Years of nuclear bomb tests in the 20th century have allowed scientists to determine the ages of vulnerable lungfish populations, scientists say.
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Australian National University geochemist Stewart Fallon said scientists would tell the age of a fish by dating their Otolith, a solid stone which forms in the inner ear of fish.
But with the vulnerable Queensland lungfish they faced a few problems: lungfish stopped growing after a certain age, getting their Otolith meant killing a vulnerable species, and their Otolith didn't form the same way as in other species, making it near unreadable.
"We could say 'Sure, we figured out how old they were but we ended up killing them all'," Dr Fallon said.
Instead, Dr Fallon said his colleagues in Queensland would take a scale from a live fish and mail the scales to him in Canberra where he would measure the amount of a nuclear isotope in it.
He said in the 20th century, nuclear bomb tests doubled the amount of the radioactive isotope carbon-14 in the Earth's atmosphere.
But the amount of carbon-14 in the atmosphere has been slowly dropping since the Nuclear Test Ban in 1963.
Scientists can measure the amount of carbon-14 each year and compare it with the amount in the human body, which is constantly absorbing it.
Dr Fallon said a fish scale grows similarly to a tree, the central part is the older and the edges are the youngest. By measuring the amount of carbon 14 in the oldest part of the scale, they could tell what year the fish was born.
"Some of the oldest ones we found were closest to 80 years old," Dr Fallon said.
But his colleagues had found there were periods in the 1970s and 80s when fewer fish were born.
"I actually don't know how they've survived in Australia for so long," Dr Fallon said.
He said aging the population of lungfish allowed conservationists to tell whether they were only finding old lungfish, which meant they needed to work on boosting Queensland's younger lungfish population.
Lungfish populations have been affected by the construction of dams in Queenland rivers, made worse by floods which can wipe away the plants along riverbanks where lungfish lay their eggs.
Dr Fallon said fossils had shown lungfish, which are also found in South America and Africa, have remained relatively unchanged for 100 million years.
"They're very primitive fish and a lot of them can have lungs as well as gills so they can breath air. They're quite interesting," Dr Fallon said.
Dr Fallon worked with Griffith University and various Queensland state government departments.
