The herring that are targeted south of the border in U.S. waters are separate stocks, and there's very little exchange of individuals between those populations. Among U.S. stocks and among U.S. stocks and Canadian stocks, there's limited exchange of individuals. There is one area, coastal New Brunswick, that does see a fair bit of movement of individuals across the border. In fact, that fishery targets—it's a very small fishery, a weir fishery—mainly migrant U.S. juveniles, but other than that, the stocks are managed separately, and there's very little exchange across the international boundary.
Evidence of meeting #5 for Fisheries and Oceans in the 43rd Parliament, 1st session. (The original version is on Parliament’s site, as are the minutes.) The winning word was information.
A recording is available from Parliament.