IV.

Fishing and Exotic Species

Although in the past the lakes have supported important commercial fisheries, the fish populations of the lakes changed dramatically in the 20th century and today there is little commercial fishing. Changes were wrought first by overfishing and then by the introduction of exotic species. Most notable of the latter was the parasitic sea lamprey, which probably entered the lakes via the Erie Canal and spread following the completion of the Welland Ship Canal in 1932. The sea lamprey virtually eliminated lake trout from Lakes Huron and Michigan. Canadian and American government programmes, instituted in the 1970s, have reduced the number of lampreys.

The decline in the lake trout population allowed another invader, the alewife, to flourish, unconstrained by any natural predators. Alewives entered the lakes through the St. Lawrence Seaway and the Welland Ship Canal. Alewife populations have been brought under control by the coho salmon, imported into the lakes in the 1970s, which has become the dominant predator and an important game fish.

In 1986, a small mollusc known as a zebra mussel was introduced from Europe, probably carried by a ship. The zebra mussel population grew rapidly. The mussels have coated pilings and clogged water intakes at power stations. Zebra mussels filter the water, consuming algae and potentially displacing other algae-feeding organisms. In removing algae from the water the mussels make the water much clearer. However, they also make the water more acidic and increase the risk of exposure for humans and wildlife to PCBs and other pollutants. As the mussels filter the lake water, they absorb the relatively low levels of toxic substances already in the water, which can then be passed along the food chain.