to be true until last week I was not aware another marten specie exist in size between the american pin marten and the volverine in north america. its very embarrassing.
in europe we have two marten - the house or white breasted marten and the european pine marten or yellow breasted marten. the house marten live near humans the pine marten avoid him. so the two are more like two subspecies in different environments and don't come in competition with each other. I think the fischer and the american pine marten have allso different niches in the eco system. the winter fur of the fischer must have been a good price back in the days when you can made a living being a trapper.

this interesting video was coming by the allgorythm - the fox and the fischer
https://www.youtube.com/watch?v=bwDeneavCjY