Earlier this month a nuclear power station at Oskarshamn, Sweden, had to shut down because a swarm of jellyfish had clogged the pipes it uses to extract cooling water from the Baltic Sea. Such incidents are becoming more common, for the world's jellyfish population is increasing. Some blame climate change. Some point the finger at overfishing, which both removes species that eat jellyfish and reduces competition for food. And some think the culprit is the run-off into the ocean of fertilisers, for jellyfish can out-compete real fish in such nutrient-rich, oxygen-poor waters.
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