A perceived recent increase in global jellyfish abundance has been
portrayed as a symptom of degraded oceans. This perception is
based primarily on a few case studies and anecdotal evidence, but
a formal analysis of global temporal trends in jellyfish populations
has been missing. Here, we analyze all available long-term datasets
on changes in jellyfish abundance across multiple coastal stations,
using linear and logistic mixed models and effect-size analysis to
showthat there is norobust evidence for a global increase in jellyfish.
Although there has been a small linear increase in jellyfish since the
1970s, this trend was unsubstantiated by effect-size analysis that
showed no difference in the proportion of increasing vs. decreasing
jellyfish populations over all time periods examined. Rather, the
strongest nonrandom trend indicated jellyfish populations undergo
larger, worldwide oscillations with an approximate 20-y periodicity,
including a rising phase during the 1990s that contributed to the perception
of a global increase in jellyfish abundance. Sustained monitoring
is required over the next decade to elucidate with statistical
confidence whether the weak increasing linear trend in jellyfish
after 1970 is an actual shift in the baseline or part of an oscillation.
Irrespective of the nature of increase, given the potential damage
posed by jellyfish blooms to fisheries, tourism, and other human
industries, our findings foretell recurrent phases of rise and fall in
jellyfish populations that society should be prepared to face.