducted from a 5 m University of Queensland research vessel. A multi-gear sampling strategy was adopted, using a bottom-set gillnet and a bottom-fished setline. The gillnet (150 m long, 2 m drop and 8·9 cm mesh size) was anchored at each end and deployed parallel to the shore in shallow water (≤2 m deep) over sandy substrata at three locations in central regions of the bay. These locations were adjacent to St Helena Island (27° 23·20′ S; 153° 12·34′ E), Horseshoe Bay, Peel Island (27° 30′ S; 153° 21·61′ E) and Deanbilla Bay, North Stradbroke Island (27° 30·86′ S; 153° 24 46′ E). The setline comprised a 400 m mainline of 4 mm braided rope. Gangions were 2 m long and consisted of a shark clip attached to 1 m of braided nylon cord and 1 m of multi-strand stainless steel wire attached to a 10/0 hook. Thirty hooks were baited with pieces of mullet Mugil cephalus L. 1758 and the gangions were spaced uniformly along the mainline. The setline was anchored at both ends and bottom-set in 2–6 m within 400 m of the gillnet, adjacent to St Helena Island (27° 23′ S; 153° 14′ E) and in Deanbilla Bay (27° 30′ S; 153° 24′ E).

The setline and gillnet were typically deployed 1 h before dawn and were fished for 4 h, weather permitting. The net was checked every 30 min while the hooks from the setline were checked and, if empty, re-baited every 2 h. Those sharks assessed as being in good condition were tagged, injected with a fluorochrome label (calcein, 5 mg kg⁻¹ body mass) and released. Sharks that were in poor condition were euthanazed and taken to the University of Queensland for related life-history studies (Taylor, 2008). The gillnet was deployed for 49 h in spring, 61 h in summer, 62 h in autumn and 54 h in winter. The setline was deployed for 45 h in spring, 61 h in summer, 62 h in autumn and 30 h in winter.

Sharks were identified to the species level, except for the Australian blacktip shark Carcharhinus tilstoni (Whitley 1950) and C. limbatus which are difficult to separate reliably in the field (Ovenden et al., 2010). All sharks were measured to the nearest mm and total length (L_T) was defined as the longest length of the shark, measured from the snout tip to the upper caudal fin (Last & Stevens, 2009). Sharks were sexed and assigned to a maturity category. Sexual maturity in males was based upon the degree of calcification and flexibility of the claspers. Sharks with small and uncalcified claspers were classified as juveniles, while sharks with calcified claspers were classified as adults (Stevens & McLoughlin, 1991).

For all sharks, except R. taylori and H. australiensis, maturity status of female sharks was based on published size-at-maturity data from northern Australia (Stevens & McLoughlin, 1991). This was because no estimates of size-at-maturity were available for the majority of species in the study region. Furthermore, sharks caught by the commercial gillnetter in this study were processed for human consumption and most of them were not available for internal examination. For R. taylori and H. australiensis, the L_T at which 50% of males and females reached maturity (L_T50) was taken from a related study in Moreton Bay (Taylor, 2008). Where present, sharks with an open or partly open umbilical scar were classed as neonates. Size at birth and timing of parturition were estimated based on the presence of neonates with an open or partly open umbilical scar.

Data analysis

The catch of Carcharhiniformes was combined from all capture methods and locations as the aim was to examine the population structure of sharks in central regions of the bay rather than to examine gear-related or spatial differences in the catch composition. The hypothesis that the sex ratio of sharks was 1:1 was tested with the χ² statistic. A significance level of P < 0·05 was required for rejection of the null hypothesis (Zar, 1999).

To assess size-related patterns of occurrence, the relative contribution (%) of each 100 mm size class and ontogenetic stage (neonate, juvenile and adult) to the total catch of each species was displayed graphically. Species with a sample size of n < 5 were excluded from the plot. Mean ±s.d. L_T was also calculated for each species.

Multivariate analysis to identify monthly patterns in the species assemblage was conducted using Primer 6.0 (Clarke & Gorley, 2006). Before analysis, numerical abundance data were square-root transformed and similarity matrices were constructed using the Bray–Curtis similarity coefficient (Clarke & Warwick, 2001). Ordination of the numerical abundance data (pooled across years) was carried out using non-metric multidimensional scaling (MDS) (Clarke & Warwick, 2001). A one-way analysis of similarities (ANOSIM) was used to examine changes in the catch composition with time of year (four seasons) as a factor. Months (pooled across years) provided replicates for their respective seasons.

Results

Species composition and sex ratios

A total of 1259 Carcharhiniformes were caught, comprising 13 species from the families Carcharhinidae, Hemigaleidae and Sphyrnidae (Table I). Of these sharks, 992 (79%) were obtained from the commercial fisher, the remainder were caught during the fisheries-independent survey. In terms of abundance and diversity, carcharhinids dominated the catch, accounting for 77% of the total number of sharks (n = 973) and 10 of the 13 species. Rhizoprionodon taylori was the most abundant shark, comprising 35% of the total catch (n = 441). Carcharhinus limbatus–C. tilstoni, H. australiensis and the dusky shark Carcharhinus obscurus (LeSueur 1818) accounted for 20% (n = 252), 19% (n = 233) and 10% (n = 131) of the catch, respectively.

Table I.  Number caught (n), mean total length (L_T), sex ratios, size of neonates and months in which neonates were caught for 13 species of Carcharhiniformes caught in shallow, central regions of Moreton Bay from 2004 to 2007. The hypothesis that the female (F) to male (M) ratio was 1:1 was tested with the χ² statistic; sex ratio comparisons were not made for species where n < 5 sharks

Species	n	Mean ±s.d. LT (mm)	Sex ratio comparisons			Neonates
			F:M	χ ^2	P	L T (mm)	Months
Carcharhinus amboinensis	69	969 ± 252	1:0·9	0·01	> 0·05	658–660	Jan., May, July
C. brevipinna	2	715 ± 36	–	–	–	–	–
C. cautus	40	965 ± 332	1:0·5	3·6	> 0·05	435–527	Dec.–Jan.
C. leucas	12	1787 ± 206	1:5	5·3	< 0·05	–	–
C. limbatus–C. tilstoni	252	841 ± 119	1:1	0·02	> 0·05	646–815	Nov.–Feb., June
C. obscurus	131	1079 ± 120	1:1·5	5·57	< 0·05	820–1040	May, Aug., Nov.
C. sorrah	16	1085 ± 132	1:2·2	2·25	> 0·05	–	–
Galeocerdo cuvier	1	2040	–	–	–	–	–
Rhizoprionodon acutus	9	713 ± 209	1:8	5·44	< 0·05	–	–
R. taylori	441	607 ± 92	1:0·8	8·44	< 0·01	–	–
Sphyrna lewini	50	751 ± 339	1:1·6	2·88	< 0·01	508	November
Hemigaleus australiensis	233	736 ± 184	1·3:1	4·67	< 0·05	–	–
Hemipristis elongata	3	1147 ± 18	–	–	–	–	–

For six of the species, sex ratios differed significantly from parity (Table I). Males were more abundant for C. leucas, C. obscurus, scalloped hammerhead Sphyrna lewini (Griffith & Smith 1834) and milk shark Rhizoprionodon acutus (Rüppell 1837), accounting for 83% (P < 0·05), 60% (P < 0·05), 62% (P < 0·01) and 89% (P < 0·05) of the total catch, respectively. For R. taylori and H. australiensis, females were more abundant, accounting for 57% (P < 0·01) and 56% (P < 0·05) of the total catch, respectively.

Size at birth and timing of parturition

Neonates were observed for five species, comprising only 6% of the total catch of all shark individuals. These were pigeye shark Carcharhinus amboinensis (Müller & Henle 1839), nervous shark Carcharhinus cautus (Whitley 1945), C. limbatus–C. tilstoni, C. obscurus and S. lewini (Table I). The majority of neonates were observed during the austral spring or summer (Table I). Three neonate C. obscurus were observed in May, August and November.

Length-related patterns of occurrence

Shark L_T ranged from a 373 mm R. taylori to a 2300 mm S. lewini (Fig. 1). Three contrasting patterns of occurrence were identified among the Carcharhiniformes. The first group consisted of smaller species that were abundant and caught throughout much of their ontogeny (C. cautus, R. taylori and H. australiensis). The second group consisted of larger species mainly caught during their neonate and juvenile stages (C. amboinensis, C. limbatus, C. obscurus and S. lewini). The third group comprised vagrant species that were caught predominantly in spring and summer [spinner shark Carcharhinus brevipinna (Müller & Henle 1839), C. leucas, spottail shark Carcharhinus sorrah (Müller & Henle 1839), tiger shark Galeocerdo cuvier (Péron & LeSueur 1822), R. acutus and fossil shark Hemipristis elongata (Klunzinger 1871)]. This group was only caught as juveniles or adults and comprised a small proportion of the total number of shark individuals.

Figure 1. The per cent contribution of size classes and ontogenetic stage for carcharhiniform sharks caught in shallow, central regions of Moreton Bay from 2004 to 2007: (a) C. leucas (n = 12), (b) C. sorrah (n = 16), (c) C. obscurus (n = 131), (d) C. amboinensis (n = 69), (e) C. limbatus–C. tilstoni (n = 252), (f) Sphyrna lewini (n = 50), (g) C. cautus (n = 40), (h) Hemigaleus australiensis (n = 233) and (i) Rhizoprionodon taylori (n = 441). Species with a sample size of ≤10 were excluded from the plot. , neonates (sharks with open or partly open umbilical scars); , juveniles; , adults.

Seasonal patterns in the assemblage

The species assemblage varied among months. Rhizoprionodon taylori and C. limbatus–C. tilstoni comprised the highest percentage (60%) of the numerical catch in spring and summer (Fig. 2). Rhizoprionodon taylori was also dominant in March and April where it comprised 62 and 73% of the catch, respectively. The species assemblage in May was dominated by H. australiensis, comprising 84% of the catch in May, while in the cooler months of June, July and August C. obscurus comprised 36, 44 and 57% of the species assemblage, respectively.

Figure 2. Monthly trends in the carcharhiniform shark assemblage in shallow, central regions of Moreton Bay from 2004 to 2007 (n = 1256). Per cent composition values relate to all sharks caught and numbers above bars represent the number of sharks caught in each month. Infrequently caught species were combined into the ‘other’ category. , Rhizoprionodon taylori; , Hemigaleus australiensis; , Carcharhinus limbatus–C. tilstoni; , C. obscurus; , C. amboinensis; , Sphyrna lewini; , other.

The MDS ordination of species composition showed that months formed two non-overlapping clusters (Fig. 3). The warmer months (September to April) formed a cluster within which successive months tended to be closer in proximity. A second cluster comprised the cooler months (June to August) while May differed considerably from all other months. ANOSIM demonstrated that the shark composition was significantly different among seasons (global r = 0·45, P < 0·01).

Figure 3. Multidimensional scaling ordination of the Bray–Curtis similarity matrices derived from the catch of 13 species of Carcharhiniformes in shallow central regions of Moreton Bay from 2004 to 2007. Stress value is shown in the top-right corner. , austral spring; , austral summer; , austral autumn; , austral winter.

Discussion

Species composition in Moreton Bay

Sub-tropical regions encompass many inshore waters adjacent to large human populations. Within Australia, these waters are known to support a high diversity of sharks (Last & White, 2011), but there is a lack of basic ecological knowledge for many species. This study examined Carcharhiniformes in Moreton Bay in southern Queensland. The capture of 13 species of Carcharhiniformes in this study equalled the number reported for a tropical bay in the Gulf of Carpentaria, Australia (Blaber et al., 1995) and was higher than those reported in Cleveland Bay, Australia (12 species; Simpfendorfer & Milward, 1993) and Herald Bight, Shark Bay, Western Australia (White & Potter, 2004). Shallow regions of Moreton Bay also provide habitat for numerous species of rays, including the blue-spotted maskray Neotrygon kuhlii (Müller & Henle 1841) and the estuarine stingray Dasyatis fluviorum Ogilby 1908 which are abundant in intertidal regions (Pierce et al., 2011). The abundance of sharks and rays in shallow regions of Moreton Bay suggest that they may influence the structure of the local ecosystem.

Carcharhinidae dominated the catch in this study and represented 10 of the 13 species. Rhizoprionodon taylori was the most abundant shark, followed by C. limbatus–C. tilstoni, H. australiensis and C. obscurus. A previous study based on setlining in a deeper (c. 15–20 m) part of Moreton Bay suggested that C. brevipinna, C. sorrah and S. lewini were the most abundant Carcharhiniformes (Johnson, 1999). Collectively, these three species formed only 5% of total catch in this study, and although some of the observed differences could be explained by gear selectivity, taken together the results suggest that the spatial separation of shark species occurs between shallow and deeper parts of the bay. Stevens et al. (2000) suggested that C. sorrah prefers deeper water, which may account for its low abundance in the nearshore habitats in Moreton Bay.

Carcharhinus obscurus has a patchy distribution in tropical and warm-temperate continental seas worldwide (Compagno, 1984). In Australia, it has been reported from the surface down to 400 m (Last & Stevens, 2009), and neonates are targeted in a demersal gillnet fishery off Western Australia (McAuley et al., 2007). The capture of juvenile C. obscurus in this study provides the first published account of this species in an Australian sub-tropical bay. Habitats for this species are generally considered to range from the surf zone to the outer continental shelf (Compagno, 1984), although small numbers of juvenile and gravid female C. obscurus have been reported in Bulls Bay, South Carolina (Castro, 1993). The presence of C. obscurus in central Moreton Bay, an ‘inshore estuarine-dominated’ environment (Davie & Hooper, 1998), and the capture of two C. obscurus in the Brisbane River (Taylor, 2008) demonstrates that this species does penetrate into potentially brackish waters. Carcharhinus obscurus has been assessed as vulnerable worldwide and near threatened in Australia by the International Union for Conservation of Nature (Simpfendorfer & Burgess, 2005) and, based on the results of this study, the reported distribution for C. obscurus should be updated to include its occurrence in shallow bays and river estuaries, particularly during periods of low rainfall and salinity.

Sex ratios

For nearly half of the species examined in this study, sex ratios significantly differed from parity. The unequal sex ratios for R. taylori, H. australiensis, C. obscurus and S. lewini are likely to be the result of sexual segregation rather than an overall population bias. Unequal sex ratios in sharks are often attributed to sexual segregation, a phenomenon widespread among elasmobranchs (Springer, 1967; Pratt & Carrier, 2001). The female-biased sex ratio for R. taylori, nearly 60% of which were mature, suggests that shallow regions of bays may provide an abundant food supply to support the rapid embryonic growth that occurs in this species (Simpfendorfer, 1992). Although the sex ratio for H. australiensis was also biased in favour of females, most sharks were immature, suggesting that the unequal distribution may have been linked to a reduction of intraspecific competition or migration of males out of the study region. Similarly, it is unlikely that factors linked with reproduction were responsible for the male-skewed population of C. obscurus and S. lewini as the catch comprised predominantly juveniles.

Size at birth and timing of parturition

Neonates were reported for C. amboinensis, C. cautus, C. limbatus–C. tilstoni, C. obscurus and S. lewini. Size at birth for S. lewini, C. amboinensis and C. obscurus from this study are comparable to previous reports in Australian waters (Stevens & Lyle, 1989; Stevens & McLoughlin, 1991; Last & Stevens, 2009), although size at birth for C. cautus appears to be larger than in northern (Lyle, 1987) and Western Australia (White et al., 2002). Based on the presence of individuals with open or partly open umbilical scars, C. cautus and C. limbatus–C. tilstoni appear to give birth in the austral summer and spring, and summer, respectively.

While the number of neonate C. obscurus observed in this study was small, the results suggest that parturition in C. obscurus in Moreton Bay occurs in late autumn and winter. The capture of a birth-size individual in November indicates that parturition may also extend into the austral spring. These observations are broadly consistent with previous reports of parturition occurring from boreal late winter to summer in the north-western Atlantic Ocean, summer and austral autumn off Western Australia and throughout the year with a peak in autumn off southern Africa (Compagno, 1984; Last & Stevens, 2009). It has been suggested that water temperature is a cue for the onset and conclusion of shark pupping and nursery season (Pratt & Carrier, 2001), and for many Carcharhiniformes parturition occurs in spring or summer as the water warms up. This makes C. obscurus somewhat unusual with birth occurring in the colder months.

The presence of neonates tentatively suggests that parts of Moreton Bay may provide nursery areas for at least five species of Carcharhiniformes; however, Heupel et al. (2007) suggested that in order to classify an area as a nursery, it must be demonstrated that sharks (1) are more abundant than in other areas, (2) have a tendency to remain or return for extended periods and (3) the area is used repeatedly across years. According to these criteria, Moreton Bay cannot be unequivocally classified as a nursery area. Further fisheries-independent sampling in other habitats and regions of the bay are needed before the nursery role of Moreton Bay can be quantitatively assessed.

Size and seasonal patterns in the shark assemblage

Patterns of occurrence were not synchronous for all species, reflecting the diversity of life-history strategies among Carcharhiniformes (Cortés, 2004). Three contrasting patterns of occurrence were observed: smaller species that were abundant and present throughout much of their ontogeny, larger species that were mainly caught as neonates or juveniles and vagrant species that were only caught during the warmer months. It is likely that these vagrant species avoid the lower temperatures in shallow regions of Moreton Bay during late autumn and winter when the temperature can drop to 16° C (Taylor, 2008).

Branstetter (1990) suggested that for sharks the risk of being preyed on is related to L_T and as such, smaller species such as R. taylori, H. australiensis and C. cautus may be prone to predation throughout much of their life cycles. Occupying shallow areas of Moreton Bay would be advantageous to these smaller species, owing to the abundance of teleost prey (Johnson, 1999) and a potential reduction in the number of predators. The capture, however, of C. leucas in excess of 2000 mm L_T during the warmer months, could pose a threat to smaller sharks as elasmobranchs form a considerable component of the diet of this species (Cliff & Dudley, 1991). This demonstrates that for small sharks, shallow regions are not entirely free of larger predators. For species such as R. taylori, this predatory risk may be largely offset by rapid growth and highly fecund characteristics (Simpfendorfer, 1992).

The fact that most of the larger shark species were only caught as neonates or juveniles suggests that shallow sub-tropical waters do not provide suitable habitat appropriate for the continued occupancy of later ontogenetic stages. The inferred shift in spatial distribution is likely to reflect an ontogenetic shift in diet coupled with the increasing energetic requirements that accompany growth and the need to reproduce. Deeper parts of the bay and offshore waters are likely to contain a greater abundance and diversity of larger prey items that would be accessible to larger sharks.

The shark species assemblage significantly differed among seasons. For much of the warmer months, the assemblage was dominated by R. taylori and C. limbatus–C. tilstoni which is consistent with results from Cleveland Bay, Queensland (Simpfendorfer & Milward, 1993). Elevated catches of juvenile sharks are often reported in sub-tropical waters during the summer (White & Potter, 2004) and in this study juvenile C. amboinensis, C. cautus, C. limbatus–C. tilstoni and S. lewini comprised a larger proportion of the species assemblage during the warmer months.

The fact that in this study C. obscurus comprised a major proportion of the species assemblage during the winter demonstrates that spatial and temporal distribution patterns are not synchronous for all sharks. Even when the water temperature drops, shallow sub-tropical waters still provide habitat for shark species. This temporal partitioning may be advantageous to juvenile C. obscurus as it appears to result in low interspecific competition for food, combined with a lower predator-density environment.

Ecological information on sharks in many sub-tropical coastal waters is limited which can complicate wise resource use and conservation efforts. This study has provided data on the species composition in a region of known high diversity where sharks interact with commercial and recreational fisheries. In the east coast of Australia, recent legislative changes have led to greater resolution in the reported catch of sharks in Queensland (Roelofs, 2011) and observer programmes have provided ongoing data on sharks caught in the ECIFFF (Harry et al., 2011b) and on large sharks caught in the New South Wales Ocean Trap and Line Fishery (Macbeth et al., 2009). More information, however, is required on the distribution, abundance and life-history parameters of Carcharhiniformes on the eastern seaboard of Australia and elsewhere in other sub-tropical coastal waters.

We are very grateful to J. Page, the commercial fisherman who provided many of the samples. We are indebted to the many volunteers who assisted in the field, particularly S. Cutmore, S. Pierce, T. Scott-Holland, S. Pardo, J. Combs, A. Gutteridge, L. Marshall, S. Pardo and C. Rohner. The PI research was supported by the William Edwards Trust U.K., a UQ Confirmation scholarship and a Queensland Smart State Award. We thank the Moreton Bay Research Station, Seaworld Research and Rescue Foundation and the Tangalooma Wild Dolphin Resort for research support. Sampling was conducted under Queensland General Fisheries Permit PRM03951I and 55543. All procedures were approved by the UQ Animal Ethics Committee.