News reports from the time period say that one billion pounds of dead fish were pulled from the lake that summer, with plenty of others washing up on the beaches, drawing flies and creating a nauseous smell. The North Side of Chicago had no access to clean drinking water because the fish clogged pumps in the lake, and the whole city was reportedly ordered to boil their water, as it was polluted with decaying fish. In the end, the piles of dead fish had to be hauled away to two dumps in Indiana that were used exclusively to dispose of the decaying alewives. Today, there is even a mural in Hyde Park, titled “Alewives and Mercury Fish,” that commemorates the strange catastrophe.

When the alewives began returning to their original numbers in 1968, states around the Great Lakes decided to launch what continues to be one of the largest bioengineering experiments in the world. Biologists introduced Chinook salmon from the Pacific Ocean into the Great Lakes as a natural predator. The initiative was a quick success, and by 1971, the alewife population had decreased significantly.

However, in the early 2000s, researchers discovered that the experiment was running wildly out of control. Two harsh winters reduced Lake Huron’s alewife population to anemic levels in 2003, but the fast-growing salmon population simply ate what was left. By 2004, the lake’s salmon population had collapsed, leaving only a few small fish struggling to adjust to an ecosystem that couldn’t support them. Experts call Lake Huron’s situation a “predator pit,” or a food web so congested with predators that the prey cannot recover.

Now, Lake Michigan seems to be following suit. Biologists report that the area is nearing an all-time low of alewife numbers, causing the number of Chinook salmon to decrease from 8.4 million in 2012 to 4.2 million in 2014. Experts say that the alewife population must reproduce in large numbers in July or this lake will collapse as well.

Such a development could be catastrophic for the fishing industry in Chicago and other cities, which has long drawn tourists interested in catching Chinook salmon. Lake Michigan would likely fare better than Lake Huron, which lost as much as 90% of its fishing businesses in some areas. However, unhappy clients are unlikely to come back for another go at the area’s famous fish, and focusing the industry on other local species like trout and bass would take time. Ironically, area officials are currently encouraging anglers to catch increasing amounts of salmon to give the alewives a chance to bounce back.

Unfortunately, there doesn’t seem to be much else that biologists and local organizations can do. Researchers say that even if every fish nursery in Michigan bred alewives all year, the stock would only feed the remaining salmon in Lake Huron for two days. And even if the state stopped stocking the lakes with salmon entirely, without a sufficient supply of alewives, the only result would be a disaster requiring combined efforts from Illinois, Michigan, Indiana and Minnesota.

Instead, researchers are waiting for July, when the alewives will hatch and have a matter of hours to find food if they are to survive. Despite these odds, however, biologists say they have seen recoveries in the past. Many are cautiously optimistic that Lake Michigan will avoid the fate of Lake Huron. However, after 66 years of trying to obliterate the fish from the lake, the area will need to cooperate if the rescue mission is to be successful.