Sei whale

  • Scientific name: Balaenoptera borealis
  • Length: Up to 16m
  • Marine mammal type: baleen
  • Family: Balaenopteridae
  • IUCN Conservation status: Endangered
  • Protected by: CITES


A pelagic baleen whale, adults measure up to 16 metres and can live for 65 years. Often confused with the fin whale, but lacks the asymmetrical colouration of the lower jaw and baleen of the latter. Bluish or very dark grey dorsally, Sei have a paler grey or whitish underside and throat grooves, numbering 32-62, and a whitish lower fringe to the baleen. Pectoral fins are dark on both sides. The slender dorsal fin is more erect than a fin whale’s and slightly further forward than other baleen whales. Sei whales can associate with Fin whales, but are sporadic visitors to UK shores. Calves are born at 4.4-4.8 metres in the winter months, becoming sexually mature at 6-8 years old. Usually travelling alone or in pairs, they feed by surface skimming plankton, copepods and euphausiids (krill), and small shoaling fish.

Where to see them

Sei whales are far ranging animals that visit the UK shore very infrequently, although may be seen during ferry crossings or far out to sea.


Due to their sheer size sei whales are generally not suitable for a refloat should they strand on the shore as they cannot be moved safely except by the tide. Their enormous weight will also begin crushing their internal organs, causing severe damage from which it is highly unlikely they will survive.

What to do if you find a stranded animal?

A whale, dolphin or porpoise stranded on the beach is obviously not a usual phenomenon. These animals do not beach themselves under normal circumstances, and they will require assistance. Please DO NOT return them to the sea as they may need treatment and or a period of recovery before they are fit enough to swim strongly.

01825 765546 (24hr)
RSPCA hotline (England & Wales): 0300 1234 999
SSPCA hotline (Scotland): 03000 999 999

You will receive further advice over the phone, but important things you can do to help are:

  • Support the animal in an upright position and dig trenches under the pectoral fins.
  • Cover the animal with wet sheets or towels (even seaweed) and keep it moist by spraying or dousing with water.
  • Do NOT cover, or let any water pass down the blowhole (nostril), sited on top of the animal’s head. This will cause the animal great distress and could even kill it.
  • Every movement around a stranded animal should be quiet, calm and gentle. Excessive noise and disturbance will only stress it further.
  • Estimate the length of the animal and look for any distinguishing features that may give clues as to the species you are dealing with.
  • Look for signs of injury and count the number of breaths (opening of the blowhole) over a minute – this can give important clues as to how stressed the animal is.
  • Take great care when handling a dolphin, porpoise or whale; keep away from the tail, as it can inflict serious injuries – this is particularly the case with whales and it is advisable to leave handling larger whales until experienced help arrives. Avoid the animal’s breath, as it may carry some potentially nasty bacteria.
  • Provide information: Give the hotline an exact location for the animal – this can save valuable and perhaps critical time. If you have a mobile, give the number to the hotline.
  • Give an accurate description of the animal, including its breathing rate, and whether it is in surf, on rocks or sand, in the shade or in the full glare of the sun.
  • Information on weather conditions and sea state are also helpful.
  • The hotline should be informed of any attempts already made to push the animal back into the sea.
  • Maintain control.
  • Keep all contact, noise and disturbance to a minimum.
  • Under no circumstances, release the animal into the sea before the rescue team has arrived. It is fine to support a smaller dolphin or porpoise in the water, as long as the blowhole is kept above the water at all times, and as long as it is carried to the water carefully, e.g. in a tarpaulin (do NOT drag it or lift it by its fins or tail).
  • However, actually releasing the animal before it has received an assessment and first aid from experienced personnel can do more harm than good.

If you find a dead cetacean

The Cetacean Strandings Investigation Programme (CSIP) collects a wide range of data on each stranding found on UK shores. If you discover a dead animal, please contact the CSIP hotline and give a description of the following where possible:

  • Location and date found
  • Species and sex
  • Overall length
  • Condition of the animal
  • Your contact details should further information be needed

Digital images are extremely helpful to identify to species, as well as ascertaining whether the body may be suitable for post-mortem examination. 

CSIP has produced a useful leaflet that can be downloaded by clicking here.

CSIP hotline: 0800 6520333. Callers are given a number of options to ensure they reach the correct department. You can also use this number to contact BDMLR as there is an option for live animal strandings that transfers directly to us.