The introduction of non-native carp to the Mississippi River has reduced the numbers of sport fish valued by anglers, a new study has found. Researchers analyzed data from 20 years of fish population monitoring on parts of the upper Mississippi River and its tributaries. The St. Croix was not included, but is likely similar in status to the uninfested Pool 3 of the Mississippi, shortly below the mouth of the St. Croix.

Although adult bighead and silver carp have been found repeatedly in the St. Croix River in the past decade, it is not believed the fish have an established population here yet. There have been no confirmed signs of reproduction.

The first two silver carp were found in the river at Prescott in 2017, with one more found in the same area this April. Bighead carp have been found as far upstream as Oak Park Heights, but silver carp have so far only been observed near the mouth of the river.

Silver carp is the species that leaps from the water when boats pass, harming passengers and damaging property. It’s also the species that was the focus of the new study, led by Dr. John Chick of the Illinois Natural History Survey’s Great Rivers Field Station in Alton, Illinois.

While the study doesn’t directly look at the St. Croix, it has serious implications.

“The alarms have been out there for a long time now,” Chick told the Associated Press. “This adds further mustard to the argument that we need to be taking these things seriously. The trends that have been established here are not the trends we want to see in other places.”

The article, recently published in the peer-reviewed journal Biological Invasions, explores how the arrival of silver carp has affected native sport fish, the species that are targeted by recreational anglers.

“Our results provide empirical evidence of a negative effect of invasive silver carp on native sport fish in the [Upper Mississippi River System],” the article’s authors wrote. “Although water temperature, suspended solid concentration, and flooding also differed substantially between control and invaded reaches, only silver carp abundance had a direct negative relationship with the abundance of adult sport fish.”

Researchers in Illinois were able to see the trends because fish population monitoring has been conducted on the river and its major tributaries consistently for more than 20 years, since before silver carp were established.

They looked at the data for six monitoring sites between 1994 and 2013 to assess the trends.

The southernmost three of the sites are now located in infested stretches of river, while the three northern sites are not. The two decades of data provided invaluable insights.

The researchers looked at changes in the fish populations in areas both infested and not infested with silver carp. They saw that infested areas saw significant declines in sport fish species, while the uninfested reaches actually saw sport fish population increases.

“We believe our analysis of 20 years of standardized monitoring data, including multiple control and invaded reaches, and 6 years of data preceding the establishment of silver carp, is one of the strongest empirical assessments of the effects of an invasive species conducted for a large and spatially complex ecosystem,” the article reads.

How and why carp affect native fish populations was the next question. Carp are not predators, so the possibility they were eating native fish was ruled out. Stealing food from sport fish seems to be the successful strategy.

The researchers say carp can outcompete juvenile walleye, bass, crappie, catfish, and pike, among other fish, for zooplankton, the tiny organisms that form the base of the aquatic food chain. This reduces the success of sport fish species making it to maturity. Sport fish population declines have followed.

The finding is interesting because it reverses earlier understanding of carp diets. It was thought that bighead and silver carp primarily ate microscopic aquatic plants called phytoplankton, but that was based on studies used to determine their usefulness to help control algae blooms at fish farms, which is why they were originally imported to the United States.

More recently, scientists have seen that carp also eat tiny aquatic animals, called zooplankton. These organisms are also critical for the young of many native game fish. When carp consume the food source, young fish are left to starve.

The team also considered how other changes in the rivers could have affected sport fish populations, but found no other differences that could explain the decline of sport fish species. Because the uninfested waters are in the northern part of the system, the water is generally cooler, but it was in part offset by warming. They also found no significant correlation with factors like flooding and sediment pollution.

The authors conclude by calling for the federal government to expand its efforts to control the spread of carp from.

“Our study suggests that in addition to focusing on the Great Lakes, efforts to prevent the spread of silver carp to new areas throughout the United States and to reduce their abundance in areas where they have already become established are worthy of federal consideration and investment to minimize ecological and economic impacts,” the authors write.