the basking shark is the second largest shark in the world, only to be behind the whale shark. the basking shark can reach lengths of up to 40 feet (12 metres), but the average adult length is 22–29 feet (6.6-8.7 m). the size at birth is believed to be between 5–6 feet (1.5-1.8 metres). the basking shark is an extremely slow-growing species and may grow to 16–20 feet (4.8–6 metres) before becoming mature.