Sei whales feed primarily on surface plankton, mainly copepods but also euphausiids. The food type of the sei whale varies according to the temperature and latitude. The most common prey of the sei whale are Calanus copepods, but moving south, close to the Antarctic convergence they are replaced by Euphausiids, amphipods and Euphausia superba in polar waters. Sei whales appear to feed opportunistically on whichever food species is most prevalent in the area, including small schooling fishes and squid in some places. Regardless of species of plankton, small fish or squid, the sei whale seems to select prey which occur in dense aggregations and in surface waters.
The food of sei whales: baby squid.
Sei whales feed by skimming and swallowing, usually in surface waters. Plankton is skimmed from the water as the whale swims forward with its mouth open. Food organisms are caught on the particularly fine fringe fibers of the baleen plates. They may also gulp mouthfuls of water containing food. Sei whales though, do not swim on their side, as other rorqual whales often do, when feeding.