If you’re over the right patch of water in the northern Gulf of California in Mexico at the right time of year, you might hear a low rumbling noise coming from the sea below. If you had a fishing net to drop over the side of your boat in the direction of the rumbling, you’d be able to fill it in one go with Gulf corvina, silvery croaker-like fish that congregate together in a noisy, rollicking spawning orgy.

“Most of the sound reflects off the surface of the water and remains underwater, but if there’s enough noise it can escape the water,” said Timothy Rowell, a graduate student at Scripps Institution of Oceanography in California, who has been studying corvina. “The rumbling noise can make you have to raise your voice, and the hull of the boat can act as a sort of speaker, amplifying the noise.”

The International Union for Conservation of Nature lists gulf corvina as vulnerable and threatened by overfishing, but few reliable population data for the fish are available. Rowell has been working with a team of researchers to find a way to nail down those population figures.

Before coming to the Gulf of California, Rowell had been studying the behavior and abundance of grouper in the Caribbean by tracking and measuring the sounds they make. It occurred to the researcher that similar acoustic techniques might be especially useful in studying the loud mating call of the Gulf corvina.

That insight has now led to acoustic methods that allow the researchers to estimate the number of Gulf corvina in a spawning aggregation just by listening. The findings, published in the journal Nature, highlight recent advances in the use of acoustics to study fisheries. They also raise the potential of using similar techniques to study other fragile, data-deficient fisheries, including one of the Gulf corvina’s neighbors, the politically controversial and critically endangered totoaba, whose bladder is a highly sought-after and highly paid-for delicacy in China.

In turn, illegal nets set for totoaba often ensnare the even more critically endangered vaquita porpoise, which is now on the brink of extinction, with just a few of these marine mammals surviving in the Gulf of California.

There are two broad ways in which researchers can use sound to find out what’s happening in the oceans: active, which involves sending out sound waves and studying how they bounce back, and passive, which involves listening. As scientists learn more about how noisy certain species of fish are and how to measure that noise, the latter is becoming increasingly useful.

Kevin Boswell, who runs the Marine Ecology and Acoustics Lab at Florida International University, said that, with respect to fish, sound is often “associated with a behavioral process, like reproduction.” Rowell’s study, in which Boswell wasn’t involved, “demonstrates that you can attribute the sound production levels to biomass.”

For their study, Rowell and the team first used an echosounder, a device that transmits sonar to detect underwater objects, to measure density and aspects of the spawning corvina. They then used underwater microphones to measure the fishes’ sounds, with the hypothesis that the louder the sound, the greater the density of the fish.

They were right.

“Quieter areas had less fish and louder areas had a lot of fish,” said Rowell. That wasn’t too surprising, he said, but the fact that the relationship between sound levels and density was so linear was.

“There’s an equation that could be used,” he said. “So, in the future, you could go out and take a sound estimate and then punch it into an equation … You could have an estimate of population size and biomass by dinnertime.”

The researchers are already conducting training workshops to help local regulators adopt the techniques. “We’re on the same side as the fishermen,” Rowell said. “We’re trying to get good information so they can keep fishing. If it turned out there were no fish a few years down the road, that would be devastating.”

Using the technique to improve the management of other hard-to-quantify species could be trickier, but the potential is there.

Totoaba are closely related to corvina, but they – their swim bladders, specifically – are a lot more valuable. Chinese demand for totoaba swim bladders has skyrocketed in recent years, spurring fishers in the Gulf of California, the only place where totoaba live, to fish for them illegally. The gill nets they use indiscriminately catch whatever swims by, including vaquita porpoise, thought to be the rarest marine mammal on Earth.

Totoaba are now listed as critically endangered, but, as with corvina, reliable population data aren’t available. It’s not clear whether conservation measures are helping them or the extent of the damage caused by illegal fishing.

Rowell said it’s safe to say researchers could get a good estimate of totoaba populations using their acoustic methods. “We could go out and do this work next year, but it’s really politically charged right now,” he noted, and that work would require government support and local buy-in.

There’s no timeline for any potential expansion of the research to the totoaba.

But elsewhere, acoustics are already being used in innovative ways to study fish movements and abundance.

Boswell, for instance, has been placing active acoustic tools at various depths to understand at very fine scales the timing and range of fish that migrate vertically through the water column. Others are putting acoustic technology on seafaring drones and sending it deeper into the ocean.

But Boswell said one of the biggest advances might be wideband technology, which offers much higher-resolution data because it utilizes a larger bandwidth and sends out a wider spectrum of frequencies, thus yielding a larger amount of potential information. That means researchers can “not only better separate fish that are closely spaced together but also learn about how they scatter sound across a spectrum of acoustic frequencies,” he said.

That means as scientists learn how different organisms “scatter” sound, they can find patterns and eventually determine whether certain species have “acoustic fingerprints.” That technology itself isn’t new, but now it’s cheaper, better and more affordable, opening the door to more new applications and a bigger pool of researchers.

Rowell says more research on using acoustics to quantify fish populations is being conducted off the coast of Texas, and some work is being conducted in the use of sound within the cod, haddock and pollock fisheries – all hugely valuable commercial species.

“There’s definitely a huge frontier here,” he said.