Just three days after the U.S. Coast Guard admiral in charge of the BP oil spill cleanup declared little recoverable surface oil remained in the Gulf of Mexico, Louisiana fishers Friday found miles-long strings of weathered oil floating toward fragile marshes on the Mississippi River delta.

The discovery, which comes as millions of birds begin moving toward the region in the fall migration, gave ammunition to groups that have insisted the government has overstated clean-up progress, and could force reclosure of key fishing areas only recently reopened.

The oil was sighted in West Bay, which covers approximately 35 square miles of open water between Southwest Pass, the main shipping channel of the river, and Tiger Pass near Venice. Boat captains working the BP clean-up effort said they have been reporting large areas of surface oil off the delta for more than a week but have seen little response from BP or the Coast Guard, which is in charge of the clean-up. The captains said most of their sightings have occurred during stretches of calm weather, similar to what the area has experienced most of this week.

Update: Coast Guard says what looks like oil is algae. Read more