Welch, D.J., Ovenden, J., Simpfendorfer, C., Tobin, A., Morgan, J.A.T., Street, R., White, J., Harry, A., Schroeder, R. and Macbeth, W.G., 2011. Stock structure of exploited shark species in north eastern Australia. Report to the Fisheries Research & Development Corporation, Project 2007/035. Fishing & Fisheries Research Centre Technical Report No. 12. James Cook University, Townsville, Australia. 130pp.  [ISBN 978-0-9808178-5-0]

Summary

The stock structures of a number of shark species commercially caught in north-eastern Australian coastal waters: scalloped hammerhead (Sphyrna lewini), milkshark (Rhizoprionodon acutus), Common blacktip shark (Carcharhinus limbatus) and Australian blacktip shark (Carcharhinus tilsoni) were investigated via a range of different analytical techniques. Biological data (e.g., length and reproductive maturity) and samples (e.g., flesh and vertebrae) were collected from a range of commercial fisheries operating in north-eastern Australian waters between Cape York in Queensland and Laurieton, NSW, including the large-shark setline fishery of northern NSW. Although milkshark is not found in NSW waters, the other three species comprise relatively minor proportions of catches of large sharks in the NSW fishery. Nevertheless, knowledge about the stock structure of those species is important to ensuring exploitation is done sustainably.

The analytical techniques used included genetic analyses (mitochondrial DNA and microsatellites), vertebral microchemistry, and analyses of life history parameters (age, growth and reproductive characteristics). This holistic approach to stock identification gave the advantage of increasing the likelihood of detecting different stocks where they existed and providing greater certainty. Genetics can inform about the evolutionary patterns as well as rates of mixing of sharks from adjacent areas, while vertebral microchemistry is directly influenced by the environment during growth and will inform about the patterns of movement during the sharks lifetime. Life history characteristics are influenced by both genetic and environmental factors.

Genetic analyses found that both scalloped hammerhead and milk shark consist of single genetic stocks along the north eastern Australian coast, while in contrast, vertebral microchemistry indicated that several separate stocks exist. Commercial catches of both species comprised largely of juveniles and adult males.

During the project it became apparent that diagnostic tools used to distinguish between the two very similar-looking blacktip shark species with certainty (mitochondrial DNA, vertebral counts, life history traits) produced conflicting and ambiguous species identifications in the cases of some individual sharks. This uncertainty in species identification meant that the ability to determine the stock structure for the two species using vertebral microchemistry and life history characteristics was not pursued further in this project. However, further-advanced genetic analyses revealed the remarkable discovery that C. limbatus and C. tilstoni are successfully mating with each other across species (‘hybridising’) all along the north eastern Australian coast. Further, 1st and 2nd generation hybrids were detected, indicating that at least some of the hybrids (genetically half C. limbatus and half C. tilstoni) are able to themselves successfully reproduce. Genetic stock structure analyses were carried out for the pure-bred subset of each blacktip species, with both species showing a distinct northern and southern stock with a boundary corresponding to the southern limit of the Great Barrier Reef.

The implications of these results for management are that, where pragmatic, management agencies should adopt spatial management of these species according to the spatial scales and boundaries identified in this study. Future monitoring and assessment of the study species within their respective fisheries should also be conducted at a regional scale. It is specifically recommended that there be further investigation into: life history traits for scalloped hammerhead and milk shark; the geographical extent of hybridisation in blacktip shark species; and blacktip hybrid fitness with respect to reproduction and ecological resilience in light of future population viability.