CHICAGO (CBS) — Anglers stumbled across a huge number of dead perch this week at a popular fishing spot on the Far South Side.

Pictures taken Tuesday at the 2,000-foot ship slip in what was once the U.S. Steel South Works at 85th Street on the lakefront showed thousands of dead perch piled up on the west end of the slip, often used by fishermen.

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By Thursday, the fish were mostly gone, as seagulls were swooping down to eat what was left. Hundreds of mergansers – fish-eating ducks – also were drawn to the slip, presumably for an easy meal.

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One fisherman wondered if the birds were in any danger from whatever killed the fish.

Photos of the dead fish were shared with the Illinois Department of Natural Resources, and a conservation officer said the fish kill was under investigation.

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Thousands of dead fish were found Tuesday in the boat slip at the old U.S. Steel South Works site on the Far South Side. [Credit: Facebook/Fishing The Area ( NWI )]