Canadian Imperialist Salmon

Some damnfool cut the nets at a 'Quoddy Bay, New Brunswick salmon farm and released 20,000 (estimate) mature Atlantic salmon. Which are now swamping the gene-pool of the few, the proud, the harried wild Atlantic salmon in Downeast Maine. Which, in turn, are about the only wild Atlantic salmon in the whole USA.

Fish-farm salmon are designed to produce meat, not survive in the wild.

I wrote a short story about this, SF rather than fantasy, about ten-fifteen years back. Nobody bought it.