Twelve ‘step-pups’ have been identified via 181 fin samples snipped from juvenile speartooth sharks (Glyphis glyphis) in the Northern Territory’s Adelaide River and Alligator Rivers region.  Identifying the half-sibling pairs will help NERP Marine Hub scientists estimate the size of the shark population, and whether it is rising or falling. They shed light on the parents’ survival, because for each half-sibling pair, at least one parent must have survived in order for its offspring to be present in the river. Secondly, the proportion of half-sibling pairs found among the sample group directly relates to the number of breeding adults in the population. Thirdly, the pairings may answer the question of how frequently the females reproduce: if no sibling pairs are found in consecutive years it probably means that females can’t reproduce every year.

The pairs were identified using nuclear genetic markers whose commonality among individual sharks indicates how closely they are related. Charles Darwin University’s Dr Pierre Feutry has been working to identify the genetic markers for speartooth sharks, as well as for the largetooth sawfish (Pristis pristis), in collaboration with CSIRO and Northern Territory Fisheries. Both species are considered threatened and protected under the EPBC Act, and they are subject to precautionary management because not enough is known about them to assess their populations. Modifications to freshwater river flows, and ongoing bycatch in fisheries are potential factors dampening their recovery.

Pierre says the half-sibling approach evolved from techniques developed by CSIRO to measure Southern Bluefin Tuna populations. But that technique required fewer genetic markers because it focused on more closely related, parent-offspring pairs. 'As we don’t know where adult speartooth shark live, we cannot get parent-offspring pairs, and instead further developed the approach to work with half-sibling pairs that can be found entirely in the juvenile population,' Pierre says. 'However, the more distantly related individuals are, the more difficult it is to find the pair with confidence. We have had to develop a rough draft of these sharks’ nuclear genome in order to identify sufficient genetic markers; a difficult and time-consuming task.’

In parallel research, Pierre and his Marine Hub colleagues have been sequencing the mitochondrial genomes of 93 speartooth sharks and 92 largetooth sawfish. ‘The mitogenome is inherited through the mother so it will tell us whether the half-sibling pairs share the same mother or father,’ Pierre says. The mitogenome sequences will also offer clues to population structure because they show differences between different river systems, suggesting the females probably don’t move much from one river to another. ‘It is important to know whether the parents are moving from one river to another, and whether males have the same patterns of dispersal as females. ‘In many shark species, the females come back to the same nursery area to breed, whereas the males move around more and that’s how genes are mixed. ‘In terms of management, it makes a big difference whether males or females or both come back to the same spot.’

If sharks freely move and reproductively mix between regions, then genetic differences will be slight, and sharks in these areas should be assessed, monitored and managed as one well-mixed population. In contrast, if mixing is restricted between regions, the populations will be genetically different, and need separate assessment, monitoring and management. The population assessments also require estimates of juvenile survival. These are being provided by an acoustic tagging program that monitors the survival of juvenile sharks over time. If the sharks are not recorded, and have not been detected leaving the river, they probably have not survived.

The preliminary analysis of 181 speartooth shark samples found some half siblings but probably not enough to have accurate estimates of population size. To advance the study, collection of speartooth shark tissue samples will continue in northern Australian rivers until the end of 2014.

Counting white sharks

Similar genetic tools are being applied to estimate population size, reproductive frequency and survival in white sharks, together with acoustic tagging to estimate survival, and aerial surveys of nursery areas.

White shark population estimates are needed to help assess the effectiveness of Australia’s national white shark recovery plan, the impact of fishing, and policies such as shark control programs.

Further reading

• Feutry P, Grewe PM, Kyne PM, Chen X. Complete mitogenomic sequence of the Critically Endangered Northern River Shark Glyphis garricki (Carcharhiniformes: Carcharhinidae). Mitochondrial DNA. 2014
• Feutry P, Pillans RD, Kyne PM, Chen X. Complete mitogenome of the Graceful Shark Carcharhinus amblyrhynchoides (Carcharhiniformes: Carcharhinidae). Mitochondrial DNA. 2014 
• Chen X, Liu M, Grewe PM, Kyne PM, Feutry P. Complete mitochondrial genome of the Critically Endangered speartooth shark Glyphis glyphis (Carcharhiniformes: Carcharhinidae). Mitochondrial DNA. 2013

Images

The speartooth shark is a protected species throughout Australia. Genetic markers are being used to identify half-sibling pairs of the shark and estimate its population size in the Northern Territory’s Adelaide River and Alligator Rivers region.

The speartooth shark is restricted to a small number of river/estuarine systems of northern Australia: the Daly River, Adelaide River and Alligator Rivers region in the Northern Territory, and the Wenlock River, Ducie River and Port Musgrave in Queensland.

Contact

Pierre Feutry, Charles Darwin University