THE ISOLATION OF LANDLOCKED SALMON IN MAINE AND QUEBEC NEAR THE END OF THE LAST ICE AGE

Landlocked salmon were first found in four Maine lakes; Sebego, Sebec, Green, and West Grand. Others are found in drainage systems along the St. Lawrence River in Quebec. Louis Agassiz was the first to identify landlocked salmon, Salmo salar Sebego, as a variety of the Atlantic salmon. Now hatchery-raised landlocked salmon are stocked in all Maine watersheds and in other parts of the United States and Canada.

While casting for Atlantic salmon on the Merrimachi River in New Brunswick, we wondered about the origin of landlocked salmon. A tome on salmon in the fishing lodge claimed that landlocked salmon were originally spawning Atlantic salmon that were cut off by an advancing lobe of ice. The geography of these lakes would require this glacial lobe to travel west or east across the whole width of the state, a distance of more than 400 km. Obviously, this lobe would have had to move across the state after the last retreat of the Wisconsin ice sheet, about 13,500 years B.P. Such a scenario made no sense to us, based on our knowledge of the glacial geology of Maine.

These four lakes in Maine are close to the limit of the glacial-marine submergence, known here as the DeGeer Sea, between about 13,500 and 12,500 B.P. It is here proposed that the Atlantic salmon were spawning in tributaries above the marine submergence, when relative sea level was lowered by crustal rebound, isolating the salmon. Once rebound began, sea level fell to 50m below the present elevation around 10,000 B.P. While the Atlantic salmon could likely leave, some barrier prevented their return. The most dramatic barrier to the return of Atlantic salmon to interbreed with the trapped salmon occurs in the Montmorency River watershed, a few kilometers northeast of Quebec City. Montmorency Falls plunges 47 meters (higher than Niagra) into the St. Lawrence River. Charles Lyell in 1845 noted the glacial-marine sediments above the falls, indicating the falls were submerged by the Champlain Sea, a Canadian analogue to Maine's DeGeer Sea. While the glacial-marine sediments in both Maine and Quebec are richly fossiliferous, we know of no examples of salmon fossils in either place.