As part of the Cetacean Strandings Investigation Programme (CSIP), we are often focused on the death of animal and can overlook the amazing lives of marine creatures before they sadly wash up along our coastlines. British waters are home to over 28 different species of whales, dolphins and porpoises, collectively known as cetaceans.
In the UK, the most numerous (and smallest) of these is the harbour porpoise (Phocoena phocoena). Unsurprisingly, these porpoises therefore make up the majority of strandings in the UK.
WARNING: This blog contains photographs of dead stranded porpoises which you may find upsetting
Coastal creatures
At home in coastal waters, the harbour porpoise feeds close to the seabed, picking off fish, squid and sand eels. But life isn’t easy for these small mammals. Porpoises live on an energetic ‘knife edge’, meaning they must feed almost constantly in order to survive cold water temperatures.
This is due to high metabolic demands associated with small, warm-blooded animals. However, research has found that porpoises have a prey catch rate of up to 97%, making them extremely successful hunters and allowing them to eat up to 10% of their own body weight every day!
Notoriously shy, harbour porpoises are difficult to see unless the sea is calm, and will actively avoid boats. Barely breaking the water, they can often be heard rather than seen when they exhale at the surface, leading them to be nicknamed ‘puffing pigs’. Harbour porpoises are different to many other cetaceans in that they are the least social. They are mostly seen alone, and occasionally in small groups.
Tough times
Despite leading the quiet life, harbour porpoises are under a variety of threats causing their populations to be in decline and listed as vulnerable on the IUCN Red List.
One of the threats they face is predation. Bottlenose dolphins and grey seals are known to hunt, attack and kill porpoises. Bottlenose dolphins have been seen throwing porpoises clean out of the water, high into the air, though the reason for such behaviour is not yet fully understood. Current thinking suggests attacks may be a result of overlapping ranges in the two species, causing increased competition for food resources.
Grey seals are much larger than harbour porpoises and have also been known to attack and feed on them. Through extracting DNA from the wounds of stranded harbour porpoises, scientists have determined grey seals as a major predator. Seals target the porpoises for their blubber as an energy-rich source. Over 10 per cent of cetaceans examined at post-mortem in the UK died as a result of such predatory attacks.
Porpoises are also highly susceptible to becoming victims of bycatch. Bycatch is the accidental capture of non-target animals in fishing gear, and represents the most common cause of mortality in cetaceans in the UK. Porpoises caught in fishing gear ultimately drown, highlighting a huge welfare issue and the need for continued monitoring and bycatch solutions.
The use of ‘pingers’ on fishing gear produce sounds to discourage porpoises from going near nets, but the effectiveness of such devices are not fully understood, nor implemented in all fisheries. There is also concern that these acoustic deterrents are adding unnatural noise to the marine environment.
As inshore animals, porpoises are also vulnerable to coastal pollution. One issue is the influx of chemical pollutants into the marine environment. These chemicals enter the food chain and become more concentrated the higher up they go. Such pollutants can cause infertility, immunosuppresion and elevated risk of disease in cetaceans.
Many of the polluting compounds were banned in the 1980s leading to a slow decline in the environment. However, there are still some that do not seem to be reducing despite the ban and consequently, the impacts of chemical pollutants on harbour porpoises was recently determined to be the second highest concern for populations in the next 20 years.
Collecting stranded harbour porpoises is essential for the CSIP. Examining them allows us to monitor the health of populations and build a picture of the challenges they face in their daily lives.
If you come across a stranded porpoise then we’d love to hear from you. This project is impossible without help from the public so please call the CSIP hotline on 0800 6520 333 or email strandings@nhm.ac.uk.
For more information, visit the Museum’s Cetacean Strandings webpage.
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.