- Jeremy Hance writes about how jellyfish could come to dominate the world’s oceans
- Overfishing has removed fish from marine ecosystems at astounding rates, which has opened the door for jellyfish to take their place.
- Eutrophication is another human-caused change in the ocean that has likely contributed to jellyfish explosions.
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It could be a plot of a (bad) science-fiction film: a man-made disaster creates spawns of millions upon millions of jellyfish that rapidly take over the ocean. Humans, starving for mahi-mahi and Chilean seabass, turn to jellyfish, which becomes the new tuna (after the tuna fishery has collapsed, of course). Fish sticks become jelly sticks, and fish-and-chips becomes jelly-and-chips. The sci-fi film could end with the ominous image of a jellyfish evolving terrestrial limbs and pulling itself onto land — readying itself for a new conquest.
While this scenario sounds ridiculous, all of it (except the last part, of course) could conceivably come to pass. Anthony Richardson calls this the “jellyfish joyride” and it is already happening in parts of the ocean: diverse fish populations are being replaced by jellyfish.
“Dense jellyfish aggregations can be a natural feature of healthy ocean ecosystems, but a clear picture is now emerging of more severe and frequent jellyfish outbreaks worldwide,” says Richardson, from the University of Queensland and CSIRO Climate Adaptation Flagship in Australia. “In recent years, jellyfish blooms have been recorded in the Mediterranean, the Gulf of Mexico, the Black and Caspian Seas, the Northeast US coast, and particularly in Far East coastal waters.”
Once jellyfish gain a foothold, and if conditions are right, Richardson says, they can establish a massive population at the expense of other ocean life. “The problem is that jellyfish might form an alternative ‘stable state,’” he says. “What this means is that parts of the ocean might switch from being dominated by fish to being dominated by jellyfish.”
In a new study appearing in Trends in Ecology and Evolution, Richardson and colleagues explore the causes behind the jellyfish infestation and the need for swift, decisive action to stem the jellyfish takeover. Jellyfish explosions are linked directly to human actions, including overfishing, the input of fertilizer and sewage into the ocean, and climate change.
Overfishing has removed fish from marine ecosystems at astounding rates. According to Richardson this has opened the door for jellyfish to take their place. “This is because small fish (e.g. anchovy, sardine, herring) appear to keep jellyfish in check by predation (on jellyfish when they are very small) and competition (for the same zooplankton food). So, once we remove fish, jellyfish can proliferate.”
Richardson points to the example of Namibia, where “intense fishing has decimated sardine stocks and jellyfish have replaced them as the dominant species.”
Eutrophication is another human-caused change in the ocean that has likely contributed to jellyfish explosions. Eutrophication is an increase of nitrogen and phosphorous in the ocean, largely caused by fertilizer and waste runoff seeping into the oceans. This leads to algae blooms, which lower oxygen in the marine ecosystem, creating so-called dead zones, which have been increasing dramatically around the world.
According to Richardson, these low-oxygen waters give jellyfish the advantage. “Fish avoid low oxygen water but jellyfish, having lower oxygen demands, not only survive but can thrive in these conditions as there is less predation and competition from fish,” he says.
Furthermore, Richardson and his colleague speculate that climate change may expand the traditional ranges of jellyfish at the expense of other marine species. “As water warms, tropical species are moving towards the Poles. This has been documented on land and in the sea. Many venomous jellyfish species are tropical (e.g. box jellyfish and irukandji) and … could move south into more densely populated subtropical and temperate regions,” Richardson says.
The deadly box and irukandji jellyfish often cause beach closures in their native northeast Australia, and there is a concern that as the water warms they will make their way to more populous southern Australia, the paper notes.
Once jellyfish appear en masse in an ecosystem they can make it very difficult for fish to stage a comeback. By feeding on fish eggs and larvae in addition to competing with fish populations for zooplankton, the jellyfish successfully “suppress fish from returning to their normal population numbers,” Richardson says. “One can thus think of two alternate states with each being stable: one dominated by fish and the other by jellyfish. Unfortunately, when there is a jellyfish dominated state then this does not support the higher trophic levels of other fish, marine mammals, and seabirds.” In other words, an ecosystem that loses fish also loses the species that depend on fish for survival.
The study describes this state as a “monoculture of jellyfish”: an apt analogy since the situation shares similarities with other monocultures. When the rich biodiversity of tropical forests is replaced by a plantation growing a single species of tree, an area of rich variety becomes a desert in terms of biodiversity, as do ocean ecosystems when jellyfish become the dominant species.
“We need to start managing the marine environment in a holistic and precautionary way to prevent more examples of what could be termed a ‘jellyfish joyride,'” Richardson says. Due to the difficulty of turning an ecosystem around once jellyfish have gained the upper hand, Richardson and his colleagues suggest focusing on “prevention rather than cure.”
They recommend a halt to overfishing small fish that are vital to keeping jellyfish in check, such as sardines, anchovies and herrings; reducing the amount of fertilizer and sewage running off into the oceans, thereby mitigating dead zones; and, finally, confronting climate change.
“Cut our greenhouse gas emissions,” Richardson says. “This would reduce the likelihood of venomous tropical species, such as box jellyfish and irukandji, from moving into subtropical and temperate areas.”
Certainly all of these recommendations would aid marine biodiversity and ocean productivity in other ways in addition to stemming the jellyfish takeover. If not tackled, a future ocean of jellyfish could have dire economic, social and, of course, ecological repercussions.
While jellyfish are edible, it is doubtful they could ever be as rich, or as diverse, a food source as marine fish. Richardson, who has tried jellyfish, says “the best types are slightly crunchy. Not a strong taste and usually had with a sauce. Excellent diet food, as it has virtually no calories!”
Far worse than the prospect of civilization forced to turn to a jellyfish diet is that of an ocean emptied of fish, big and small, predator and prey; an ocean that no longer supports birds in great flocks or marine mammals at all. This would be an alien ocean, better suited for (bad) science-fiction than for our actual future.
CITATION: Anthony J. Richardson, Andrew Bakun, Graeme C. Hays, and Mark J. Gibbons. The jellyfish joyride: causes, consequences and management responses to a more gelatinous future. Trends in Ecology and Evolution. June 2009, Volume 24, Number 6.
This story originally appeared on Mongabay June 16, 2009 and later appeared in Jeremy Hance’s book, Life is Good. Photos were updated when this story was re-posted in November 2018.