Advanced genetic and statistical techniques have been combined to develop a reliable way of monitoring rare but wide-ranging species such as sharks and sawfish and has the potential for extension to other species including dugongs and turtles. The method has evolved from a simpler version developed by CSIRO in 2012 that now underlies the management of Southern Bluefin Tuna.
Image: Speartooth Shark pup from the Adelaide River, NT. Image: Charlotte Klempin, Charles Darwin University
It is based on ‘mark-recapture’: the principle that if some animals in a given area are caught and marked, the proportion of animals in a later, second round of captures that are caught twice can be used to estimate the total population.
The new monitoring technique extends the mark-recapture principle to use natural genetic ‘marks’ to identify animals that are close-kin pairs (full or half-siblings, or parent and offspring). These paired relationships are identified from tissue samples taken from live or dead animals. Importantly, because the mark is a natural piece of the inherited DNA, no first round of captures to place artificial marks is required.
The ability to find half-siblings (that share the genetic mark from only one parent) has only become possible in the past five years thanks to improved genetic technology, and it means that juvenile samples can be used to study the adult population. This is a breakthrough for many sharks and sawfish species for which adults cannot be caught in large numbers. Data from other sources such as acoustic tagging provide additional information on factors including age-specific mortality rates and movement patterns. Where feasible, these data can supplement the close-kin population estimate to pinpoint when and where population-limiting mortality occurs.
Key challenges tackled during this project have been to improve the reliability of the genetic techniques and to devise a statistical and demographic framework for close-kin mark-recapture that incorporates information from additional sources. The good news is that the genetic techniques appear to be working. Dozens of full and half-siblings have been identified among hundreds of Speartooth Shark and White Shark samples, and for these two species sufficient data exist to make preliminary abundance estimates.
Besides adult abundance, other parameters that can be estimated from close-kin mark-recapture include adult survival rates, breeding frequency, female reproductive parameters and stock structure. These parameters are important for evidence-based management, and have been impossible to estimate for rare but wide-ranging shark and sawfish species.
Ongoing challenges are to optimise the identification of close-kin pairs, and to scope the potential of emerging genetic tools. It also remains to develop abundance and survival rate estimates for two shark species, and examples of how the outputs can provide evidence for the recovery of threatened and endangered species.
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