Although the name is French there is a Japanese slant to La Petite Poisonnerie. On the Insta page sakayana means fishmonger in Japanese, as you ask. It has a fish bar by the same name in Marylebone with a menu of pastas, salads and tapas. In the shop, down at the Camden market side of Primrose Hill, the iced fish slab takes centre stage, usually fine whole fish and shellfish, sushi grade on request. The shelves around have supportive items like sushi rice, seaweeds and a chill counter for dressed crab etc. Everything for fish in fact.
