When a call comes in to the Cetacean Strandings Investigation Programme (CSIP) hotline, the details of a stranding can often be minimal and somewhat vague. An animal may be highly decomposed, inaccessible or lack features for it to be identified. As a research assistant for CSIP, my job is to investigate a stranding and try to gather as much information as possible so an accurate identification can be made. This blog post provides a simple guide for everyone to try and identify dolphins, whales and porpoises commonly washed up on British coastlines.
WARNING: This blog contains photographs of dead and injured stranded cetaceans which you may find upsetting
- Stranded juvenile common dolphin (Delphinus delphis), first thought to be a harbour porpoise due to its size and beak damage. Credit: Tom Mustill (SW2018/75).
Dolphin or porpoise?
One of the most common reports received over the phone is “I think I’ve found a porpoise…or maybe a baby dolphin?”. Dolphins and porpoises are part of the same parvorder, the odontocetes, meaning they have teeth instead of baleen plates (difference between the two explained later) and are common in British waters, making identification confusing if you don’t know what you are looking for! Here are a few simple steps to help you decide between the two…
- Is the head blunt in shape? Or does it have a long beak?
- Porpoises have flat, blunt heads whereas common dolphins have ‘dog-like’ faces, with distinct beaks
- What shape are the teeth?
- Porpoises have small, spade like teeth (up to 28 pairs) compared to common dolphins which have conical, needle like teeth (up to 50 pairs)
- How long is the animal (from the tip of the nose to the tail)?
- Porpoises tend to be between 1.3 – 1.9 m (4 – 6 ft) and common dolphins between 2.1 – 2.4 m (6.5 – 8 ft)
- Does the animal have any colouration, or patterns on the body?
- Porpoises are black on top, with a white underbelly compared to common dolphins which usually have a ‘figure of eight’ on the side, with yellow colouration towards the head (though this varies between individuals)
Harbour porpoise (Phocoena phocoena). Credit: Steve Williams (SW2018/29). 
Common dolphin (Delphinus delphis). Credit: Lesley Dockrell (SW2018/25).
- Porpoises are black on top, with a white underbelly compared to common dolphins which usually have a ‘figure of eight’ on the side, with yellow colouration towards the head (though this varies between individuals)
Common or striped dolphin?
There is also often confusion (even within the CSIP team itself!) when trying to tell the difference between common and striped dolphins. These species have similar patterns on their flanks and are of similar size and tooth morphology making identification between the two quite hard unless a stranded animal is very fresh!
Identification comes down to the pattern of stripes along the body. Common dolphins have a black stripe beginning at the flipper and extending up and under the lower jaw. Confusingly, striped dolphins have the same, except the stripe extends to the eye rather than under the jaw. Striped dolphins also have a second black stripe from the eye extending along the flanks of the body, to the underbelly, which common dolphins don’t have, instead this species display a ‘figure of eight’ pattern on the flanks.
It is important to remember each individual can vary in colour and pattern, so identification is not always straightforward!
Baleen whales
Generally, there are two groups of cetaceans – the odontocetes (toothed whales such as orca and dolphins) and the mysticetes (baleen whales such as the blue whale). Mysticetes, or baleen whales, tend to be much larger than toothed whales and can be identified by the presence of baleen plates in the mouth instead of teeth. Baleen is made up of keratin, the same protein as in our hair and nails and allows whales to filter feed on small fish or crustaceans. All baleen whales also have two blowholes as opposed to one blowhole found in odontocete species.
To identify between different baleen species, the colour of baleen and the number of throat grooves should be noted when reporting a stranding. For example, Sei whales have 30 – 65 throat grooves compared to the blue whale which has 50 – 70.
Baby cetaceans
Occasionally, baby or neonatal animals wash up on the shoreline. These animals are very interesting for the project and are often collected for post mortem. Though it may be sad to see such animals dead, these specimens provide insight into the early life of cetaceans and allow monitoring of disease and environmental contaminants that may be passed to the calf maternally (via the mother).
To identify if an animal is neonatal look for creases (also know as fetal folds) down the animal’s body, a floppy or collapsed dorsal fin and occasionally hairs on the nose (cetaceans are mammals after all!).
Get involved
CSIP depends largely on members of the public reporting strandings. If you come across a stranded cetacean (or seal, marine turtle or large-bodied shark) then please call the CSIP hotline on 0800 6520 333 or send an email to strandings@nhm.ac.uk and provide the following details:
- Species (if known)
- Approximate length of the animal
- Location of the animal (grid reference or longitude/latitude)
- State of decomposition
- Sex of the animal
- Degree of accessibility if the animal were to be picked up for post mortem
- Your name and contact number/email
Photographs are extremely valuable in allowing us to identify the animal correctly so please take pictures from different angles and attach to your email!
A guide on how to sex a cetacean and determine the degree of decomposition can be found here: CSIP strandings leaflet.
Kate Swindells is a research assistant at NHM, a partner organisation of CSIP. She completed her Masters degree in Wildlife Health & Conservation at the University of Bristol and also has an undergraduate degree in Marine Biology. She joined the museum in January 2018.