A shark attacked a surfer and tore off a "good chunk" of his arm off the French Indian Ocean island of La Reunion on Wednesday, according to officials and witnesses.

The man in his 40s, who was not named, was described as a "regular surfer" who was in "clear water" and in "very good weather conditions", said lifeguards on the scene.

Witnesses said the man had "a good chunk of his arm torn off" in the attack, the 18th in those waters since 2011 -- seven of which have been fatal.

Emergency doctors applied a tourniquet to the rest of the arm and he was taken to hospital still conscious.

La Reunion was plunged into shock in April when a shark attacked and killed a 13-year-old boy named Elio Canestri said to be a keen and talented surfer.

After that incident, France approved increased fishing of tiger and bull sharks in the marine reserve off La Reunion.

The plans included increased surveillance and protection nets, as well as a 50-percent boost in authorised shark-fishing trips.