It may be unclear whether jellyfish numbers are rising globally (Nature 482, 20–21; 2012), but this should not distract us from taking urgent action to control populations in those degraded ecosystems where particular species have undeniably increased.

You mention the “paradigm” that jellyfish are increasing globally, but this consensus view is not held by the scientific community because jellyfish time series are scarce. Even with new database syntheses of scientific data, supplemented by anecdotal information from fishermen, news reports, historical research cruises and modelled reconstructions of historic ecosystems, we are unlikely to reach a consensus in the near future.

There is compelling evidence that some jellyfish species pose a risk in particular marine systems, so we believe that precautionary action should be taken now. Efforts could focus on increasing surveillance (currently scanty for jellyfish) and minimizing habitat eutrophication, overfishing and species translocation, all thought to cause jellyfish outbreaks.