In the 2006 eco-doc Sharkwater, Canadian activist film-maker Rob Stewart gave us a heartfelt plea to save the planet’s sharks. He was on a mission to reduce overfishing and rehabilitate the creatures’ reputation as stone-cold killers – if only we could love sharks as much as we love cuddly pandas we’d do more to protect them. Back then, you couldn’t help feeling that Stewart wanted us to love him too, with all the shots of himself in tiny Speedos. Watching the sequel, I experienced a sharp stab of self-reproach. Stewart died in a diving accident while shooting this film – he was 37. Sharkwater: Extinction has been scrappily put together from footage he’d already shot.

And there are some striking images here. Since the first film, many countries have banned “finning” ­– the practice of hacking off the fins then tossing the shark’s body back into the sea. But it still happens. In Costa Rica, Stewart uses a drone to film a warehouse packed with them. Shark fin soup is a delicacy in China, which drives the illegal market. And it’s not just finning that’s the problem. In California, he captures upsetting footage of a graceful thresher shark tangled up in a mile-long net intended for swordfish.

The movie would have worked better if the film-makers had been upfront about Stewart’s death – we only find out about it 65 minutes in. But there are poignant clips of him on the voiceover; he talks about the danger of his work with the scary magical thinking of a war photographer. His parents worry, but he’s convinced that all his past near-misses have made him indestructible. He also speaks honestly about struggling with distrust and dislike of people – those of us sitting on our hands while the planet tips into catastrophe. It’s powerful stuff, in places.