The tench farm, in St-Alexandre, 60 kilometres southeast of Montreal, proved a disaster. The fish had low growth and survival rates — and marketing trials showed Quebecers didn’t want to eat them. In the early 1990s, the businessman drained the ponds and some of the tench escaped into nearby agricultural streams and into the Richelieu River.

The rest is aquatic-invasive-species history.

The tench took hold in the river and then spread through much of the St. Lawrence River, threatening local fish such as the copper redhorse and perch, and disrupting sports and commercial fisheries in Quebec. Now, the invaders are moving west toward an even juicier target: the Great Lakes.