The film unfolds like a mystery, with Mr Clover as detective. The film-makers travel to Nova Scotia, the Bahamas, Tokyo and the Mediterranean, as well as to a small village in west Africa whose supply of fish has been halved in recent years as a result of over-fishing. Ignorance, corruption and greed all play a part.

As Robert Redford, founder of the Sundance film festival, says: “Just explaining the problem isn’t enough. You have to show what people can do about the problem.”