This story is not a red herring but rather about medieval herrings and the trade in them in the Baltic region. Research suggests that rather than this developing round about 1200 with the emergence of what was to become the Hanseatic League it was actually in existence by about 800. This is based on an analysis of fish bones from Viking related sites in Poland and from the fact that different species of herring can be identified from their bones and DNA. The evidence suggests that herring from the more saline western part of the Baltic were being traded eastwards into areas where that particular species did not flourish. As a result trading networks were set up and endured, linking communities and fostering economic activity.
The online article about this pattern is set out in an article from phys.org which can be seen at Ancient DNA pushes herring trade back to the Viking age