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Using environmental DNA in the search for critically endangered sawfish

Traces of fish DNA can remain in water for days, making detection a useful tool for biologists
Using environmental DNA in the search for critically endangered sawfish hero image

Sawfish used to be so common in the Persian Gulf that fishers used their tooth-spiked snouts to build fences. They were abundant, too, off the coast of Costa Rica, where they swam from the ocean into murky estuaries. And they mingled with mighty bluefin tuna off the coast of Mauritania.

Today, there are just a few strongholds in the world for the sawfish—Florida and northern Australia among them—and a few more places where they are hanging on, such as off the coasts of Sudan, Brazil, the Bahamas, and Papua New Guinea.

The largetooth sawfish is listed as critically endangered worldwide. They can grow up to 25 feet long and weigh in excess of 1,100 pounds. The fish’s saw, technically known as a rostrum, can be one-third of the total length of the fish and have as many as two dozen equally spaced teeth. The fish use them to find and stun prey.

Sawfish are the most endangered of all marine fish. They grow slowly and their giant blades can easily become entangled in fishing gear. They have legal protection in more than a dozen countries and their international trade is banned. And in a desperate quest to find them, researchers are scanning waters looking for traces of their DNA.

That DNA hunt will look for environmental DNA (eDNA), or “genetic material obtained directly from environmental samples (soil, sediment, water, etc.) without any obvious signs of biological source material,” notes an early article on the tool in Biological Conservation, which said the practice is an efficient and easy-to-standardize approach to sampling for traces of hard-to-find species.

The practice is being put to use in Costa Rica, notes National Geographic. The concept behind eDNA states that as animals move through their environment, they shed genetic material in the form of DNA—it could be feces, urine, mucus, or material byproducts of decomposition. Trapped in ice or soil, eDNA can persist for eons, but in water it lasts for a matter of days, meaning eDNA hunting for fish like the sawfish can provide highly useful information about the recent presence of the animals.

eDNA samples can offer remarkable glimpses into aquatic life otherwise difficult if not impossible to measure, although studies have shown that a number of variables can affect results. One recent study of eDNA from a lake in Mexico suggested the lake harbored species not previously known to live there. A 2018 study of eDNA sampling in a river in France found that eDNA could be detected as much as 100 km downstream from its deposition area. A 2016 study described the use of eDNA to find two rare species of sturgeon in Alabama.

In Oregon, the practice is being refined using synthetic DNA to see how the particles spread in water, reports NPR.

“DNA will travel differently depending on the habitat and the species,” noted NPR. “Where and how much DNA there is can be affected by the speed and direction of the water, the amount of sunlight, the number of DNA-eating bacteria and the season it is. And this is only a partial list. There are multiple variables that still need to be tested. Scientists are only really beginning to scratch the surface. Even so, the technology is beginning to prove its worth for bull trout monitoring in the West and for keeping track of invasive Asian carp, which pose a huge threat to the Great Lakes region.”

In Costa Rica, saving sawfish will help not just those fish but coastal communities nearby and in other developing regions, asserted National Geographic. Problems faced by those communities include food security and “blue carbon”—an effort to combat climate change by removing carbon dioxide from the atmosphere using ocean ecosystems such as salt marshes, mangroves, and seagrass.

“If you can solve the sawfish problem, you’ve solved the blue carbon and food security problem,” Nick Dulvy, a marine ecologist at Simon Fraser University in Vancouver, told National Geographic. “Because if you've solved sawfish, you’ve figured out how to make tropical fisheries appropriately selective and sustainable.”

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