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 (LT) 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 LT at which 50% of males and females reached maturity (LT50) 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.
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. LT 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.