Marine photographer Brian Skerry, 54, started his career with the National Geographic in 1998, documenting the beauty of the oceans, immortalising in his frames the dance of soft coral, the lazy strokes of manatees and the sparkle of gleaming fish.
But in the years since then, he has seen the underwater world become less and less pretty.
It is under attack, and he has witnessed first-hand the impact that threats such as climate change and overfishing are having on the world's oceans.
But the marine conservationist is fighting back, using his camera as a weapon.
"Nature photography has always been about showing the beauty of nature, which is important.
"But the problem with that is, if all we ever see are the pretty pictures, we never know that there is a problem," said the American photojournalist.
So he turned his attention to the "problem pictures", which he hopes give people a better idea of the threats the oceans face.
PICTURES OF REALITY
Nature photography has always been about showing the beauty of nature, which is important... But the problem with that is, if all we ever see are the pretty pictures, we never know that there is a problem. It's important to make both the beautiful pictures and the problem pictures, so people have an idea of what is real, and maybe how we can fix it.
MARINE PHOTOGRAPHER BRIAN SKERRY, on using his camera as a weapon against climate change and overfishing
One of his most iconic photographs, which depicts unsustainable fishing practices, shows a dead thresher shark caught in a gill net, its fins spread sideways in what looks like a crucifixion.
In 2002, he photographed the hunting of harp seals in Canada, where they are clubbed to death for their fur, and documented the loss of sea ice there.
"It's important to make both the beautiful pictures and the problem pictures, so people have an idea of what is real," said Mr Skerry, who is married with two daughters and lives in the United States.
He spoke to The Straits Times last month, when he was in Singapore to give a talk on marine conservation at the Esplanade Concert Hall.
He has clocked more than 10,000 hours - more than a year - underwater in his three-decade career, starting from when he was a shipwreck photographer.
Although the work is challenging, the marine conservationist does it as he believes photographs can spread the conservation message.
Human beings are visual creatures who react emotionally to photographs, he says.
"Since the invention of photography, human beings have marked every important event, whether good or bad, with photographs," he added, citing examples such as the moon landing, the Tiananmen Square protests and the Kennedy assassination.
His work is gruelling, taking him from playing peekaboo with harp seals in icy waters one day to swimming in shark-infested waters the next. But his efforts have paid off.
He has seen some of his work translate directly into conservation action. After a story on the North Atlantic right whale, a species endangered due to whaling, was published in 2008, the US passed laws to protect the whales.
The measures included slowing ship traffic in areas where mothers and calves live, so that the whales would not be affected by passing ships.
While it is not always possible to influence policy so directly, he believes people are made more aware of the situation through the stories and photographs in the National Geographic.
He said: "What we do as photographers and journalists is try to shine a light on problems and hopefully people will change their behaviour because of that."
He noted that many people do not realise the harm that overfishing is causing to the sea.
He said: "What's happening with seafood is that it is the biggest harvest of wildlife on the planet. And in many cases, it is being done in indiscriminate and unsustainable ways."
He pointed to the practice of dragging trawl nets through the water, trapping and killing anything that is caught in it, as an example.
"But nobody sees that as it is happening below the ocean waves, out of the view of most people."
The end result, he added, is that 90 per cent of the big fish in the ocean - including tuna and sharks - are gone.
He is selective about the type of seafood he eats as he does not want to create demand for marine species which are under threat, such as shark and tuna.
Ms Kathy Xu, 33, the founder and director of Singapore-based eco- tourism company The Dorsal Effect and a volunteer at marine conservation group Shark Savers, said she found his work inspiring.
She said: "I saw his photograph of the thresher shark caught in a gill net some years back, and was very moved by it. It reminded me of why I do the work I do with The Dorsal Effect and Shark Savers, even when it can feel like a huge uphill task sometimes."