Boulogne is France's leading fishing port by volume — more fish passes through here than any other port in the country, with the catch unloaded, processed, and dispatched across France and Europe within hours. The fish market on the quayside opens early (around 6am on weekdays) and the spectacle of the auction, the stacking crates, and the trucks queuing at the wholesale hall is genuinely impressive even if you are not buying.
For visitors, the retail fish market on Place Dalton in the lower town is the practical option: stalls selling the day's catch, oysters, smoked herring, dressed crab, and ready-to-eat seafood at prices significantly lower than Paris. The herring is particularly associated with Boulogne — the city has smoked and salted herring since the Middle Ages, and the local variety (hareng fumé de Boulogne) has EU protected status.