There is no definitive answer to this question as it largely depends on the definition of “largest brain”. For example, if brain size is measured by weight, then the whale shark has the largest brain of any fish. However, if brain size is measured by relative size to body size (i.e. encephalization quotient), then the manta ray has the largest fish brain.
In terms of absolute brain size, the whale shark has the largest known fish brain. The average adult whale shark weighs in at around 21 metric tons (46,000 lb) and has a total length of 12 meters (40 ft). Their brains weigh an average of 8 kg (18 lb), making them about 0.04% of their total body weight. In comparison, human brains make up around 2% of our total body weight.
The manta ray also has a relatively large brain for its body size when compared to other fish species. Manta rays can grow up to 7 meters (23 ft) in width and weigh up to 1 metric ton (2,200 lb). Their brains only make up around 0.007% of their total body weight but they are still larger than most other fish species when scaled for body size.
