Heavy oil from the Gulf of Mexico spill threatened Louisiana marshlands on Thursday after washing ashore for the first time since a BP-operated rig exploded a month ago, sparking ecological disaster.

Calling it a “day that we have all been fearing,” Louisiana Governor Bobby Jindal said on Wednesday that heavy oil — not simply tar balls or sheen — had entered the state’s prized wetlands.

“It’s already here but we know more is coming,” he said.

The marshes are the nurseries for shrimp, oysters, crabs and fish that make Louisiana the leading producer of commercial seafood in the continental United States. A large no-fishing zone in Gulf waters seen as affected by the spill has been imposed.