by Carlos M. Duarte, Rob Condon & Kylie Pitt
First published in The Conversation
It’s turning out to be a great summer for jellyfish. Spectacular blooms of ‘blue blubbers’ are occurring in Moreton Bay, Queensland; Cable Beach near Broome was littered with tonnes of stranded ‘sea tomatoes’ and ‘jimbles’ are thriving in the waters of Sydney. Indeed blooms of jellies are a conspicuous feature of coastal waters throughout the world and are notorious for interfering with tourism, fishing and industries, such as desalination and power generation, depending on seawater intakes.
Increased reporting of jellyfish blooms by the media has fuelled a perception that jellyfish blooms are on the rise. Indeed, even the scientific literature regularly reports that jellyfish are increasing globally as a symptom of a degrading ocean. But are the blooms of jellyfish really increasing? Claims of a global increase have been largely inferred by extrapolating from several case studies that indicate jellyfish have increased in some regions of the world. Until now, however, a rigorous analysis of all available time series data on jellyfish, has been missing.
The Global Jellyfish Group, a consortium of experts on gelatinous organisms, climatology, oceanography, time-series analyses and socioeconomics, met regularly over the past three years at the National Center for Ecological Analysis and Synthesis, a cross-discipline ecological and data synthesis research centre affiliated with the University of California, Santa Barbara, receiving funding from NSF, to undertake the study. The group assembled all available time series data on jellyfish from around the world, to provide the first formal test of whether available data support the hypothesis that jellyfish blooms are increasing. The data set stretched back more than 200 years and contained 1,140 observation-years of jellyfish abundance. The surprising results of the study have just been published in Proceedings of the National Academy of Sciences (Condon et al. 2012).
The key finding was that globally, jellyfish populations undergo synchronous oscillations with successive decadal periods of rise and fall, including a rising phase in the 1990s and early 2000s that contributed to the current perception of a global increase in jellyfish abundance. The previous increasing phase of jellyfish populations, which occurred in the 1970s, went largely unnoticed, probably because fewer people were studying jellyfish, there was less awareness of global-scale problems, and, without the internet, there was less capacity to share information. There is, however, just a hint that jellyfish populations could be starting to increase because the most recent minimum in the time series was well above the preceding minima. This slight trend, however, was countered by the observation that there is no difference in the proportion of jellyfish populations that have increased versus decreased over time and the uncertainty in interpreting a small baseline shift against an order of magnitude larger increase as part of the cycle. Thus, confirmation of whether we are now seeing the start of an emerging trend will have to wait until we observe where the next minima in the time series will fall.
Natural long-term cycles are not a new phenomenon in nature. North American cicadas invade en mass every 17 years, tree-ring exhibit multi-decadal growth patterns, and even oceanic oxygen concentrations generated by phytoplankton production rise and fall over 20 year periods. The most pressing question, however, is how do anthropogenic practices, such as excess fossil fuel burning and increased urbanization along coastlines, compound or synergistically interact with natural oscillations to cause potential shift in these baselines. Of course, without long-term monitoring or data to analyse this is difficult to answer, but this underscores the importance of oceanic time-series programs for they enable interpretation of the baseline over appropriate spatiotemporal scales.
The realisation that jellyfish populations synchronously rise and fall around the world should now redirect researchers to search for the long-term natural and climate drivers of jellyfish populations. Moreover, the analysis also revealed regions of the world, such as the open ocean, and much of the southern hemisphere, where data are scarce and so highlights where new research efforts should be directed.
Although we found little evidence for a global rise in jellyfish, there are regions of the world where blooms have indeed increased over time, including the Sea of Japan, the North Atlantic shelf regions and parts of the Mediterranean Sea. For these regions, sustained increases in jellyfish populations continue to present problems for coastal industries and research on how to mitigate the effects of jellyfish blooms must be prioritised, including a search for drivers, such as the growth in artificial surfaces around the coast, which may provide habitats for polyps producing jellyfish or climate change, which can alter the phenology or timing of seasonal blooms.
If the global oscillations in jellyfish populations that have occurred for hundreds of years persist, there will continue to be periods in the future where jellyfish abound and coastal industries should prepare for these, as every new rising phase meet a society that is interacting with the oceans not only more intensively but also in new ways and, therefore, more vulnerable. Importantly, however, we now have a solid baseline from which to assess future changes in jellyfish populations.
Rob Condon is an Assistant Professor at UNCW. He studies jellyfish blooms, global oil spills, climate change, carbon cycles& marine food webs. He is also interested in elementary outreach & education, and runs a program called the Young Scientist Academy.