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.
Fin whales have a varied diet.
In some areas or at certain times of year, they feed heavily on pelagic crustaceans, mainly euphausiids but also copepods occasionally. In other areas or at other times, small schooling fishes such as capelin, herring, sand lance, and anchovies are the staple fare. Cod, whiting, mackerel, and squid are sometimes eaten. Tagging data and morphological differences suggest the existence of at least two separate populations of fin whales off eastern Canada, one of which summers on the Scotian Shelf and in the Gulf of St. Lawrence, and the other in the Labrador Sea. There may be a small degree of mixing between these two populations.