TRAVERSE CITY, Mich. (AP) — The Trump administration has delayed indefinitely a plan for strengthening defenses on a crucial Chicago-area waterway to prevent Asian carp from reaching the Great Lakes, where scientists say they could cause severe harm to native fish populations, officials said Tuesday.

The U.S. Army Corps of Engineers had been scheduled to release draft results of a study begun in April 2015 on possible structural or technological upgrades at the Brandon Road Lock and Dam near Joliet, Illinois, a few miles downstream of a network of electrical barriers intended to repel the carp. A final version of the plan was to be released in early 2019.

Allen Marshall, spokesman for the Corps district at Rock Island, Illinois, said Tuesday the draft study release “has been deferred pending further coordination” with government officials and advocacy groups.

The study is “a very large, complex undertaking,” Marshall said, adding that more consultation was needed “to address things like the range of diverse ecosystems and national economies that could be impacted by outcomes of the study.”

Sen. Debbie Stabenow, a Michigan Democrat, said the decision was “alarming” and demanded a fuller explanation.

“After a lengthy review process, which included extensive outreach and collaboration with a broad range of stakeholders, it seems that the administration has decided to side with a narrow group of special interests intent on preventing actions to address the movement of Asian Carp toward Lake Michigan,” Stabenow said.

Asian carp were imported in the early 1970s to feed on scummy algae and plants in Southern fish farms and sewage ponds. They escaped into the Mississippi River and have migrated northward since, reaching dozens of tributaries.

Two Asian species — bighead and silver carp — feed on huge volumes of plankton that forms the base of aquatic food chains. They have infested the Illinois River, which connects with other waterways that reach Lake Michigan at Chicago.

The question of how to keep the carp from reaching the lake has divided the region. Some states, including Michigan, want barriers placed in the Chicago waterways, which Illinois and Indiana say would disrupt cargo shipping.

The Army Corps has been looking into a variety of measures that could be taken at Brandon Road, perhaps including installation of additional electric barriers.

In a Feb. 23 letter to President Donald Trump, 16 Republicans in Congress — mostly from Illinois and Indiana — called for delaying the Army Corps study until a new assistant secretary of the Army for civil works is appointed.

They argued that more than 5 million pounds of Asian carp had been removed from the waterway in recent years and the leading edge of the Asian carp population was still well below Brandon Road.

The Army Corps “should not hastily recommend a structural alternative that could negatively impact the economy and the safety of towboat crews,” said the lawmakers, who included Reps. Rodney Davis of Illinois and Todd Rokita of Indiana.

Molly Flanagan of the Alliance for the Great Lakes, an environmental group, said there was “no credible reason” for further delay of the draft report.