The small pond six miles deep in Everglades National Park suddenly began bubbling like a pot aboil -- a telltale sign of air-slurping walking catfish.

Dave Hallac, the park's chief biologist, dipped a net into the muddy commotion and hauled up a mess of wriggling slime so hefty it surprised even him. He counted out 56 fish from a single scoop.

Walking catfish, along with other species originally imported for somebody's tank or table, outnumbered natives in this shallow, shady bayhead by an unhealthy margin. Unlike giant python, the Glades' most notorious invader, these dinky denizens don't draw attention to their presence by, say, swallowing an alligator and exploding. But for park scientists, their spread is no small concern.

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