'Shellfish' was a British naval plan to intercept U-boats and German surface warships through the use of telegraph cables as navigation aids (22 April/late April 1940).

The plan was implemented by Admiral the Hon. Sir Reginald Plunkett-Ernle-Erle-Drax’s Nore Command, and began on 22 April as one patrol sloop was stationed in the area between 52° 37.5' N/02° 5.25' E and 52° 39.25' N/02° 32.5' E, and another between the Haisborough light vessel and a point 10 miles (16 km) 285° from the light vessel. The 627-ton anti-submarine yacht Mollusc patrolled the area round the Haisborough light vessel with two motor torpedo boats in tow, one of which she had herself towed to the area.