Illinois biologists cheered a recent fish die-off, but now their high hopes are dashed.

The Asian carp spent the past decade out-eating and out-spawning other fish in the Illinois River.

So biologists shed few tears when they learned thousands of them turned belly-up last week. But the biologists’ hey-day was short-lived.

Ecologists trawled 110 miles of the river, and haven’t spotted any more dead Asian carp.

Now the state’s turning to federal wildlife pathologists to get to the bottom of the fish-kill mystery. Some biologists guess a virus attacked the carp, but they’re running tests to be sure.

The state’s turning over 86 live carp to the feds, and they expect results within a few weeks.

Meanwhile, biologists are still watching the Asian carp’s progress in moving up the Illinois River. They worry the carp could enter Lake Michigan and disrupt its food chain, too.