Japweed or wire weed (Sargassum Muticum) is one of the varieties affecting the island.

Its spread is thought to be a global problem, and is already affecting beaches in the Mediterranean.

Dr Cynthia Trowbridge, from Oregan State University, is in Alderney to map the spread of the seaweed and says it poses a serious threat.

She said: "The weed spreads on oysters on the bottoms of boats and even on luggage.

"It's a natural process to disperse, but it is not natural to spread between ocean basins and the rate at which they have spread."

In many cases the seaweed displaces native species and attack shellfish.

Ms Trowbridge said: "When the photosynthesise they make oxygen and become very buoyant and rip the shellfish off the seafloor and wash up on the beach and kill the shellfish."