17 August
2006 - 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,
bloodless…
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.
