FRESHWATER FISHES
E. J. Crossman and E. Holm
GLACIAL REFUGIA AND POSTGLACIAL REINVASION
For several thousand years prior to the last stages of the Wisconsinan Glaciation the Mixedwood Plains Ecozone was under approximately one kilometre of ice, and no freshwater habitats existed. Fishes which had inhabited aquatic environments in this area prior to that glacial period were either killed with the advance of the Wisconsinan ice sheet, or they moved as it advanced to one or more unglaciated areas referred to as glacial refugia. For the fish species living today in this Ecozone the most important were the Mississippian and the Atlantic Coastal refugia (Fig. FF-1).
As the ice sheet receded, portions of what were eventually to become the lower Great Lakes and their tributary waters began to appear. This process began approximately 14,500 years ago with what are now lakes Michigan and Erie. Meltwater from the receding glacier filled various depressions and ran off, over a long period of time, by a changing series of drainages. At different times the developing lakes were connected directly to the Mississippian Refugium, or indirectly via a huge glacial Lake Agassiz, of which the Manitoba Great Lakes are remnants. At other times the outfall of the developing lakes was south through what is now the Hudson River, and later through the opened St. Lawrence River. All of the many outfalls of the developing lakes and streams provided access to different groups of what are now the indigenous and native species of fishes of the waters of the Ecozone.
A secondary influence was the incursion of marine water, in what is known as the Champlain Sea. This extended inland to approximately the present location of the confluence of the Ottawa and St. Lawrence rivers. This provided access to inland areas to representatives of marine or anadromous groups of fishes. With the gradual freshening of what remained of the Champlain Sea some of these species adapted to completing the whole of their life history in freshwater. See Mandrak and Crossman (1992b) and Legendre and Legendre (1984) for detailed summaries of postglacial dispersal into the Ecozone. The first aquatic form to be extirpated from the Ecozone, the unique, non-anadromous population of Atlantic salmon of Lake Ontario, is an example of fishes which made their way into the Mixedwood Plains Ecozone by this process.