Thousands of fish were killed in an Australian river, possibly in a matter of hours, shocking people who feed the friendly swimmers.

The Mandurah Coastal Times reported people living near the Murray River, which runs through an area called Yunderup on the coast of Western Australia, near the city of Mandurah and about 50 miles south of Perth, woke up Wednesday to huge numbers of dead fish on the river’s banks.

Read: These Fish Can Teach Us a Lot if We Would Stop Eating Them

“I was feeding them last night and now they’re dead,” resident Bernie Lydiate told the publication, saying he had been close with the fish community for a long time. “I don’t catch them. I feed them. They’re a beautiful fish and they trust you. They come to me and I buy special food for them. I’ve been doing that for years.”

The newspaper reported the local water department confirmed hundreds of fish, including the popular recreational and commercial species black bream, were killed in the river — though residents say it was closer to thousands, as did another publication, the Mandurah Mail. The water department is investigating but has not yet determined what is behind the fish kill.

“There’s also some slime on the water,” neighbor Marg McBride told the Mandurah Coastal Times.

There is other marine life in the river as well, including dolphins, crabs, cormorants and pelicans, but it was unclear how they were affected.

Read: Hideous, Slime-Covered Fish Eats Things from the Inside Like an Alien

The U.S. Geological Survey says fish kills have multiple possible causes. In warmer weather, it’s often that the amount of dissolved oxygen in the waterway, which fish need to breathe and survive, has dipped because warm water does not hold as much of that dissolved gas as cold water does. It can be the result of natural events in the water or can be exacerbated by people adding certain nutrients to the water supply, like nitrogen, making algae grow faster and consume a larger share of the oxygen. Dredging and decreased water circulation also can affect oxygen levels.

Another cause of fish kills, though less common, is something toxic released into the waterway.

“In order for this to occur, the toxic compound must be fairly highly concentrated,” the USGS said. “In a large water body … this would require a very large amount of the toxic compound, and a release site fairly close to the affected fish.”

The Mandurah Coastal Times said there was a fish kill in the Murray River a few months ago after a big rainstorm and an earlier one in 2013 at a bay in the same town.

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