The Basking Shark (Cetorhinus maximus) is the world’s second-largest fish, dwarfed only by the Whale Shark. As a massive, slow-moving filter feeder, its size complicates the logistics of weighing an animal that can reach the size of a small bus. Understanding the weight of this migratory species, which cruises temperate waters globally, requires specialized scientific approaches to accurately determine its mass.
Typical Adult Weight and Size
A mature adult Basking Shark typically weighs between 4,000 and 6,000 kilograms (4 to 6 metric tons), or approximately 8,800 to 13,200 pounds. This massive weight corresponds to a length generally measuring between 6.7 and 8 meters (22 to 26 feet). The average adult is often cited as weighing about 4.65 metric tons (5.13 short tons) while reaching 7.9 meters (26 feet).
The Basking Shark’s weight places it among the heaviest animals on the planet, exceeding that of a large African bush elephant. Its overall body plan, characterized by a conical snout and enormous gill slits, supports its immense bulk as it filters plankton from the water.
Record Weights and Maximum Potential
While typical adults are massive, historical records point to larger, outlier specimens that push the maximum potential weight. The largest reliably documented Basking Shark was caught in a fishing net in the Bay of Fundy, Canada, in 1851, measuring 12.27 meters (40.3 feet) in length. Although it was impossible to weigh the animal directly, its mass was later estimated to be around 16 metric tons (about 35,000 pounds).
Reports of sharks exceeding 12 meters (40 feet) are often based on visual estimates from historical whaling or fishing logs, which lack rigorous verification. Some unconfirmed historical sightings suggest sizes up to 14 meters (46 feet), but contemporary scientific studies suggest that individuals exceeding 10 meters (33 feet) are exceedingly rare.
Factors Influencing Basking Shark Weight
The weight of an individual Basking Shark fluctuates significantly due to biological and environmental factors. Seasonal feeding plays a major role, as the sharks migrate to temperate coastal waters in summer to feed on dense concentrations of zooplankton. They gain substantial mass during these plankton-rich months, and their weight naturally decreases during the winter when food is scarce and they move to deeper waters.
Age and maturity also dictate the shark’s final weight; females generally reach sexual maturity later than males, at lengths between 8 and 10 meters. Another element contributing to their immense weight is the liver, which can account for up to 25% of the shark’s total body mass. This large, oil-filled liver provides buoyancy in the water column, which is necessary for such a massive, cartilaginous fish.
Methods for Estimating Basking Shark Mass
Because Basking Sharks are rarely captured or weighed directly, scientists rely on non-invasive and mathematical techniques to estimate their mass. The primary method involves allometric scaling, a length-weight relationship formula expressed as W = aL^b (where W is weight and L is length). Researchers use species-specific constants (a and b) to convert a measured length into a reliable weight estimate.
Non-invasive visual techniques, such as laser photogrammetry, are increasingly used to obtain accurate length measurements in the field. This method involves projecting two parallel lasers a known distance apart onto a free-swimming shark. By capturing a photograph with the laser dots visible, researchers use the fixed distance between the dots as a scale to precisely measure the shark’s length, which is then fed into the allometric formula to estimate mass. In rare cases, such as when a shark is caught as bycatch, the total weight of the catch is measured, and an average weight—sometimes around 1,725 kilograms—is used to estimate the number of sharks caught.