656 basking sharks were spotted from Gwennap Head in 2007
Cornwall Wildlife Trust is calling for volunteers to help with a basking shark survey this summer.
The Seaquest Project aims to collect important data on the sharks and to create awareness of the creatures and the threats they face.
The project is supported by the BBC Wildlife Fund.
SeaWatch SW surveys recorded 656 basking sharks from Gwennap Head in 2007. Six species of cetacean were also recorded, including a fin whale.
Russell Wynn, SeaWatch SW co-ordinator, said: "The results of this survey are starting to help us understand a lot more about basking sharks off Cornwall, and identify the areas which are important for them.
"This data will be useful in influencing the development of a network of Marine Protected Areas around Cornwall, which the Finding Sanctuary project is currently working towards."
The trust says previous experience is useful but not essential as training can be provided.
The basking shark is a regular visitor to the Cornish coast but little is understood about the status of their population or about their behaviour.
They do not pose a risk to humans since they eat only plankton, but can grow up to 12m (39ft) in length and weigh up to 7 tonnes.