An increase in the number of recorded sightings of Portuguese man o’ war jellyfish around British coastlines, have led researchers to speculate whether it is down to changes to our marine environments driven by climate change or simply recent strong winds that have blown them off course and away from their usual stomping grounds in the Atlantic Ocean over the last year.

According to the results of the Marine Conservation Society’s annual Wildlife Sighting report – released to coincide with World Jellyfish Day earlier this month – citizen scientists participating in the survey have recorded a 16% increase in Portuguese man o’ war sightings on the year prior.

Also known as ‘Bluebottles’, the species – which is technically not actually a jellyfish but a siphonophore – are typically found in warm, tropical, and subtropical waters and most commonly seen in the Atlantic Ocean along the eastern coast of the United States, the Caribbean Sea, and the Gulf of Mexico.

A recorded total of 280 sightings of the Portuguese man o’ war along British coastlines in the 12 months between October 2023 and September 2024 has, therefore, led to much speculation as to what has brought the species this side of the Atlantic.

The species – which differs from jellyfish by being a colony of specialised organisms working as one, rather than a single animal – was the second-most recorded species by the Marine Conservation Society’s volunteers this year, making up a fifth of sightings. It was also the most abundant species recorded from October to January, accounting for 80% of sightings in October.

Callum Duncan, head of policy and advocacy at the Marine Conservation Society, said: “It’s really important to report jellyfish and turtle sightings, especially if it’s something unusual, so we can understand how our marine environment is changing.

“The rise in Portuguese man o’ war sightings in October may be attributed to strong westerly winds bridging them to our shores. Historically, these creatures have not been prevalent in UK and Irish waters, so their increase could be related to shifts in our climate or alterations in the marine ecosystem.”

The charity’s wildlife sightings project focuses on gathering reports of jellyfish and marine turtles which feed on jellyfish. These marine animals are vital in supporting ocean biodiversity and are indicators of changes in our ocean over time.

A very similar number of compass jellyfish (280) and moon jellyfish (286) were also recorded this year, joining the Portuguese man o’ war as top spots. However, both moon and compass jellyfish live in British waters so are frequently spotted. Despite this, their numbers fluctuate depending on several factors.

Research has suggested that an increase in some jellyfish and siphonophore numbers around the UK and Ireland could be related to rising sea temperatures as a result of climate change, but there isn’t currently enough evidence to make this link.

Jellyfish play a vital role in the marine ecosystem, helping to transfer carbon while providing food and shelter for other fish. They also play a significant role in controlling fish populations. The Marine Conservation Society’s Wildlife Sightings programme aims to collect long term data which can be used as a reference to study the reality of jellyfish trends in UK and Irish waters. 

The charity also asks volunteers to submit sightings of marine turtles. This year, nine sightings of marine turtle were reported, three of which were live leatherbacks. Leatherbacks were spotted in summer months (June and July), which is when this species heads to our waters to feed on jellyfish. 

Leatherbacks are the largest marine turtle species, and the only turtle with a soft leathery shell instead of a hard one. This year, three dead Kemp’s ridley turtles and two dead loggerhead turtles were also reported in winter months, with another unidentified. Kemp’s ridley turtles are the rarest and most threatened species of marine turtle, currently listed as critically endangered. 

Sightings of these hard-shelled marine turtles are usually juveniles which get carried here by strong winds and currents. They’re not suited to the UK and Ireland’s cold waters, and their frequency can be an indication of stormy weather. 

The Marine Conservation Society works with other organisations to help rehabilitate stranded hard-shelled turtles where possible, that are often suffering from cold shock.