Which fish species are the oldest?

Lake Agassiz

This map from Saint John's University shows the boundaries of ancient Lake Agassiz, a pro-glacial lake that covered large portions of North America, including Red Lake,  soon after the ice age, 12,000 years ago.
This is recent history -- there were people here then, for instance.
There were probably many lakes that formed as the Ice Age came to an end. They would have created temporary inland seas, only to disappear when ice dams finally melted through at low points. Wherever there is clay soil, there once was one of these lakes. The clay is just the sediment at the bottom of such a lake.
As the SJ site points out, the fish found throughout this entire area tells you the kind of fish there were back then. They just got trapped into smaller lakes as the glaciers retreated northward and eventually disappeared.
The native larger fish in Lake Agassiz were: northern pike, lake trout, whitefish, goldeye and lake sturgeon.
But notice the land around Lake Superior that was not part of Lake Agassiz. This area is just higher. Fish in lakes in this area would have got there from an even larger and earlier post glacial lake. I know from previous research that fish native to these highlands of the Lake Superior watershed are: northern pike, whitefish and lake trout.
So they were actually here first. They are the oldest.
All other species in the Lake Superior watershed have either swam up the Great Lakes from the south or have been purposely introduced. Walleye is an example. There are walleye in Lake Superior but for any other lake that drains into it, there had to be a direct, navigable link for walleye to be there naturally.
In the Thunder Bay area, walleye introductions began very recently, starting in the 1940s. Mostly the Ministry of Natural Resources took adult walleye from Lake Superior and released them into a suitable lake. Within about 10 years there was a self-sustaining walleye population. In most cases, the walleye have all but wiped out the whitefish in these lakes.
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