Swarming jellyfish dampening the holiday spirit
Although swarming is a normal part of the lifecycle of jellyfish, this year’s frequency and extent is unusual. Alarmingly, it points to underlying changes in Europe’s marine ecosystems resulting from a combination of climate change, nutrient enrichment, tourism and fishing. Fishing has indirectly helped jellyfish to thrive. As larger fish are removed from the food chain, jellyfish face less competition for their own food. Also, climate change has brought warmer seawater and increased salinity, improving conditions for jellyfish growth. And inputs of nutrients into the sea from industry, households, and agriculture have increased the plankton population on which jellyfish feed.
Boneless, brainless and bloodless yet successfully ocean-going for millions of years, jellyfish are creatures people find both fascinating and disgusting. While smaller jellyfish may feed on food particles in the water, larger species are predators. Competing with fish for food, larger jellyfish have thousands of trigger and stinger mechanisms on their tentacles making them perfectly geared for hunting and defence.
European jellyfish are not deadly. However, some species can harm humans by direct contact with their toxins or indirect allergic reactions provoked by stings. The Portuguese-Man-o-War, which is not a jellyfish but a floating colony of organisms, is very harmful. And on rare occasions, its sting has been fatal. Found in European waters, the Portuguese Man-o-War usually lives out in the open sea, away from beaches and does not appear to swarm. However, single colonies have been known to hide among swarms of other species.
Marine ecosystem changes
Jellyfish have a seasonal lifecycle. Attaching themselves to rocks for part of the year, Jellyfish then swarm in the summer months. The effect of human activities on marine ecosystems and coastal regions exaggerates this natural cycle and can lead to enormous swarms suddenly appearing on Europe’s shores. Moreover, the economic and ecosystem effects of jellyfish swarms are not unsubstantial with disruptions to industry and tourism often resulting. The Swedish nuclear power plant Oscarshamn had to close down a reactor on 29 August last summer following an accumulation of jellyfish in the cooling water. Fish farms in Scotland and Shetland have seen thousands of salmon in jellyfish-clogged fish cages die from lack of oxygen. Several Spanish beaches have had to close down this summer as the waters around the Costa del Sol, the Costa Blanca, the Costa Brava and the Balearic Islands are infested by large numbers of jellyfish.
Although some jellyfish are eaten by fish, most fall prey to other species of jellyfish. Therefore, an independent and relatively stable food web of interlinked gelatinous predators exists. This stable food web or jellyweb can accommodate massive long-term changes within Europe’s marine ecosystems
Relevant EEA reports and information:
The changing faces of Europe’s coastal areas: http://reports.eea.europa.eu/eea_report_2006_6/en/eea_report_6_2006.pdf
The European Environment - State and Outlook http://www.eea.europa.eu/Highlights/20051122115248
Impacts of Europe’s changing climate (chapter 3.4.) http://reports.eea.europa.eu/climate_report_2_2004/en/climate_change_pda.pdf Nutrients in European ecosystems http://reports.eea.europa.eu/ENVIASSRP04/en/enviassrp04.pdf