Global warming has begun to sting literally. Millions of baby mauve stinger jellyfish, in a swarm 26 square kilometres in area and 10 metres deep, drifted into salmon farm in the Irish Sea, killing 100,000 fish.

Fish farmers are left helpless to defend against such predators, as small jellyfish – or the tentacles of larger ones, detached but still stinging – are swept by currents into the salmon cages, where the fish die of stress. Additional swarms have been sighted along the British coastlines as far north as Shetland. The mauve stinger, a Mediterranean species, has been increasingly turning appearing in UK waters in recent years, but this autumn’s numbers are unprecedented. What’s more, this isn’t to be the season babies. 

It is now Warmer seawaters are boosting mauve stinger numbers in the Med by increasing the breeding season. It is also allowing them to move north. 

Jellyfish are prospering out of global warming, not only because of warmer water and favourable winds but also because carbon dioxide has made seawater more acidic, which adversely affects small creatures with acid-soluble shells that compete with jellyfish. 

Over fishing is removing vertebrates that eat jellyfish. The animals also eat baby fish, preventing stocks from replenishing. Fisheries scientists have warned for years that we are replacing an ocean full with one full of jellyfish-which few creatures eat – meaning this could adversely affect the global carbon budget. 

The present problem is for the 107 million tonnes of fish and shellfish people eat each year. With wild fisheries maxed out, the UN Food and Agriculture Organisation warned, additional 37 million tonnes will be needed by 2030 to feed the predicted human population will have to come through fish farms, even if existing wild fisheries stay the same. If wild fish stocks fall, that will increase. 

It’s a vicious circle. Over-fishing means we need more fish farms and it also boosts population of jellyfish, which damage fish farms. As the growing human population requires additional food, it exacerbates warming, and inedible jellyfish prosper. The final irony is that small planktons-eating fish, which compete most directly with jellyfish and whose decline aids those most, are also being over fished – largely to make fish meat, the main food for fish farms.