The shutdown of Xcel Energy’s Monticello Nuclear Power Plant on Friday is hurting fish and wildlife along the Mississippi River.

The flow of warm water from the plant stopped after the unplanned shutdown, and by Saturday the water temperature downstream had dropped from 75 degrees to just 35 degrees, said Joe Stewig, Department of Natural Resources area fisheries manager.

About 200 fish were found dead along the river over the weekend, likely killed by thermal shock, said Stewig. More than half were catfish, 18 to 24 inches long, he said. Xcel will have to pay the state restitution, based on the number and size of fish. But Stewig said he expects no long-term impact to the fishery.

Meanwhile, about 2,000 trumpeter swans usually winter in the open water near Monticello, but without the influx of warm water from the plant that open area has shrunk and become slushy, and some swans could leave, said Lori Naumann of the DNR’s nongame wildlife program.

“It’s hard to say what’s going to happen, especially with this cold weather,’’ she said.

Jim Lawrence, who lives on the river and feeds the swans, said he’s not concerned. There are other open-water spots in the region where the swans will go.

“I think they’ll be fine,’’ Lawrence said. “We will have fewer birds, possibly. But they’ll come on back.’’

Xcel officials said Monday that the plant could remain shut down for several days as workers repair a leaking heat exchanger on the reactor’s coolant system. Operators shut the plant down Friday night after detecting greater-than-normal levels of water in a system that cools plant components, Xcel said. The problem turned out to be a leak inside a heat exchanger.