The National Oceanic and Atmospheric administration said yesterday that it had closed 4,200 square miles of federal waters in the Gulf of Mexico to royal red shrimping after a commercial shrimper discovered tar balls in his net.

NOAA said in a statement that it had taken the measure "out of an abundance of caution" and that it only applied to royal red shrimp, which are found at the deep depths where the tar balls of weathered oil apparently were trawled from.

The tar balls are being analysed by the US Coast Guard to determine if they are from the catastrophic BP oil spill.

It was the first closure of a fishery in the Gulf since 12 July, said NOAA spokeswoman Karrie Carnes. NOAA has since gradually reopened most of the area that was closed to commercial and recreational fishing in the wake of the BP spill that saw almost 5m barrels of oil surge into the sea.

NOAA said that at its peak in early June, more than 88,000 square miles of Gulf waters had been closed to fishing.

"We are taking this situation seriously," said Roy Crabtree, assistant NOAA administrator for NOAA's Fisheries Service southeast region. "This fishery is the only trawl fishery that operates at the deep depths where the tar balls were found and we have not received reports of any other gear or fishery interactions with tar balls.

"Our primary concerns are public safety and ensuring the integrity of the Gulf's seafood supply."

NOAA also is sending vessels to the area to re-sample for royal red shrimp to determine if it is safe for human consumption. The area is off the coasts of Louisiana, Mississippi, and Alabama.

An area covering 1,041 square miles around the blown wellhead still remains closed to all commercial and recreational fishing.