Almost 30 years ago, Fiona Darcy received an unusual gift.

Fishermen in Limmen Bight River, north of Borroloola in the Gulf of Carpentaria, had been hauling in a net when they noticed a strange creature had become entangled in it.

Sawfish are bottom-dwelling sharks recognisable for their distinct snouts, or rostrum, which are long and narrow appendages barbed with teeth.

The sawfish the fishermen hauled was already dead but they removed its rostrum which was kept as a keepsake.

"It was given to this fellow who a year later gave it to me. I prize it," Ms Darcy said.

The Darwin resident is one of many people to have souvenired rostrums over the years, and to some researchers they are a useful relic of a species in sharp decline.

Fiona Darcy was gifted the rostrum in 1989. ( ABC Radio Darwin: Jesse Thompson )

Going to the pub, for science

Annmarie Fearing is a masters student undertaking an international search for sawfish rostrum.

The hunt has taken her to museum collections, aquariums, zoos and the living rooms of families who have passed them down through generations.

She has just spent a month in the UK and found 500 of them, despite the fact sawfish are not native there.

"People have them hanging up in their homes," Ms Fearing said.

"Sometimes they're in pubs, so that's always fun to go look for."

Ms Fearing takes the large rostrum outside and gets to work. ( ABC Radio Darwin: Jesse Thompson )

Owners often regard them as trophies, but the premium placed on these snouts has also contributed to the species' decline.

According to Peter Kyne, a conservation biologist and Ms Fearing's collaborator at Charles Darwin University, sawfish rank among the most threatened marine species in the world.

"The large, long bill or snout, it's got these sharp teeth studded down the side of it, and that means it's easily entangled in nets — gill nets, trawl nets, that sort of thing," Dr Kyne said.

Captures are often incidental, but it's known that sawfish are sometimes harvested, a practice outlawed in Australia.

All five species are considered endangered or critically endangered, but the northern Australian population has endured.

The origin, species and tooth count of each fish is recorded. ( ABC Radio Darwin: Jesse Thompson )

"We still have four species left here and in decent numbers," Dr Kyne said.

"They've declined, but we've still got viable populations in northern Australia."

Ms Fearing believes this is one reason why so many local rostrum owners have contacted them.

A careful extraction of tissue

Christy Davies from Charles Darwin University sat with Ms Darcy in her trinket-filled living room as Ms Fearing got to work.

"This is definitely one of the biggest I've seen," Ms Fearing said, studiously noting of the number of teeth — 53.

"It's always exciting, because you never know what species they have or how big it is, where it's from."

Ms Fearing carefully carves a small piece of tissue off the rostrum. ( ABC Radio Darwin: Jesse Thompson )

In fact, the rostrum belonged to a pristis zijsron (green sawfish), which are found throughout the northern Indian Ocean and listed as vulnerable in the Northern Territory.

"And this is only a third of the actual animal, so if you imagine this three times, this one was probably close to seven metres," Ms Fearing said.

Dealing with treasured artefacts — occasionally those in museum collections — demands a non-invasive procedure, and Ms Fearing nimbly sets about the extraction.

She takes out a box cutter, some forceps and a collection of small plastic tubes.

The underside of a green sawfish rostrum, which is about 1.5 metres long and has 53 teeth. ( ABC Radio Darwin: Jesse Thompson )

"We cut basically a little bit of tissue away from the already-cut end of the saw, and we usually take it from the underside," she said.

"That way, even if I do a sample, no-one can tell. We don't go in there with a saw and chop stuff up."

Where do sawfish stand today?

A few minutes later, Ms Fearing had carefully carved off about 60 milligrams of off-white tissue, wafer thin and smaller than a five cent coin — by her reckoning the "cleanest bit of tissue you can find".

"That gives us, usually, around three DNA extractions," she said as she held up the tube containing the tissue.

Given the age of the objects, the DNA tends to be old. Care must be taken to find samples that haven't been exposed to the elements.

The extracted tissue is smaller than a five cent piece. ( ABC Radio Darwin: Jesse Thompson )

"It's not great DNA but we can still get it out, and it can still tell us something about helping populations now," she said.

The samples will return to a lab at Charles Darwin University until they are shipped to the University of Southern Mississippi.

"There's a fair bit of paperwork involved, but we'll get there," Ms Davies said.

Then, Ms Fearing and her colleagues will work with funding from the Shark Conservation Fund and Save Our Seas Foundation to translate the mantlepieces into important scientific data.

They are specifically interested in comparing current levels of genetic diversity with those of historic sawfish populations.

"If you have a lot of genetic diversity in a population, populations are typically healthier than when they don't have a lot of genetic diversity," she said.

"But the thing is, we don't know with historic populations of sawfish — we don't know anything about their genetics at all."

The samples will eventually travel to the US for analysis. ( ABC Radio Darwin: Jesse Thompson )

By looking at historic populations scattered across the globe, they are trying to get a sense of what the data-poor sawfish population looked like before human activity contributed to their decline.

"That can kind of help us understand where sawfish stand today," Ms Fearing said.