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Are Jellyfish Blooms Increasing? Two Studies Reach Different Conclusions

Giant Nomura’s jellyfish – which can weigh up to 200 kilograms (440 pounds) – are seen here clogging a fishing net in Japan Shin-ichi Uye

With recent media accounts of jellyfish blooms so large and dense they impede commercial fishing operations, decimate farmed fish stocks, and force nuclear power plants to shut down by clogging the water inlets, public concern over an apparent proliferation of jellyfish has grown. Indeed there is overwhelming evidence that jellyfish numbers have increased in some regions, with human factors likely playing an important role. Some regions, on the other hand, have seen a decline in jellyfish numbers. Various scientific researchers, meanwhile, have suggested that a variety of human activities such as overfishing, nutrient pollution, ocean warming and acidification, and species translocation may, if unchecked, lead to dramatic increases in jellyfish on a global scale. 

Two recent studies now revisit the issue. The first, published in BioScience by Robert Condon and 16 other researchers affiliated with the Global Jellyfish Group, is based on an expansive review of the data available on jellyfish populations and concludes that any global increase in gelatinous zooplankton (jellyfish) cannot be substantiated. Condon’s group does not deny the possibility that jellyfish numbers are increasing. But it cites a wide range of deficiencies in the information available about jellyfish, as well as factors that may lead to misconceptions of current trends in global jellyfish populations. In the former case, for example, they point to the very few long-term (more than 20 years) records of jellyfish abundance and distribution.

Contrasting the above assertion is a study published in Hydrobiologia by Lucas Brotz of the University of British Columbia and colleagues who conclude that, “jellyfish populations appear to be increasing in the majority of the world’s coastal ecosystems and seas.” To reach this verdict Brotz and team examined accounts of jellyfish blooms from 1950 onwards, drawing from a wide variety of sources, both scientific and anecdotal. Accounts were grouped together into different “jellyfish chronicles” depending on the locations in which they occurred, which were divided into Large Marine Ecosystems (LMEs). After they had been grouped by LME, each individual account was analyzed by fuzzy logic for reliability, temporal coverage, and spatial coverage. Using the fuzzy logic system ensured that information from reliable sources such as peer reviewed journal articles had a greater influence on the data than more casual accounts, and permitted conclusions with varying degrees of certainty to be formed about jellyfish populations in the different LMEs. Out of the 45 LMEs covered in the study, 28 exhibited increasing jellyfish population trends, three showed decreasing population trends, and the rest remained relatively constant. Among the LMEs with increasing jellyfish populations, 10 (21 per cent of the total area analyzed) were classified as “high certainty”. Only data that documented changes (or the absence of change) in jellyfish populations over the period of several years or more were included, and sporadic or isolated bloom events were omitted to minimize reporting bias, making the findings as reliable as possible given the heterogeneity of information sources.

As both papers concede, there remain large gaps to be filled in our understanding of jellyfish, their population dynamics, and their roles in marine ecosystems. In a separate letter to Nature one of the co-authors of the Hydrobiologia paper, Daniel Pauly, along with two colleagues, conclude that, “[W]e 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.”

Sources: 

1. Brotz, L. et al.  In press.  Increasing jellyfish populations: trends in Large Marine Ecosystems. Hydrobiologia. 
Open Access:  http://www.springerlink.com/content/h2m74376448540r8/fulltext.pdf

Contact:  Lucas Brotz, Fisheries Centre, University of British Columbia, Vancouver 
E-mail:  lucasbrotz@gmail.com

2. Condon, R.H. et al.  2012.  Questioning the rise of gelatinous zooplankton in the world’s oceans. BioScience 62(2): 160-169.

Contact:  Robert Condon, Dauphin Island Sea Lab, Dauphin Island, Alabama  
E-mail:  rcondon@disl.org

3. Richardson, A.J. et al.  2012.  Degraded ecosystems: Keep jellyfish numbers in check.  Nature 483(7388): 158.         

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