It was back in the 1920s that the Great Lakes fish population took a drastic drop. Overfishing, deforestation (which changed the river habitats), introduction of invasive species like the lamprey and alewife, and even pollution contributed to this calamity. Some fish species -- like the blue pike -- were wiped out. Others -- especially the Lake Sturgeon which must live 20 years before it can reproduce -- were almost eradicated.
By the 1960s, the alwife had become the main fish species in the lakes because there were few predator fish to eat them. It was in the 60s that large scale hatcheries were finally established to 'grow up' large numbers of fish (mostly trout and salmon) to 'seed' back into the lakes.
I visited the Wolf Lake Hatchery just west of Kalamazoo to see their operation. The hatchery gathers eggs and sperm from salmon and steelhead migrating early in the spring and grows them up for almost a year (steelhead are 7-8 inches long by then). Then, they 'plant' them back in the rivers and streams which feed into the Great Lakes.
It's quite an operation. The Great Lakes ecosystem is managed and monitored, and the DNR plays an important role in this. Hopefully, the Great Lakes Restoration Initiative can bring the lakes back to a healthier state than they are in currently.