Though some reports suggest jellyfish are taking over the world’s
oceans, long-term records of these gelatinous animals fail to show a
global increase in jellyfish blooms likely caused by pollution, warming,
coastal development and other human influences.
While the analysis of a team of researchers who have pulled together
records of jellyfish presence going back to the 19th century don’t
support a rising gelatinous menace, the team did find a surprise: roughly 20-year cycles in the abundance of jellies.
Part of a recent rise-and-fall cycle may have prompted the perception
of a global swell in jellyfish, according to the international team,
whose researchers are part of the Global Jellyfish Group. They point
specifically to the rising phase that began in 1993 and peaked in 2004.
Blamed for stinging swimmers, clogging fishing nets, overrunning ecosystems
and wreaking other havoc, jellyfish blooms — when these animals appear
in massive numbers — have caught the attention of the media and
scientists alike. A number of research papers have suggested that not
only are blooms increasing on a global scale, but humans are likely
responsible, because humans alter the oceans in ways that favor
jellyfish. (See Stunning Photos of Jellyfish Blooms)
However, others have maintained information on jellyfish populations just isn’t sufficient to draw such conclusions.
This most recent study drew upon 37 data sets, each of which included at
least 10 years of records of jellyfish presence in an area.
Even though the records don’t evenly represent the ocean — the majority
came from the Northern Hemisphere, particularly in the Atlantic Ocean
and the Mediterranean Sea — they include all available annual
measurements, including datasets used to support work indicating
increases in jellyfish, the authors write in a study published online today (Dec. 31) in the journal Proceedings of the National Academy of Sciences.
Language has added to the challenge. In common use, the term jellyfish
lumps together organisms that can be quite different from one another.
For the purposes of this study, the researchers included records for
true jellies, the type most familiar to beachgoers; their relatives the
hydrozoans; comb jellies, which use tiny hairs, called cilia, to swim;
and another group of free-swimming invertebrates called salps.
From around 1940 to present, the records show the 20-year rising and
falling cycles. Prior to that, researchers saw signs of oscillations in
regions where data were available; however, this isn’t enough
information to draw conclusions about global patterns, said lead
researcher Rob Condon, a marine scientist at Dauphin Island Sea
Laboratory in Alabama.
Multidecadal cycles are not uncommon in nature, whether in organisms’
growth patterns and populations, or physical phenomena, such as the
oxygen concentration of the oceans.
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