String jellyfish

Since 2025, string jellyfish have been multiplying in seas around Norway and Scotland, where they have caused mass deaths of farmed salmon.1

Each string jellyfish is a colony of specialised tiny organisms called zooids. They are permanently joined and cannot survive on their own.1

In rough seas, or where there are physical barriers, a colony can break into smaller pieces. These pieces can get into fish cages to reach farmed salmon.1

Rising sea temperatures and waste dumped from fish farms, which the jellyfish feed on, have been blamed for the increase.1

Colonies can be found at the surface and at depths of around 1,000 m.1

In September 2007, string jellyfish were spotted off South Devon and South East Cornwall, and for the first time off Plymouth. Changes in Atlantic ocean currents are the most likely cause.1