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A different catch

Majorca is a Mediterranean Island hugely popular with European tourists. This year, visitors lounging on its beaches may notice an unusual number of fishing boats sweeping back and forth just offshore. This level of activity might surprise tourists who have heard of recent declines in European fish populations, especially hard pressed nearshore stocks in the Mediterranean. The solution to this conundrum is that the boats are after jellyfish, not fish. Jellyfish are a bit of a spoiler for a beach holiday, particularly the stinging variety. In the Mediterranean, Pelagia noctiluca or the mauve stinger, is notorious and especially prone to blooms. Swarms pushed inshore by wind and tide can clear the sea of bathers almost as fast as a shout of ‘shark'! Sometimes beaches have to be closed for days. In the heavily polluted Adriatic Sea east of Italy, great heaps of jellyfish are occasionally thrown onto beaches where they expire and putrify in the sun. Should such blooms threaten Majorcan beaches, the hope is that they will not make it ashore. Forty fishing boats are on standby throughout the summer to catch jellyfish if the need arises. Plagues of jellyfish have become a regular phenomenon in places where fish have been overexploited and nutrient pollution fertilises the ocean. Overexploitation of fish reduces predation on jellyfish and raised nutrients stimulate plankton growth. Both conditions apply in the Mediterranean. There coastal fish populations are among the most overexploited in the world having been subject to commercial fishing since antiquity, and nutrient pollution is high from agricultural runoff, industrial effluents and sewage (much of it from the millions of visitors). As a result, the region often suffers serious outbreaks of jellyfish and other gelatinous zooplankton. Increased jellyfish numbers can pose problems for fish stocks. In the North Sea, favourable climatic conditions in the early 1970s led to high abundances of jellyfish. Jellyfish prey on fish larvae and compete with them for other zooplankton food. The outbreak helped precipitate the collapse of herring stocks in the North Sea putting many fishers out of work. Aquaculture facilities are also at risk. Last year, more than 100,000 salmon were wiped out in an Irish fish farm after a ten square mile swarm of mauve stingers swept inshore. There have always been outbreaks of jellyfish and always will be. Animals that can put on 10-20% of their body weight every day soon become a nuisance when conditions favour. But the increased frequency and severity of outbreaks today is a human problem the cure for which is for us to fish and pollute less. For the past century or two, fishing has been simplifying aquatic food webs and in many places has reduced the abundance of large fish more than 10-fold. Complex food webs afford fewer opportunities for outbreaks of animals like jellyfish than simplified ones. We see this truth all around us on farmland where a few species of plants dominate and chemicals must be used to control pest and weed outbreaks. We have little recourse to chemicals in the sea so must instead change the way we manage ocean resources. Instead of thinking only of what we can extract from the sea, we need to rebuild the abundance, variety and complexity of life. In such a world, Majorcan fishers could concentrate on catching fish, not jellyfish. ———- Callum Roberts is Professor of Marine Conservation at the University of York in England and the author of The Unnatural History of the Sea. Click here to visit his website.