字幕表 動画を再生する 英語字幕をプリント This is me. I'm about to go scuba diving for the first time ever. I went in expecting muffled peace and quiet. I mean, one of the earliest documentaries on underwater life was literally called “The Silent World”. “They roamed deep under the Mediterranean, the Red Sea, and the Indian Ocean, in a mysterious realm, a silent world.” But as soon as I got down a few yards, I couldn't help but notice sound all around me. It was hard to tell what direction it was coming from, or how far away it was. It was coming from boats above me. As far as I can tell, the Earth's water is not as quiet as I thought. I've been binging this podcast all about sound, and they've been looking into this too "From DeFacto Sound, you're listening to Twenty Thousand Hertz: The stories behind the world's most recognizable and interesting sounds. I'm Dallas Taylor." So Dallas — life underwater can be pretty loud, right? It can! Just listen to these toadfish— they oscillate their swim bladders to make sounds that are loud enough to keep houseboat residents in Sausalito, California up at night. Or these Barred Grunt fish, that make a grinding sound with the teeth in their gullet. Or these snapping shrimp — they produce a sound by creating tiny popping bubbles with their claws. It's been measured at 200 decibels. That's louder than a gunshot. Light doesn't penetrate very deep in water, so a lot of ocean life has evolved to use sound as its primary sense. Because water particles are more densely packed together than air, sound travels farther and faster in water. That makes it an efficient medium of communication at any depth or time of day. “Let's say we had 440, like the middle key of your piano, or whatever.” That's John Hildebrand, he's a professor at the Scripps Institution of Oceanography. He's an expert in the field of underwater sound and how it's used by and how it affects marine mammals. “If you looked at the wavelength of that sound in air, that note, the 440 note, would be a little bit less than a meter long. In water, it's more like several meters long.” What that means is that noise travels about four times faster and farther in water than it does in air. In an experiment in 1991, sound emitted from Heard Island was picked up at 16 sites in the Atlantic, Pacific, and Indian Oceans. It turns out that one of the most common noises in the entire ocean that can be picked up almost anywhere these days, sounds like this: It's the sound of boats. Noise from ship traffic has doubled every decade since the 1960s. “Basically, anywhere you go, the ambient noise in the ocean is dominated by anthropogenic sound.” “Let's say I made a measurement 30 years ago and now if I went to the same place and made the same measurement, it would be 10 dBs or more higher sound level than what I measured when I started my career. That's spooky.” That's a huge problem for animals that use sound as their primary sense. Just listen to this audio of how noise from a passing boat totally drowns out dolphin communication. But arguably the worst culprit of underwater sound pollution is a process that sounds like this: This is seismic surveying. It's a process that allows companies to locate spots on the ocean floor where they can drill for fossil fuels. Boats with about 30-40 airguns that all go off at once will move back and forth over large parts of the ocean. Bubbles from the horns expand and contract typically every 10 seconds, creating a huge amount of acoustic energy that maps geological structures deep in the ocean floor. And it's about as loud as a jet at takeoff. This can go on for weeks at a time. A study of seismic survey noise between 1999 and 2009 found that airgun sounds were recorded almost 2500 miles away from the survey ship itself. At some locations, they were recorded on 80 percent of days for over a year. And that changes how animals behave. A study by the Institute of Marine Science at the University of North Carolina found that reef fish abundance decreased by 78 percent during seismic surveying. This is what the reef looked like before seismic surveying in the area. And this is what it looked like after. “Seismic surveying is a constroversial topic at the coast and new research is only adding to the conversation. “One of those impacted is local fisherman Jack Cox. He's seen firsthand the impact seismic surveying has on fish.” “It does something, that — we just don't catch fish. For animals like whales — who rely on complex sound communication systems to socialize, find food, and mate — that poses a huge problem. If you watch this heat map from a study of endangered North Atlantic right whales, you can see that shipping paths near Boston squeeze the whales into a tiny space where communication is possible — it's about a third of what it was without the ships. Susan Parks, a biology professor at Syracuse, recorded right whale sounds throughout the early 2000s. And when she compared her audio to some that had been recorded in 1956, she noticed that the older sounds were much deeper than her high-pitched recordings. It turns out that the whales had started calling at higher and higher frequencies in order to hear each other over the hum of ship noise. Apart from habitat displacement and communication changes, there's also evidence that boat noise simply stresses these whales out. After the September 11th attacks, researchers in Canada's Bay of Fundy compared underwater noise levels during a period of reduced ship traffic to the stress-related hormone levels in the right whale population. They discovered a 6 decibel noise decrease in the bay after 9/11, which correlated to lower baseline levels of stress related hormones. Effects like this go all the way down the food chain. A 2017 study found that there were two to three times more dead zooplankton after after a day of blasts from a single airgun. And the larvae of krill, which whales rely on for food, were totally destroyed. In April 2017, Donald Trump signed an executive order to open offshore drilling in the Atlantic. It directed the Interior Department to consider allowing seismic exploration by 5 companies that had been blocked under the Obama administration. It's getting widespread pushback in Congress — there's a bipartisan House bill and Democrat-led Senate bill to ban seismic testing — but if those authorizations go through, companies could be conducting seismic testing in the Atlantic by Fall 2017. So… is there any good news? Yes, there is. In 2014, the International Maritime Organization adopted guidelines for reducing commercial ship noise levels with things like noise-muffling propellers and insulated engines — though they're not mandatory yet. A year later, the US Navy agreed to limit sonar testing — which has been linked to whales stranding themselves on beaches in habitats near Southern California and Hawaii — following a lawsuit from environmental groups. And in June 2016, the National Oceanic and Atmospheric Administration laid out an Ocean Noise Strategy Roadmap. It's the start of a 10-year plan to assess the human impact on underwater sound and the measures required to reduce it. Seismic surveys can work better, too. Norway has started multi-client surveys, so that seismic data from a particular area is only collected once. US authorities are considering a similar process. The ocean is huge. We've explored less than 5 percent of it so far. So it's easy to forget that what we do on the surface affects everything down below. But like plastic, chemicals, and waste, noise pollutes our oceans. Understanding that is key to doing our part to protect it. Thank you so much for watching, we loved doing this collaboration with Twenty Thousand Hertz. They create super highly produced podcasts that tell stories all about sound. If you enjoyed this video, you should absolutely go check out their podcast on the exact same topic. You can find that and subscribe at applepodcasts.com/20k.