Each time I’ve seen shark fin soup on a restaurant menu, it sounded conceptually sketchy. “The fin is considered a delicacy,” I recall being told, and there was the rub: Only the fin? What about the rest of the shark?

Well, turns out it’s as bad as I thought. “Sharkwater Extinction,” the third feature film directed by the scuba diver and activist Rob Stewart, reveals a cruel market in which tens of millions of sharks are caught every year, their fins cut off and their bodies thrown back in the water to die.

In his narration, Stewart recounts how he thought that if his films could make people love these animals, he could push popular opinion against their being hunted. He doesn’t quite pull this off here, despite impressive footage of him swimming with sharks. He does, however, convince us that these superpredators are important to oceanic ecosystems and that because they are so indiscriminate in their eating habits, they are full of toxins. Even if there were a way to fish for them humanely (there isn’t), Stewart argues sharks ought not be eaten at all.