How robust is the evidence that human activities cause jellyfish blooms?

By Professor Kylie Pitt

(Originally Published at: Griffith Sea Jellies Research Lab)

Pitt KA, Lucas CH, Condon RH, Duarte CM, Stewart-Koster B (2018) Claims that anthropogenic stressors facilitate jellyfish blooms have been amplified beyond the available evidence: A systematic review.  Frontiers in Marine Science doi: 10.3389/fmars.2018.00451

Sea jellies have bloomed in the oceans for more than 500 million years and are an integral part of healthy ocean ecosystems.  In some regions, however, sea jelly blooms have increased and severely impact coastal industries by clogging cooling water intakes and fishing nets and stinging beach-users.  The economic cost to individual tourism and fishing industries has been estimated at 10s – 100s of millions of dollars annually.

Melbourne power plant
The common Blue blubber clogging a cooling water intake.
Photo: 

Sea jelly populations have mostly increased along the world’s densely populated coastlines where the adjoining coastal waters are polluted with nutrients and pesticides and where artificial infrastructure, such as jetty pilings may be increasing the availability of habitat for polyps (the tiny stage of the sea jelly life cycle that lives on the sea floor).  These observations have seeded claims in the scientific literature that human activities may be exacerbating sea jelly blooms.

We were curious about how often scientific papers claim that human activities cause sea jelly blooms and what sort of evidence authors used to support their claims.

We discovered that almost half the papers published on the topic of “jellyfish blooms” contained statements that human activities caused sea jelly blooms, so our perception that such claims are widespread in the scientific literature was correct.

Catostylus bloom Moreton Bay by Jacob Gruythuysen 1
A spectacular bloom of Blue blubbers in Moreton Bay, Queensland.
Photo: Jacob Gruythuysen

When we analysed the evidence authors used to substantiate their claims that human activities cause sea jelly blooms we discovered three things:

  1. That most authors used as evidence studies of just a couple of highly invasive species of sea jellies.  Those species might be uniquely able to thrive in degraded ocean environments and are probably not typical of most sea jelly species.  An analogy would be extrapolating from studies of rats or rabbits to understand how bilbies or snow leopards might respond to human activities.
  2. Authors often cited correlative studies to support their claims. An example, might be that sea jellies were more abundant in areas polluted with nutrients.  A problem with correlations is that we don’t know if one observation is caused by the other, so correlative evidence needs to be interpreted cautiously.
  3. Finally, authors also frequently cited the same handful of studies to support their claims that human activities cause sea jelly blooms.  The studies cited (which were predominantly review papers) mainly contained circumstantial evidence and proposed conceptual ideas about how sea jellies might respond to human activities rather than robust evidence that human activities cause sea jelly blooms.

We conclude that, although human activities could enhance sea jelly populations, robust evidence is limited.  We need to use rigorous experiments and study how many different species of sea jellies respond to human activities before concluding that ‘sea jellies’ as a group thrive in degraded ocean ecosystems.

 

Blubber_Jellyfish
Blue Blubber Jellyfish. Photo: Wiki Commons

For more stories from Kylie Pitt and her team working with Sea Jellies check out the Blog at Griffith Sea Jellies Research Lab. 

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