Giant Jellyfish Blooms Signal Shifting Marine Ecosystems Around the UK
London – A surge in sightings of barrel jellyfish, also known as Portuguese man o’ war, is among the key findings of the Marine Conservation Society’s (MCS) annual Wildlife Sightings Report, released today. The report, compiled from public submissions, indicates meaningful changes occurring in British and irish waters, largely attributed to rising sea temperatures and shifting ocean currents.
The report details 1,327 jellyfish sightings across the UK and Ireland between October 1, 2023, and September 30, 2024 – a 6% decrease in total sightings compared to the previous year. However, the proportion of large jellyfish blooms has notably increased. Approximately 20% of moon jellyfish sightings involved groups of 100 or more, the highest proportion recorded since 2016.
These shifts align with a broader trend of warming seas; the average surface temperature of British waters has risen by roughly one degree Celsius since the 1980s, with recent summers registering among the warmest on record. This warming is drawing species typically found further south northward, with increased reports of triggerfish and short-snouted seahorses along the English Channel.
octopus sightings have also climbed sharply, with common octopus reported off the coasts of Cornwall and Devon in numbers not seen for decades, and smaller species like the curled octopus appearing more frequently along the Welsh coast. Leatherback turtles, which migrate to feed on jellyfish, have been observed off the south-west coast and even in the Hebrides. This year,12 turtle sightings were confirmed – three more than the previous year,with nine identified as leatherbacks,eight of which were alive,primarily spotted off the south-west. A single live loggerhead turtle was also recorded in the Outer Hebrides.
“Volunteer reports show just how dynamic and ever-changing our coastal ecosystems truly are,” said Anna Bunney, citizen science program manager at the marine Conservation Society. “The surge in barrel jellyfish sightings is particularly engaging. Whilst we need more records and data to help us understand these fluctuations, it might potentially be linked to warmer sea temperatures and changing ocean currents. That’s why our public sightings are so vital: they help build a clearer picture of what’s happening in our seas and allow scientists to track changes in real time.”
