As spring arrives, so do the floods. The National Ocean and Atmospheric Administration (NOAA) is forecasting intense flooding in the Midwest. The good news is that the floods are predicted to be less severe than those in 2019 that cost the Midwest $6.2 billion, but they’re still coming — and they may bring in some unwanted visitors.

Scientists have long feared the arrival of silver and bighead carp — invasive fish native to Asia — to the Great Lakes. Invasive carp are already found in many Midwestern states in the Mississippi River Basin and its tributaries. Another extreme flooding event could allow carp to move further upstream and head toward the Great Lakes, where scientists worry they could overtake the native fish species. Just a few carp in a new area can start a new a population.

Silver and bighead carp can grow up to 60 inches long and weigh over 100 pounds. They can eat up to 40 percent of their bodyweight in plankton, snails, and grasses in one day, which is why they pose such a big threat to an ecosystem. They were imported to the Midwest in the 1970s for aquaculture and were used to control algae in fish farms, but during the Great Flood of 1993 they escaped into the Mississippi River and moved north.

In 2019, floods inundated 6.5 million acres near the Mississippi River, stretching from Missouri to Louisiana. The dams on the Mississippi are meant to maintain water flow for commercial navigation; as an unintended effect, they also slow the movement of fish with hydraulic pressure and gates that make it challenging to swim upstream. During the floods, however, many dams were deluged and at risk of physical damage due to high-pressure conditions. The dams were opened, allowing water — and fish — to flow through.

A study recently published in the Journal of Freshwater Ecology found that silver carp have an affinity for breeding in flood waters. The researchers examined habitats created by flooding alongside the Mississippi river and captured and identified over 12,000 larval fish, the early developmental stages of fish. Ninety-seven percent of the fish identified were larval silver carp, indicating that not only do the fish move upstream during floods, they can also reproduce prolifically in flooded areas.

New populations of silver carp established themselves in Minnesota and Wisconsin thanks to last year's floods, threatening the Great Lakes. The Mississippi watershed is a flood pulse system, flooding predictably in the spring to early summer and experiencing low water in late summer. There is a window of time where the perfect combination of river flow and spring water temperatures are a “stimulus for reproduction” for carp, warned Andrew Casper, a research biologist at the Illinois Natural History Survey. The flow allows eggs to stay suspended in the water column so they can develop into larvae. This in turn lets the population increase from a few carp to a cohort that is primed to survive the winter. 

A 2019 study that Casper co-authored showed Asian carp have caused a significant decline of native fish in the Mississippi over the past 20 years. Silver and bighead carp are planktivores, and these voracious feeders can outcompete sport fish like yellow perch and bluegil,l and culinary favorites like catfish. The carp “starves out” other species, eating up their food supply and taking over their breeding grounds.

As a part of a long-term monitoring project, the researchers sampled sections of the Mississippi and Illinois Rivers in Illinois. They found silver carp populations grew exponentially from 2000 through 2010. It is not yet known what the effects will be in the Great Lakes, but researchers from the Illinois Natural History Survey have found enough evidence to be worried about the ecosystems.

“Theoretically if carp get in in high numbers, they may have a bigger impact on the Great Lakes, a less productive ecosystem than the Mississippi,” said John Chick, a freshwater ecologist at the Illinois Natural History Survey and co-author on the 2019 study as well as the new paper. “We think — we hope — it will be harder for carp to reproduce in the lake itself, because they look for the rise in water. But let’s not take the risk.”

Integrated pest management, where constant pressures are applied year-round, is considered one of the best management practices to slow the further spread of silver and bighead carp to the Great Lakes. The National Park Service has partnered with local and state agencies to slow the spread of carp and are actively monitoring the presence of carp while investigating removal techniques. An electric barrier was installed in the upper Illinois River in an attempt to prevent the spread of carp to Lake Michigan. The Illinois Department of Natural Resources has also deployed fisherman to collect carp with nets to slow their numbers.

But ultimately, the best way to keep carp at bay is to keep them out. “The best we can do in a lot of situations is protect what we have left,” said Casper. “We don’t want to lose all our native species. Once they’re in, they aren’t coming out."