Global warming is starting to sting – literally. Last week millions of baby mauve stinger jellyfish, in a swarm 26 square kilometres in area and 10 metres deep, drifted into a salmon farm in the Irish Sea, killing all 100,000 fish.
Fish farmers can do almost nothing to defend against such swarms, 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.
More swarms have been spotted along British coastlines as far north as Shetland. The mauve stinger, a Mediterranean species, has been increasingly turning up in UK waters in recent years, but this autumn’s numbers are unprecedented. What’s more, this isn’t supposed to be the season for babies.
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It is now. Warmer seawater is boosting mauve stinger numbers in the Med by increasing winter survival and perhaps lengthening the breeding season. It is also allowing them to move north.
Jellyfish in general are doing well out of global warming, not only because of warmer water and favourable winds but also because carbon dioxide has made seawater more acidic, which harms small creatures with acid-soluble shells that compete with jellyfish.
Meanwhile overfishing is removing vertebrates that eat jellyfish. The animals also gobble baby fish, preventing stocks from recovering. Fisheries scientists have warned for years that we are replacing an ocean full of fish with one full of jellyfish – which few creatures eat – meaning this could even affect the global carbon budget.
The more immediate worry is for the 107 million tonnes of fish and shellfish people eat each year. With wild fisheries maxed out, the UN Food and Agriculture Organization warned last week, the additional 37 million tonnes we will need by 2030 to feed the predicted human population will have to come from fish farms, even if existing wild fisheries stay the same. If wild fish stocks decline, that need will increase.
It’s a vicious circle. Overfishing means we need more fish farms and it also boost populations of jellyfish, which damage fish farms. As the growing human population needs more food, it exacerbates warming, and inedible jellyfish prosper. The final irony is that small plankton-eating fish, which compete most directly with jellyfish and whose decline aids them most, are also being overfished – largely to make fishmeal, the main food for fish farms.
