They are abundant around Iceland and off the east and west coasts of Greenland. In the North Pacific, fin whales occur from the Chukchi Sea in the north, south to the Gulf of California in the east and Japan in the west. They have also been reported from the mid-Pacific. It is difficult for an inexperienced observer to distinguish between the fin whale and the sei whale. The fin whale's asymmetric head colouration is its most reliable and least subtle field mark.
Behavioural differences can also help in making an identification.
The fin whale has a characteristic roll. The head usually breaks the surface first, and the animal blows just before the dorsal fin appears. The fin whale usually arches its back high out of the water on the terminal dive of a series, but it almost never flukes-up. The sei whale usually approaches the surface at a flatter angle, and its prominent dorsal fin can sometimes be seen while the whale is blowing. Sei whales usually do not arch the back high as they dive.