The 1 thing BBC legend David Attenborough says can save the planet that must happen now

Don MacNeish, a conservationist who has been diving for years off the Isle of Aran in Scotland, was truly shocked. For the first time, he was seeing the devastating effect on the once-pristine seabed of the little-known, but immensely destructive fishing practice of bottom trawling.

The technique, whereby a trawler drags a massive metal bar across the ocean floor and forces anything it disturbs into a net behind, leaves annihilation in its wake. The trails of carnage can be seen from space.

Bottom trawling is the fishing method you never knew about that is wreaking havoc in our oceans. “The first time I dived over an area that a dredger had just been over, it was heartbreaking,” sighs Don. “All sorts of animals were just smashed to pieces. It was like swimming over the Garden of Eden during a nuclear winter.”

“They were just taking the future out of the sea, and the island community would be just left with wreckage. It is difficult to try and explain to people just exactly how abundant it once was here and just how much has been lost.”

Fortunately, Don now has the most powerful ally on the planet in Sir David Attenborough after featuring in his potent new documentary, Ocean, which features the first ever footage of bottom trawling and airs tonight ahead of tomorrow’s UN Ocean Conference in Nice. The documentary, which the 99-year-old conservationist has called one of the most important films of his career, was shot in Turkey with the help of a cooperative government, and in Plymouth, as part of a highly controlled scientific experiment run by the Marine Biological Association.

It is genuinely upsetting to watch the dredger shatter every living creature in its path and, in so doing, churn up a toxic cloud of sediment. Keith Scholey, the co-director and executive producer of the film, says: “I remember getting back the first rushes of bottom trawling and feeling sick. “I started diving in the 70s, when there was pristine sea moss off the British coast. But the level of destruction now is really terrible. What we're left with is rubble.”

Referring to the apocalyptic demolition of the ocean floor, David says: “From the surface, you would have no idea this is happening. It remained hidden from view until now. But bottom trawling smashes its way across the seabed destroying nearly everything in its path, often in the hunt for just a single species.”

The appalling truth is that over three-quarters of a bottom trawler’s catch is frequently thrown away. As David says: “It’s hard to imagine a more wasteful way to catch fish.”

The sheer scale of bottom trawling takes your breath away. An area equivalent to the entire Amazon rainforest is ploughed every year. A lot of the seabed is repeatedly trawled, throwing up sediment which releases enormous amounts of CO2 into the atmosphere. This in turn plays a huge and very unwelcome role in the warming of our planet.

This exceedingly harmful fishing practice happens thousands of times every day right around the world. “Very few places are safe from this,” says David. “Some $20 billion is spent every year supporting overfishing on an industrial scale. Vast factories now travel the seas. They work day and night, further and faster than ever. It seems nowhere is off limits, even the open ocean.”

What is especially alarming is that all our lives depend on the health of our seas.

Unfortunately, bottom trawling is not the only menace to our waters – lines of baited hooks 50 miles long are killing millions of sharks every year. We have now wiped out two-thirds of all large predatory fish. Sharks and turtles lived through the extinction of the dinosaurs, but may not survive this.

Another aspect placing our oceans in mortal danger is severe over-fishing. More than 400,000 industrialised vessels now hunt every nook and cranny of our seas.

They even scour the waters of the Antarctic, where they are voraciously hoovering up krill vital for the survival of blue whales, penguins and other species. “With so few fish left in the ocean, we are now seeking other prey in the furthest corners of the world,” says David. Gigantic fishing boats, some of the largest industrial units on the sea, “Suck hundreds of tons of krill into vast nets. It’s then boiled and processed for fish farms, health foods and most recently pet food.

“How can wildlife compete with this? Some claim this is sustainable, but we may now be removing the foundations of an entire ecosystem.”

These practices also have the very serious knock-on effect of leaving many seabird colonies on the brink of collapse, jeopardising the entire food chain.

And yet, all hope is not lost. New jungles of giant kelp, the tallest living elements in the ocean, are being discovered all the time. They now border a quarter of the world’s coastlines. The game-changer is that they absorb far more carbon than rainforests on land.

Even more positively, recent developments demonstrate that the ocean can recover much more quickly than scientists ever believed possible.

This is borne out by the fact that it has already happened in many different places across the globe. For instance, in the Channel Islands off the coast of California, an area where marine animals were hunted intensively for over 200 years, a decision was made to halt fishing in a 300-square-mile reserve.

The result is that in this No Take Zone, nature’s balance has been restored. “In just five years the forests were once again thriving and with them a bustling neighbourhood,” says David.

Just as importantly, saved from hunting, fish had time to grow much larger and spawn youngsters that could swim away and settle outside the No Take Zone.

“If protecting a small portion of the sea from fishing has such a large effect, imagine the potential of doing it across much larger areas,” reflects David. “Whenever we have given the ocean time and space, it has recovered faster and on a greater scale than we dared to imagine possible. It has the power to go even further to defend itself against the greatest threats of our time.”

At tomorrow’s UN Ocean Conference, delegates from every country will be urged to create Marine Protection Areas that cover 30% of their waters by 2030. In the UN’s Decade of the Ocean, that would give our seas a colossal and much-needed boost. At present, less than 3% of them are protected.

It is only fitting that Sir David has the final word. “It’s my great hope that we all come to see the ocean not as a dark and distant place with no relevance to our land, but as the lifeblood of our home. I’m sure that nothing is more important,” he says.

“For if we save the sea, we save our world.”

Ocean with David Attenborough airs at 8pm tonight as part of World Oceans Day, on National Geographic. It will stream on Disney+