The photo from the Billy Stevenson Collection shows Harry 'pistol' Small & Mr William S Stevenson with a large sturgeon (archive reference 657).
The section in the catalogue about the fish trade contains items about the early fish sales on the beach and the hawkers and fishwives who sold the fish. There are papers about the development of the fish market from the first trawl quay built at the foot of the North pier to the later fish markets. There is data about the firms associated with the fish trade including Stevenson & Sons, Harvey & Sons, Suttons, BJ Ridge and others.
PDF link: Fish Trade Archive