Aspects of the reproductive biology of dusky, spinner and sandbar sharks (Family Carcharhinidae) from the Tasman Sea

Increased harvest pressure exerted on sharks worldwide has created a necessity for more information concerning the basic biology of targeted species. This study marks the first dedicated assessment of the reproductive biology of Carcharhinus obscurus, C. brevipinna and C. plumbeus in eastern Australian waters, where these species support a demersal longline fishery. We found all three to be late maturing species of low reproductive output, suggestive of an overarching susceptibility to stock depletion in the region. Length- (L₅₀, cm LT) and age-at-maturity (A₅₀, years), for females and males were respectively 281.1 and 15.5, and 271.9 and 14.6 for C. obscurus; 224.9 and 10.1, and 208.9 and 8.5 for C. brevipinna; and, 174.8 and 9.5, and 164.5 and 7.0 for C. plumbeus. Uterine fecundity (i.e. litter size) was observed at 5-12 (x = 9.6), 5-14 (x = 10.6) and 3-12 (x = 7.8) for the same three species respectively, and increased significantly with maternal length in C. brevipinna. Length-at-birth (L0, cm LT) ranged from 92 to 96 for C. obscurus, 79-82 for C. brevipinna and 66-76 for C. plumbeus, and all three species exhibited lengthy gestation periods, overall embryonic sex ratios of 1:1 and synchronous parturition in autumn. However, given limitations with respect to sample size and temporal distribution inherent in this study, the reproductive parameters defined herein are necessarily preliminary. Nevertheless, this research challenges a range of findings emanating from other parts of the world and, in doing so, raises pertinent questions relating to the resilience to targeted fishing activities of these species in New South Wales waters compared to other oceanic regions. Moreover, it reinforces the importance of locally derived demographic parameters for population modelling and stock assessment.13 page(s

Life history of the common blacktip shark, Carcharhinus limbatus, from central eastern Australia and comparative demography of a cryptic shark complex

Common and Australian blacktip sharks (Carcharhinus limbatus and Carcharhinus tilstoni) occur sympatrically in Australia, where they are reported as a complex because of their morphological similarities. This study provides the first description of the life history of C. limbatus using samples from central eastern Australia, where C. tilstoni is rare. Females (68-267 cm total length (TL); n = 183) and males (65-255 cm TL; n = 292) both matured at 8.3 years and 200 cm TL, which exceeds the maximum length of C. tilstoni. Vertebral ageing revealed that female and male C. limbatus lived to 22 and 24 years respectively, exceeding known longevity in C. tilstoni. The mean (±s.d.) intrinsic rate of population increase calculated using a Euler-Lotka demographic method was 0.11 ± 0.02 year for C. limbatus, compared with 0.17 ± 0.02 and 0.20 ± 0.03 year for two C. tilstoni stocks. Despite their similar appearance, these species differed in both their biological productivity and susceptibility to fishing activities. Monitoring of relative abundance should be a priority given they are likely to have divergent responses to fishing

Genetic structure and diversity of two highly vulnerable carcharhinids in Australian waters

Molecular techniques were employed to investigate genetic structure and diversity in dusky Carcharhinus obscurus and sandbar C. plumbeus sharks in the Indo-Australian region. Tissue samples of 423 C. obscurus and 442 C. plumbeus defined 18 and 11 mtDNA ND4 haplo-types, respectively. For C. obscurus, weak genetic differentiation was detected be tween eastern and western Australian waters ( pairwise Phi(ST) = 0.04437, p 100 at any particular location. Off Australia's temperate east coast, C. obscurus and C. plumbeus exhibited strong similarities in genetic structure, suggestive of similar evolutionary histories in the region. In addition, genetic validation revealed observers to be highly accurate in the identification of both target species in an eastern Australian shark fishery. Our findings contribute valuable information for the management and conservation of both species

Detection of interspecies hybridisation in Chondrichthyes: hybrids and hybrid offspring between Australian (Carcharhinus tilstoni) and common (C. limbatus) blacktip shark found in an Australian fishery

Abstract Interspecies hybridisation in nature is a wellstudied phenomenon, but it has not been analysed using genetic markers in the class Chondrichthyes (sharks, rays and chimeras). Two black-tip whaler shark species (Australian, Carcharhinus tilstoni; Common, C. limbatus) have overlapping distributions in Australia, distinct mitochondrial DNA sequence (ND4, COI, control region) and distinct morphological features such as length at sexual maturity, length at birth and number of vertebrae. A mismatch was observed between species identification using mtDNA sequence and species identification using morphological characters. To test whether hybridisation between the two species was responsible, a nuclear gene with species-specific mutations was sequenced. Extensive interspecies hybridisation was found to be occurring. Hybrids were found from five locations on the eastern Australian coastline, spanning 2,000 km. If hybrid fitness is low and hybrids are common, then fisheries recruitment may be overestimated and the productivity of the black-tip shark fishery may be well below that required to support commercial exploitation. To guard against identification errors, the likelihood of hybridisation and subsequent introgression should be assessed prior to using mtDNA (e.g. barcoding) to identify shark species. The C. limbatus-C. tilstoni species complex provides a unique opportunity to investigate the ability of sharks to adapt to environmental change, in particular, the impact of hybridization on species distributions which favour C. tilstoni along the north and C. limbatus along the south eastern Australian coastline

Length-frequency distribution of individuals from which tissues were sampled.

<p>Length-frequency distribution of individuals from which tissues were sampled.</p

Maximum likelihood phylogenetic tree for <i>Carcharhinus</i><i>brevipinna</i> haplotypes.

<p>Nodal bootstrap support is displayed where ≥ 70%. Scale represents the proportion of polymorphic sites between haplotypes.</p

Rarefaction exact curves for sample locations.

<p>Rarefaction exact curves for sample locations.</p

<i>Carcharhinus</i><i>brevipinna</i> mitochondrial DNA ND4 haplotype network.

<p>Sizes of circles correspond to the number of individuals displaying each haplotype. Shading indicates the proportion observed from each of the four putative populations. (−) = mutational steps/missing haplotypes.</p

Detection of interspecies hybridisation in Chondrichthyes: Hybrids and hybrid offspring between Australian (Carcharhinus tilstoni) and common (C. limbatus) blacktip shark found in an Australian fishery

Interspecies hybridisation in nature is a well-studied phenomenon, but it has not been analysed using genetic markers in the class Chondrichthyes (sharks, rays and chimeras). Two black-tip whaler shark species (Australian, Carcharhinus tilstoni; Common, C. limbatus) have overlapping distributions in Australia, distinct mitochondrial DNA sequence (ND4, COI, control region) and distinct morphological features such as length at sexual maturity, length at birth and number of vertebrae. A mismatch was observed between species identification using mtDNA sequence and species identification using morphological characters. To test whether hybridisation between the two species was responsible, a nuclear gene with species-specific mutations was sequenced. Extensive interspecies hybridisation was found to be occurring. Hybrids were found from five locations on the eastern Australian coastline, spanning 2,000 km. If hybrid fitness is low and hybrids are common, then fisheries recruitment may be overestimated and the productivity of the black-tip shark fishery may be well below that required to support commercial exploitation. To guard against identification errors, the likelihood of hybridisation and subsequent introgression should be assessed prior to using mtDNA (e.g. barcoding) to identify shark species. The C. limbatus-C. tilstoni species complex provides a unique opportunity to investigate the ability of sharks to adapt to environmental change, in particular, the impact of hybridization on species distributions which favour C. tilstoni along the north and C. limbatus along the south eastern Australian coastline