Final leg of Mediterranean cetacean survey: a window into whales’ noisy underwater world

Judith Matz (IFAW trainee, Marine Conservation programme) updates from Song of the Whale surveying off the Libyan coast. This is the third and final instalment of the ACCOBAMS survey initiative blog series, outlining IFAW supported research looking into cetacean presence and distribution within the region.

Mediterranean cetacean survey on ocean noise

We are on the tenth day of this research leg, navigating the Mediterranean from east to west in a zig-zag pattern, where we are surveying the waters off Libya for marine megafauna. So far, most of our encounters have been limited to shearwaters, migrating songbirds and lots of insects, but we have also spotted some striped dolphins - and have heard much more. That's because of the hydrophone trailing off the back of the boat allowing us to see and listen to sounds underwater through computer monitors and headphones.

We are a mixed team of sailors, scientists, students and campaigners, from as far afield as Ireland, Libya, Spain and Germany. At all times, one person is at the helm, one is stationed at the computer monitoring incoming data, and as long as weather permits, two people are stationed on the back deck to spot cetacean activity (but mostly reporting plastic debris sightings).

We have tracked striped dolphins and took photos of them leaping. Every now and then, their maraca-like chatter zooms in and out of our acoustic range, and yesterday, we clearly hear their enthusiastic whistles. A few days ago, we even tracked a sperm whale whose metronome-like clicks had been picked up as it was diving deep to hunt for squid.

grey herons in the Mediterranean sea

Mostly we hear the whispering of the water and the faint sound of our vessel making its way through it. When another vessel passes by in the distance and we pick up the hum of their engine or the "woosh woosh" of the cavitation of their propeller: air bubbles building and loudly bursting due to pressure differences around the propeller blades.

Whilst conducting research for IFAW over the summer, my attention was brought to the issue of underwater noise pollution in an investigation that was primarily focused on ship strikes and vessel speeds in the Strait of Gibraltar, the entrance to the Mediterranean from the Atlantic Ocean. Even though I'm an avid scuba diver and know about the underwater world, nothing could prepare me for the barrage of noise when I put on the headphones one night whilst four vessels were around us on all sides. So unbearable was the noise that I could barely make it through two minutes of listening, and felt incredibly relieved when I could finally take them off and just enjoy the (relative) silence of the world above the surface. When I reported my shock to the team, they commented that this was nothing in comparison to anthropogenic-induced noise off the coast of Portugal or in the Strait. While surveying more heavily trafficked areas, listening effort must sometimes be paused in order not to hurt the ears of the team member who is listening.

sunset during Mediterranean cetacean survey on ocean noise

While we have the luxury to just take off our headphones cetaceans, and in fact any sea creature, are permanently exposed to the underwater cacophony produced by humans. Areas like the Strait of Gibraltar are heavily populated by cetaceans due to the prevalent currents providing nutrient-rich waters. At the same time, around 110 000 voyages occur in the Strait every year. That equates to around 300 voyages per day or 12 per hour. I was already showing signs of distress after listening to four vessels for two minutes, imagine the impact of this permanent noise level animals in the Strait of Gibraltar are exposed to. This might explain why observed potential fin whale feeding grounds in the Sea of Alboran, just east of Gibraltar, are bereft of the deep-diving cetaceans, as they are too stressed by this human activity to feed.

It is time that anthropogenic noise is taken seriously by all and effectively regulated by reducing ship speeds and implementing more efficient hulls and propellers in vessels. This would keep noise to a minimum (and, whilst we're on the subject, reduce ship strikes at the same time).

-- JM

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