Fears of a global ‘jellyfish invasion’ of the oceans may not hold water
Like a karmic device come to punish our planetary transgressions, jellyfish thrive on the environmental chaos humans create. Is the age of the jellyfish upon us?
Another British summer, another set of fear-mongering headlines about swarms of “deadly” jellyfish set to ruin your holiday. But news that jellyfish numbers may be rising carries implications far beyond the interrupted pastimes of the sunburnt masses.
Like a karmic device come to punish our planetary transgressions, jellyfish thrive on the chaos humans create. Overfishing wipes out their competitors and predators; warmer water from climate change encourages the spread of some jellies; pollution from fertilisers causes the ocean to lose its oxygen, a deprivation to which jellyfish are uniquely tolerant; coastal developments provide convenient, safe habitat for their polyps to hide. In addition, the great mixing of species transported across the world in the ballasts of ships opens up new, vulnerable ecosystems to these super-adaptors.
“They’ve got this unique life cycle where they can tolerate harsh conditions and then rapidly thrive when conditions are favourable. So when a stressor like climate change or overfishing opens up a niche for them they can really take advantage of that and rapidly proliferate,” said Lucas Brotz, a researcher at the University of British Columbia. Not all species of jelly benefit, rather there tends to be a reduction in the diversity of species and vast, homogenous masses emerge.
“They can make millions and millions of copies of themselves and clone asexually. That’s when you get these massive blooms. I think that’s the secret to the success of jellyfish, the reason they’ve been around for hundreds of millions of years.”
The latest flurry of “jellyfish invasion” stories was spawned by an announcement by the UK’s Marine Conservation Society (MCS) that their annual survey of UK waters for 2015 was looking like being a ‘record-breaker’.
The problem is there aren’t many records to break. The MCS survey has only run for 12 years, not enough to identify a strong trend. Further confusing the issue, the MCS study relies on members of the public for its data. Citizen science is a wonderful tool but adds extra uncertainty — the society admits it cannot be sure how much of the growth is due to more jellyfish and how much is people becoming aware of their survey and reporting to it.
Even so, the charity’s jellyfish expert Dr Peter Richardson said climate change and overfishing were fundamentally changing the ocean.
“At the same time we seem to be witnessing increases in jellyfish around the UK. Is this an anomaly, a coincidence, or are the jellyfish telling us something?” he asked.
Are these British blooms harbingers of the age of the jelly? Globally, jellyfish are little surveyed and their numbers vary wildly from year to year for reasons not entirely understood. Because of the paucity of historical records, jellyfish experts are hesitant about whether a global trend exists.
But Priscilla Licandro, a researcher at the Sir Alister Hardy Foundation for Ocean Science, said in the few places where decent data exists, populations of jellyfish tend to be rising.
“There is not very much reliable information, but from what we’ve got, at least at a regional level, there is an increase in the persistence and the occurrence,” she said. In the Mediterranean Sea, where records date back some 200 years, natural cycles of boom and bust have been replaced by a constant, large presence.
“[In the Mediterranean] there have been changes in the last 15 to 20 years that haven’t been seen for two centuries,” she said.
Similar increases in the northeastern Atlantic seem to support the MCS’s more anecdotal findings.
The links between human activity and local jellyfish blooms are strong. In the Black Sea, invasive comb jellies dumped from the ballast of tankers have spawned deliriously and destroyed the region’s fishing industry.
In the Sea of Japan, fertilizer run-off has left an oxygen-depleted sea where little other than jellies can thrive.
But aside from these regional observations, Mark Gibbons, a zoologist at the University of the Western Cape, said the evidence to support a global trend was still patchy.
“Whether there is strong evidence of a global increase in jellyfish populations [now] is difficult to answer. Certainly in some coastal systems there have been increases but in others there have not – or at least the background data with which to measure change are absent or scant, so it is hard to say,” he said.
Other researchers, including Steven Haddock from the Monterey Bay Aquarium Research Institute, believe the current observed rise may represent a natural cycle.
But scientists agree that the continued abuse of our oceans is leading us down a road where fish and other marine animals fail and jellies win.
“In all likelihood, owing to the factors that you have listed, we are going to see more jellyfish,” said Gibbons.
However, said Brotz, “we don’t want to be too quick to say that jellyfish are this evil plague, or the cockroaches of the sea or anything like that. Jellyfish are these beautiful, amazing creatures that play a very important role in the ecosystem.”
To vilify them would be to miss the ultimate lesson: that a future of oceans filled with swarms of gelatinous beasts will not be a jellyfish apocalypse, but a human one.