It’s been ages since I took limnology but I can’t help but answer a trout shaped bat signal. This is a very famous case in my field so I can weigh in here despite my general ignorance about the mighty Midwest. The Tanner experiment case is one of the most important examples we look at of what are called “large scale ecological experiments” of the same scale/impact/cost as something like Everglades restoration or Yellowstone wolf release, all similar experimental thinking underpins these efforts. The Great Lakes were originally a cold water ecosystem with the top predator being the lake trout. Lake trout grow slowly relative to other fish, and live in deeper water further from shore. They were at the top of a foodweb that had some remarkably productive commercial fisheries (for lake trout, whitefish, ciscoes (lake herring). Two stressors collapsed the lake trout dominated system: invasive sea lampreys in the early 1900s and overly intense commercial fishing, so few lake trout (large predators) remained. Alewives from the Atlantic coast reached the Great Lakes over time, with their population exploding. Alewives were the the new top-of-the-foodweb dominant fish by the 1960s, causing huge seasonal die offs that would wreck beaches, close swimming areas, and tank tourism. Alewives had the seasonal die offs because Great Lakes temperature swings just weren't right for a fish that would (on their native East coast) spend winters in the ocean and spring closer to shore or up river. Howard Tanner, who ran Michigan's fisheries program at the time (60s) was asked by the governor to so something flashy to fix the die offs and maybe bring back a sport fishery. He looked into restoring lake trout and realized because they grow slow (~20 years) this would take pretty long to do. He needed a predator with abundant eggs that he could buy from another state's hatcheries. He realized Pacific salmon, like coho and Chinook, would be perfect becayse they only took 2 years to get big, they'd aggressively feed on the alewives preventing the annual die offs/beach closures, and they're fun sport fish because they jump and put up a fight when they're hooked. He got the idea from his time spent working fisheries in Colorado and introducing coho and kokanee into reservoirs, so he knew they could be landlocked for life and still complete their life cycle (usually they'd need to spend some time in the ocean). At this time, Oregon developed a new hatchery tech called the Oregon Moist Pellet (a new type of high quality feed) that resulted in much higher survival of juvenile fish, so they were making more eggs than they needed. Tanner bought a million coho egg from Oregon and releasted them into tributaries of Lake Michigan. He was looking to create what's called a "trophic cascade" in ecology, or a chain reaction where predators reduce prey, then the prey's food increases, which impacts the next level down. So they needed a predator to eat alewife so the alewives would eat less zooplankton so that the zooplankton could live to graze down algae (and prevent algal blooms). Fewer alewives also allowed the "assemblage" or groups of fish further down the food chain to come back since they weren't eating all their larva. This is considered one of the first major large scale experimental trophic cascade experiments implemented by an American fish and wildlife agency in a freshwater system that worked. His experiment led to a world class sport fishery, major reductions to beach closures/die offs, better controls of algal blooms, and the original ideas that we might want to manage fish for recreation and ecosystems simultaneously. The system today is what we call heavily managed (so there was never a return to the native lake trout at the top of the foodweb). The state agency stocks Chinook, coho, steelhead, and lake trout. They use really neat predator-prey models to determine stocking decisions and keep alewives at preferred levels. This case completely transformed what Americans think of when we think of the capacity of a state's hatcheries. Working across state lines for eggs, stocking millions of fish annually, determine assemblages based on what anglers like, and using hatchery raised fish for ecological management were all tried here at a scale that hadn't really been done before, and it worked. One of the prestige fisheries scholarships in America is named after Tanner at Michigan State, and he has a research vessel or two named after him which is a huge honor.