Stock structure of exploited shark species in north eastern Australia
David Welch (Fishing and Fisheries Research Centre, James Cook University)
Jess Morgan, Jenny Ovenden, Raewyn Street, Will Macbeth, Colin Simpfendorfer, Andrew Tobin, Alastair Harry, Ron Schroeder, Jimmy White
Supported by funding from the on behalf of the Australian Government ( 2007/035)
In a time of widespread concern for the sustainability of shark fisheries, there is an urgent need for stock structure information on selected shark species. This project determined the stock structure of four key target shark species in northern Australian net fisheries and defined appropriate management units for their sustainable management.
The four shark species for this study were milk shark (Rhizoprionodon acutus), scalloped hammerhead (Sphyrna lewini), Australian black tip shark (Carcharhinus tilstoni), and the common black tip shark (C. limbatus). The project sampling regions covered the Queensland east coast however during the project collaborations were established with New South Wales Fisheries scientists and the range of the study was extended into northern New South Wales giving a more comprehensive study of stock structure for the Australian eastern coast.
We used multiple techniques concurrently to determine the stock structure of each species. These techniques were: genetic analyses (mitochondrial DNA and microsatellites), vertebral microchemistry, and life history parameters.
Both scalloped hammerhead and milk sharks were found to consist of single genetic stocks along the north-eastern Australian coast, however vertebral microchemistry revealed that several separate stocks.
We found that diagnostic tools (mitochondrial DNA, vertebral counts, life history traits) used to distinguish between the two morphologically indistinct black tip shark species (C. limbatus and C. tilstoni) was ambiguous for some individuals. Further analyses revealed the remarkable discovery that C. tilstoni and C. limbatus are hybridising all along the eastern Australian coast with crosses occurring both ways. This meant that the ability to determine the stock structure for these two species was compromised and so stock structure using vertebral microchemistry, life history and population genetics was not pursued further in this project.
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