Hartlepool fisherman Stan Rennie has been called the next Mr Bates. For nearly 50 years Rennie has fished the same stretch of northeast coastline – then in October 2021, thousands of crabs, starfish and lobsters washed up dead and dying on the beaches. “It was like something out of a horror film,” says one local. Rennie and other fishermen suspected industrial pollution, but an investigation by the Department for Environment, Food and Rural Affairs concluded that the most likely cause of the die-off was “algal bloom”. For Rennie and the other fisherman, it didn’t add up. So, they crowdfunded their own independent scientific study.

This documentary from Jeanie Finlay (Your Fat Friend) tells the story, and it’s maddening. The fishermen are close to bankruptcy, they’ve got mortgages and kids, catches are down 80%. The cost of diesel is higher than the price of a haul, and most put their boats up for sale. “Any jobs in Lidl?” reads one placard as they protest through town. What the fishermen suspect is that toxic sediment dredged from the River Tees and dumped in the sea is to blame. Their investigation finds evidence of a link to the chemical pyridine, a byproduct of steelmaking.

The reason for the dredging is the development of the then Conservative government’s flagship freeport (a zone with tax and customs breaks to attract investment) in the Tees Valley. The freeport was seen as key to the Tories’ “levelling-up” agenda after Brexit – and there’s footage of Boris Johnson at the site in his favourite dressing-up costume, a hi-vis vest. The gaslighting of fishermen and their concerns by public officials is enraging. Tees Valley mayor Ben Houchen brushes them off as conspiracy theorists, crackpots and – a word he delivers with a sneer – activists. But, like Alan Bates, Rennie refuses to give up. “If an activist is someone who is trying to stop the ecosystem being killed, I’m an activist.”

• All Rivers Spill Their Stories to the Sea is in UK and Irish cinemas from 23 October