This image shows giant jellyfish (Nemopilema nomurai) clogging fishing nets in Japan. Credit: Dr. Shin-ichi Uye
SOUTHAMPTON, England -- Scientists have cast
doubt on the widely held perception that there has been a global
increase in jellyfish.
Blooms, or proliferations, of jellyfish can show
a substantial, visible impact on coastal populations – clogged nets for
fishermen, stinging waters for tourists, even choked cooling intake
pipes for power plants – and recent media reports have created a
perception that the world's oceans
are experiencing trending increases in jellyfish. Now, a new
multinational collaborative study, involving the University of
Southampton, suggests these trends may be overstated, finding that there
is no robust evidence for a global increase in jellyfish over the past
two centuries.
The results of the study, which
includes lead co-author Dr Cathy Lucas, a marine biologist at the
University of Southampton, appear in the latest issue of Proceedings of
the National Academy of Science (PNAS manuscript # 2012-10920R).
The key finding of the study shows global
jellyfish populations undergo concurrent fluctuations with successive
decadal periods of rise and fall, including a rising phase in the 1990s
and early 2000s that has contributed to the current perception of a
global increase in jellyfish abundance. The previous period of high
jellyfish numbers during the 1970s went unnoticed due to limited
research on jellyfish at the time, less awareness of global-scale
problems and a lower capacity for information sharing (e.g. no
Internet).
While there has been no increase over the
long-term, the authors detected a hint of a slight increase in jellyfish
since 1970, although this trend was countered by the observation that
there was no difference in the proportion of increasing vs. decreasing
jellyfish populations over time.
Dr Cathy Lucas, who is based at the National Oceanography
Centre, Southampton, says: "Sustained monitoring is now required over
the next decade to shed light with statistical confidence whether the
weak increasing linear trend in jellyfish populations after 1970 is an
actual shift in the baseline or part of a larger oscillation."
To date, media and scientific opinion for the
current perception of a global increase in jellyfish was evidenced by a
few local and regional case studies. Although there are areas where
jellyfish have increased; the situation with the Giant Jellyfish in
Japan and parts of the Mediterranean are classic examples, there are
also areas where jellyfish numbers have remained stable, fluctuated over
decadal periods, or actually decreased over time.
Increased speculation and discrepancies about
current and future jellyfish blooms by the media and in climate and
science reports formed the motivation for the study. "There are major
consequences for getting the answer correct for tourism, fisheries and
management decisions as they relate to climate change and changing ocean environments,"
says Dr Lucas. "The important aspect about our work is that we have
provided the long-term baseline backed with all data available to
science, which will enable scientists to build on and eventually repeat
these analyses in a decade or two from now to determine whether there
has been a real increase in jellyfish."
"The realization that jellyfish synchronously
rise and fall around the world should now lead researchers to search for
the long-term natural and climate drivers of jellyfish populations, in
addition to begin monitoring jellyfish in open ocean and Southern
Hemisphere regions that are underrepresented in our analyses," says lead
author Dr Rob Condon, marine scientist at the Dauphin Island Sea Lab (DISL) in Alabama.
Given the potential damage posed by jellyfish
blooms to fisheries, tourism and other human industries, the findings of
the group foretell recurrent phases of rise and fall in jellyfish
populations that society should be prepared to face.
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