"It is like meeting an old neighbor," he recalls, speaking of the touching moment that is indescribably wonderful when he returns after a gap of a few days or even a couple months to see the fish he previously photographed still there.

Having documented the underwater world for the past decade, Zhang has seen how climate change and human behavior have influenced the ocean.

"Corals are among the first indicators of climate change. With the rise in temperature, corals have undergone a process called 'bleaching', during which they lose their color and are dying. It is heartbreaking."

Besides global warming, he adds that overfishing and plastic pollution are also challenges faced by the ocean ecosystem. "Our planet is changing and the beautiful things that I love are dying. I hope that my photos and videos can act as a wake-up call to protect our oceans," Zhang says.