In the Gulf of California, when springtime love is in the air, the male Gulf corvina sings a sexy love song that has been likened to "a really loud machine gun". It sounds like this:

But these fish don't sing solo. Most of the year, they are spread throughout the Gulf, but in spring, the adults – all of them – gather together for a huge mating extravaganza in the Colorado River Delta. This "spawning aggregation" can include millions of fish spread across an area 27 kilometres (16 miles) wide. As you might expect, things get loud.

"The collective chorus sounds like a crowd cheering at a stadium or perhaps a really loud beehive," said Timothy Rowell of the Scripps Institution of Oceanography in a press release.

And he would know. In the spring of 2014, Rowell and colleague Brad Erisman of the University of Texas at Austin joined roughly 1.5 million corvinas in their mating frenzy – an event the researchers call a "wildlife spectacle" – in order to measure just how loud these feisty fish can get.

"[The mating calls] reverberate through the hulls of small, fibreglass fishing boats (pangas) and are audible to the naked ear," they describe in their paper. The sound is so intense that it can drown out the din of boat engines, making it an easy task for fishers to follow their ears collecting the assembled corvinas.

"These spawning events are among the loudest wildlife events found on planet Earth," Rowell said. (And you thought your college dorm neighbours were bad.)

Armed with specialised underwater recording gear – echosounders and hydrophones – the scientists recorded the fish, documenting for the first time just how cacophonous they get. They found that the fish create a noise of over 200 decibels, 21 times louder than the normal background noise of the delta. And this can go on for days.

Noise is no small matter in the ocean. Loud sounds – such as the motors of boats – can really interrupt the lives of sea creatures, so much so that marine conservation specialists have developed noise guidelines for human activity in the water. But the corvinas care not for these rules: just two hours of their chorus exceeds an entire day's recommended noise level.

The Gulf corvina lives only in the Gulf of California, and after a long history of overfishing, its populations are declining. Image: Brad Erisman

For nearby animals, this sound might be more than a disturbance: it has the potential to cause permanent damage. For marine mammals – the researchers saw lots of dolphins and California sea lions dining on the fish swarms – listening to noise at that volume can cause temporary hearing loss, and after a full day of exposure, permanent deafness. Hopefully the hungry mammals know not to stick around too long (and the spawning only lasts a few days), but who knows how long their ears ring afterwards!

It's the sort of once-a-year event that would seem right at home in an awe-inspiring episode of Blue Planet. But the researchers are concerned that it might not be around very long. Remember those pangas (fishing boats) that come to collect the fish? The researchers explain that "a single panga with one net can catch two tons of corvina within minutes, and the local fleet of 500 pangas harvests up to 5,900 tons (two million corvina!) in 20 days of fishing each year."

This unique species of fish lives only in the Gulf of California, and after a long history of overfishing, plus the fact that the Colorado River hasn't flowed consistently in over fifty years, there is evidence of the corvina populations declining, and the fish themselves decreasing in average size, in past decades.

The researchers suggest that management of fishing will be important to preserve the corvinas into the future. We can all agree that it would be a shame to lose the strange spectacle of the world's loudest fish orgies.  

For more on the spawning aggregations of the Gulf corvina – and its vulnerability to overfishing – check out this great video from science communications initiative Natural Numbers: