Just off the coast of Miami a man is driving a yacht towards the open sea. His T-shirt is emblazoned with a giant shark’s jaw and the words ‘Shark Hunter’ in the centre, yet he looks more like an ageing bodybuilder than a fisherman. Meet ‘Mark the Shark’. He’s taking film director and activist Rob Stewart along with him on one of his fishing trips, where excited customers pay to catch sharks. “How many sharks have you killed, do you think?” Stewart asks. “Oh, man,” ‘Mark the Shark’ replies. “Well, they say 100,000, but I don’t think that’s true. I’d say more like 50,000.”

When one of his customers catches a hammerhead, dragging its bloodied and limp body onto the deck to take photos with it, we witness one of the most disturbing scenes of the film. When the shark is pushed back into the sea we do not know whether it will survive, but by its violent thrashing we assume it is unlikely. With compelling patience, Stewart continues to question ‘Mark the Shark’ about why people like fishing for sharks. “Sharks are the apex predator, you know,” is the reply. “They always have been and they always will be… This is a creature put on Earth for what reason? For man to eat!”

Months after recording this scene, Stewart would tragically die in a diving accident while filming in Florida. Sharkwater Extinction was then finished by a dedicated team of producers, editors and loyal supporters, who continue to advocate for his cause. Stewart’s first film, Sharkwater, brought a huge amount of attention to the plight of sharks, gathering support from influential figures and giving him worldwide notoriety. At the present time more than 90 countries have banned shark finning or the trade of shark products, but there is still more work to be done. The shark population has dropped by more than 90% in the last 30 years, and 100–150 million sharks are killed every year.

Sharkwater Extinction carries on the legacy of Sharkwater but also pays tribute to Stewart’s life and to his career as an activist. Although it is obvious why the people who finished the film have chosen to combine these things, it is also the central reason why the film doesn’t quite work as a journey. The narrative often feels stitched together, and several scenes feel unfinished. As memoir it certainly gives us a glimpse of Stewart’s life and work, but the scenes where he is interacting in a nuanced way, like the scene with ‘Mark the Shark’, are too few and far between. The result is problematic in that different themes are vying for our attention. Stewart’s death is revealed dramatically towards the end, yet we know throughout the film that it is coming, and although we are sympathetic there are far too many selfie-type shots of him that disturb the main message of the film.

In a medium that is already saturated with cruelty and barbarism, the most powerful parts of this film are not of sharks being mutilated – although this is of course incredibly distressing – but those where Stewart interacts with people with the compassion and patience of someone who is willing to listen rather than shout down. When the film is not trying to shock, it shocks the most.

If the film doesn’t quite hold together as a piece of cinema, what it does do well is deliver stark messages. Shark meat is ending up not just on our plates, but also in pet food, fertiliser and even beauty products. What is even more concerning is that no one knows the result of effectively killing off the top predator in the sea, and there is still a long way to go in the campaign to get more countries to stop both the finning of sharks and the import of shark meat. Rob Stewart’s life shows what one dedicated person can do for the cause of a single species and leaves us asking what a greater mass of people could do collectively to fight the ongoing extinction of our planet.

Huw Wahl is a film-maker and artist. His latest project, The Republics, a film made in collaboration with the poet Stephen Watts, will be released in late 2019.