As many as 40,000 African elephants are being slaughtered every year for their ivory.

And since there are only an estimated 415,000 African elephants remaining in the wild — down from possibly 5 million at the turn of the 20th century — poaching is a key threat to the future of the species.

However, efforts to stamp out poaching by prosecuting poachers have so far proven largely unsuccessful.

Prosecutions are usually based on seizures of a single, relatively small shipment and sentences are often accordingly light, says Professor Samuel Wasser from the University of Washington.

Professor Wasser (left) sampling ivory seized in Malaysia in 2014. ( Supplied: Malaysia Department of National Parks )

Professor Wasser used genetic analysis to link multiple ivory shipments back to the same three export operations, shipping out of Togo, Kenya, and Uganda.

This could allow prosecutors to tie numerous seized shipments of ivory back to the same operators, and to better focus their policing at these export points, he says.

"If we are able to link many different seizures to the same cartel … it provides a trail where law enforcement can go in and get the [shipping receipt] and follow the money," he said.

"It opens up the opportunity of increasing the severity of the sentence; now they're doing multiple crimes and so you have the opportunity to try them for major transnational crime."

His research is published today in Science Advances.

Links between seized shipments discovered by 'a bit of luck'

Professor Wasser and team looked at tusks from seized ivory shipments from ports in Dubai, Sri Lanka, Malaysia, Hong Kong, and several that were intercepted before leaving Africa, between 2006 and 2015.

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They discovered "by a bit of luck" that around half of the tusks in each shipment were unpaired; meaning there was only one, rather than two tusks from the same elephant present.

When they genetically analysed the tusks, the researchers found that even though matching tusks weren't always shipped together, they were always shipped from the same port in Africa, and all but one tusk was exported less than 10 months from its matching pair.

Based on earlier genetic work, they also found that ivory shipped from each port had a roughly consistent source — the ivory was being sourced from elephants living in the same location.

They deduced from this that cartels have their network of poachers working in the field, and that tusks are being separated as they work their way from those poachers up the supply chain, but eventually end up in the hands of the same cartels.

Tusks from the same elephant were often shipped in separate consignments of ivory. ( Supplied: Centre for Conservation Biology/University of Washington )

Linking a cartel to one shipment that has a matching tusk in another seized shipment, provides strong evidence that one cartel is responsible for both.

Using this method, they were able to create daisy chains of links between multiple ivory seizures, according to Professor Wasser.

"In almost every case, the ivory is poached out of a different country from where it is shipped out of," he said.

"We found what we believe to be three of the largest ivory cartels moving ivory out of Africa, and we also found that there are links between [the cartels]."

Poachers 'pay off' rangers for information on patrols

Trying to police shipments once they're in transit can be complicated, according to Dr Matthew Holden from the University of Queensland, who wasn't involved in the study.

"There's a lot of middlemen in the illegal wildlife trade," he said.

"International shipments often go through different countries on different routes, which one might guess is in order to try to avoid detection."

Elephant poaching is believed to be increasing. ( Supplied: Karl Ammann )

Dr Holden uses mathematical modelling to assess the most effective measures for tackling wildlife poaching, including poaching of elephants for ivory.

While it is important to look at tackling demand for ivory products, research that improves the ability of authorities to stop exports before they are loaded onto ships could be highly valuable, according to Dr Holden.

This research builds on earlier work from Professor Wasser and his colleagues, where they compared seized ivory genotypes against a database of wild elephant genotypes from across Africa, to establish where each elephant had been poached from.

They found that almost all of the ivory was coming from four regions in Africa, and that savannah elephant tusks were coming almost exclusively from Tanzania and Mozambique.

While they were hopeful that identifying the regions where elephants were being killed for their ivory would allow authorities to crack down on the poachers, that didn't have the desired effect, according to Professor Wasser.

"The poachers in those areas know the area well and they only have as much ivory as they can carry," he said.

"When you catch them, there's 10 others waiting to take their place and they pay off rangers so they know where the patrols are."

Rangers are hired to try to stop poachers on the ground. ( Supplied: Art Wolfe )

While policing the poachers is very difficult, and finding shipments once they've left port is also complicated, Professor Wasser hopes that closing in on the cartels that are amassing the ivory in Africa could be the sweet spot.

"We've tried to develop methods to attack the trade before the ivory gets into transit … where it becomes so difficult and expensive to trace," he said.

"It allows law enforcement to understand the importance of focussing on these big guys."