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Deep sea research with microphone

‘Even at the deepest point in the ocean you can still hear the noise from boats,' says biologist Hans Slabbekoorn. ‘And that's while sound is the most important means of communication for underwater life.' What is the effect of all that underwater noise on fish and other animals? Slabbekoorn is on board the Pelagia, a research vessel, looking for answers.

‘We'll never see that again!' Researcher Hans Slabbekoorn watches with some concern as the underwater microphone (also known as a hydrophone) worth around ten thousand euros disappears in the deep water of the ocean. But this is no accident: Slabbekoorn is conducting research on sounds under water. He is on the research vessel Pelagia in the middle of the Atlantic Ocean, making recordings. As far as sound is concerned, there are a lot of changes going on in the ocean, he comments. 'Even at the deepest point of the ocean you can still hear boats.' 

Fish use sound to communicate

Even in the most tropical locations with clear water it's impossible - even for marine animals - to see further than ten metres. That means underwater life mainly relies on sound for communication. There are over 800 known species of fish that make sounds. 'And that's just a small part of all the fish species studied; the actual number will be much higher,' Slabbekoorn comments. Fish generally produce sound using the muscle around their swim bladder, but they can also use their jaws, and they can express air from their intestines and oral cavities. Rasping noises, squeaks, rumblings, creaks and buzzing: fish can make all these kinds of sounds.

Hans Slabbekoorn fixing the hydrophone so it can rest at a depth of two kilometres below the surface.
Hans Slabbekoorn fixing the hydrophone so it can rest at a depth of two kilometres below the surface.

‘Fish would be amazed if they knew we also use sound above water,' Slabbekoorn laughs. Water is the ideal medium for communicating with sound because sound travels so effectively through water. Some whales communicate with one another at distances over more than a thousand kilometres. But other sounds, like boats, also stretch over a long distance. 'And that can disrupt natural processes,' Slabbekoorn explains. 'Boats mainly produce low-frequency sounds and you can hear these sounds during a storm. Given the growth in marine shipping, there are fewer and fewer quiet days underwater. And the whales then? Nowadays they can only be heard up to about ten kilometres away. We don't know what effect this has on social cohesion or on the choice of partner.’

Under-water noise

Then it's time for some recordings. Attached to a metal frame weighing 450 kilos, the advanced microphone sinks within 15 minutes to a depth of 2,300 metres. It will stay there for 24 hours recording what is known as a ‘soundscape’. That's a complete picture of all the sounds in the environment: those produced by animals and plants (biotic), by the sea bed and the water (abiotic) and by humans (anthropogenic). ‘It's hard to imagine that the microphone is over two kilometres deep,' Slabbekoorn repeats, shaking his head in disbelief. 'I could never have tried something like this alone.' The team on the Pelagia know exactly what they are doing. A release mechanism makes it possible to free the microphone and buoys raise it to the surface. And there's even a radio beacon and a lamp attached so that the microphone can be located at night and picked up. So, how is it released? With a simple remote control so that the microphone is already waiting on the surface when the research vessel arrives.  

The cage with the camera and hydrophone are raised above the water at night.
The cage with the camera and hydrophone are raised above the water at night.

Hot springs deep under the sea

The research is part of the NICO expedition ‘Changing Oceans’ (see box). Other researchers on the expedition will be filming near to hydrothermal vents: a kind of underwater hot spring. The vents burst out of the ground and expel hot and mineral-rich water into the ocean. This makes the environment around the source rich in marine life: they are colourful oases of unusual animal life in the deep-sea desert. As well as suddenly exploding into life, a vent can die out again equally suddenly. Once the source of energy has disappeared, the fauna around the vent  have to look for a new source. Slabbekoorn suspects that the animals use noise to locate a new place to go. 'These vents can be heard from some distance away.' When a camera was lowered into the sea to investigate the vents, a hydrophone for Slabbekoorn also went with it.

To date, Dutch researchers have never managed to film active vents. So, when the first images appeared on board the Pelagia of a plume of an exploding hot-water spring, everyone was euphoric. But when the camera passed through the plume, everything suddenly stopped and the camera image went black. 'How hot are these plumes?' someone asked. 'Around 300 degrees,' came the answer. For a minute, everyone thought the equipment had melted.  'Are we insured for this?' someone asked hesitantly, but just after that the image was restored. A short while later, the camera and microphone were back on board the boat and everyone breathed a sigh of relief. The unique recordings were a success. 

First steps in the research

After studying sound in air for ten years, Slabbekoorn  started to focus on sound under water. 'One of my first experiments was funny and painful at the same time. While I was still doing my PhD research, we hung an expensive directional microphone packed in a condom in an aquarium containing courting cichlids - African  freshwater fish,' he recounts, smiling broadly. 'Then we used the condom for a second time - something you should never do - and certainly not in combination with expensive recording equipment. The condom slowly filled with water and the microphone was destroyed - before our very eyes.' It was cold comfort, but Slabbekoorn's assumption was later proved right: the fish did communicate using sounds specific to the species.    

Interest in underwater sounds is growing. And that's just as well, because the impact of sound on flora and fauna is becoming    increasingly clear. ‘Take the zebra fish,' Slabbekoorn begins to explain. 'These fish use vision to hunt, but when we fed them water fleas while at the same time making a noise in the aquarium, the fish tried to bite into the fleas, but often missed their target, and once they had caught the fleas they often released them - more often than would happen in a quiet aquarium.' The impact of noise  can go much further than only masking relevant sounds or influencing the tasks for which the fish need sound.'

Research vessel Pelargia moored off the coast of  the Azores. In the background the Pico volcano.
Research vessel Pelargia moored off the coast of the Azores. In the background the Pico volcano.

Noise impacts ecosystems

Slabbekoorn also gives an example from above the water. Jays spread seeds, he explains, and are easily frightened off by noise. Mice eat seeds, but they don't hear low noises very well, so they're not easily disturbed. Sound has an impact on the presence of jays and mice, and consequently on the spread and destruction of seeds. You can tell from the plants in an area whether or not there is a lot of noise in that area. Altogether, 'acoustic pollution' can change a whole ecosystem of animals and plants.  

That's precisely what Slabbekoorn wants to explore in his deep-sea studies using soundscapes. The recordings teach us more about our disturbance of unspoilt marine environments, even those more than two kilometres deep. The sound of boats can be heard everywhere, and the research vessel itself can also be heard clearly on the recordings. But animal sounds have been recorded, too. The whole crew sits around the laptop and listens tensely. A clicking sound comes from the speakers. 'It's a kind of echo chamber,' says Slabbekoorn. 'It sounds like a whale - but still different. What it was is still unknown. The recordings and the anthropogenic disturbance will be investigated further once ashore. The sound recordings with the camera images are sometimes easier to interpret. After viewing the images, a sudden thump the researchers had heard turns out to be a shark swimming close to the hydrophone!

NICO expedition

The NICO expedition (Netherlands Initiative for Changing Oceans) is a collaborative initiative led by the Royal Netherlands Institute for Marine Research (NIOZ) and funded by NWO. Around a hundred researchers from twenty different organisations are on the Pelargia research vessel for one or several stages of the trip. The expedition to the Azores, where Leiden biologist Hans Slabbekoorn carried out his sound research, was the latest stage in the expedition. More information on the NICO expedition.

Text and photos: Auke-Florian Hiemstra/Leiden University
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