Study: Carp Have Caused Sport Fish Decline

A new study based on almost 20 years of fish-sampling data gathered by the Corps of Engineers in the Mississippi River says that the spread of Asian carp has resulted in drastic declines in Mississippi River populations of sport fish like crappie, yellow perch and bluegill.

Using Corps data from electrofishing gathered between 1994 and 2013, the scientists found that sport fish had declined by 30 percent on two areas on the Mississippi River and one on the Illinois Waterway.

At the same time, populations of those fish in areas of the Upper Mississippi where the carp have not yet penetrated have jumped by 35 percent.

The study appeared in the November issue of the peer-reviewed journal Biological Invasions. The lead author is John Chick, a fisheries biologist who runs a University of Illinois field station in Alton, Ill. It provides additional evidence for what many scientists have long suspected: that the fast-growing and voracious carp out-compete other fish for the plankton and algae that they all feed on.

Four varieties of Asian carp were imported into the U.S. in the late 1960s and early 1970s to clear algae and weeds from fish farms. During floods, some escaped into the river systems and gradually spread northwards. The study focuses on silver carp, the most numerous of the four varieties.

It concluded, “We found empirical evidence of a negative influence of silver carp on the abundance of sport fish in the [Upper Mississippi River System]. … We believe our analysis of 20 years of standardized monitoring data, including multiple control and invaded reaches, and six 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 study considered other factors that could have affected fish populations, including flooding, water temperatures and sedimentation, but found that none of the other factors could explain the decline in sport fish populations.

Proponents of strong measures to keep Asian carp out of Lake Michigan frequently invoke their threat to the lake’s multibillion sport fishing industry.

Some experts cautioned that the study, while strongly suggestive, doesn’t definitively prove causation.