Key difference: Although they appear very similar on the surface, fishes and whales are different from each other. The most basic difference between the two is that whales are warm blooded mammals, whereas fishes are marine animals.

Similar in their looks and environment, fishes and whales are generally confused to be under the same category. In fact, many people believe that whales are a type of fish. Though, such is not the case. Whales are not fish! Whales are mammals, whereas fishes are aquatic animals.   

Fishes and whales are both vertebrates, and live in similar aquatic environments. Except for a few species, whales only live in the ocean water, whereas fishes inhabit in both fresh and salt water. Whales are classified in the class ‘mammalia’ and order Cetacea in the animal kingdom. The term ‘whale’ is sometimes used to refer to all cetaceans, which include the sperm whale, killer whale, pilot whale, and beluga whale. On the other hand, fishes are the members of paraphyletic group, which includes all gill-bearing aquatic animals. This group includes all cartilaginous and bony fishes, lampreys and living hagfishes, etc.

Whales are said to be the largest animals on Earth, and there are some fishes which can be among the smallest of other animals. Being mammals, whales are warm-blooded, and are required to maintain a constant warm body temperature. Fishes, on the other hand, are cold blooded, which means their body temperature changes according to their surrounding water.

Whales have smooth and almost hairless skin; there is a thick layer of fat called blubber under the skin, which helps in retaining their body heat. On the other hand, fishes have very little body fat, which is the main reason for why they are prized as food. Also to keep themselves from freezing in the cold water, fishes have an anti-freeze substance in their blood, and a slimy material over their skin, which protects them from salt and other chemicals.

Though whales and fishes are from different classes, they do have similar behavior and share some similar characteristics too. They are similar in their body characteristics such as both have fins and a tail, which helps them swim and stay upright in the water. They both have similar scaly skin, backbones and brains. However, these similarities end at their appearances.

There are certain points which easily distinguish fishes from whales. Whales move their body through the water with the help of their horizontal tail, which flaps up and down, whereas fishes have vertical tails, which move from side to side. Their breathing technique also differs, wherein whales breathe through their blowholes, which are basically nostrils on the top of their heads, fishes breathe in oxygen from the water through their gills. Also, fishes lay their eggs in water, whereas whales give birth to their young ones and nurse them to growth.

Thus, from the above article, one can defintely agree that whales are surely not a type of fish, and they certainly are not fishes. 

Comparison between Fish and Whale:

Image Courtesy: whalesanddolphinsofbc.blogspot.com, www.hongkiat.com