Jellyfish could supplant fish in the world's oceans, according to experts who speak of "a global regime shift from a fish to a jellyfish ocean".
Surges in populations of jellyfish may be one of the reasons behind the fall in fish stocks seen in the Mediterranean and Black Sea, according to a report published by the Food and Agriculture Organisation of the UN (FAO).
Overfishing, which removes top predators from the sea, is one of the factors behind jellyfish blooms. A "vicious circle" can then follow in which large numbers of medusae feed on fish larvae and juveniles, and "further reduce the resilience of fish populations already impacted by overfishing," according to the report.
In addition to overfishing, the report also notes the effects of global warming, which enhances the species that thrive at tropical latitudes to span out; eutrophication, which increases nutrients in the water; the widespread use of sea walls — built to prevent coastal erosion — are also ideal habitats for those jellyfish which go through a stage as polyps in their early lives.
The severe effect jellyfish can have on fish stocks was demonstrated in the early 1980s when a jellyfish known as the Sea walnut (Mnemiopsis leidyi), normally native to western Atlantic coastal waters, was accidentally introduced into the Black Sea and had such "overwhelming" impact on fish populations that fisheries were put "on their knees".
The problem was only resolved after another invader species, Beroe ovate, which feeds on Mnemiopsis, also arrived in the Black sea, and numbers stabilised.
In the Adriatic a drop in fish populations was also observed 20-30 years ago with a successive surges of Mauve stingers (Pelagia noctiluca), which deliver a vicious sting. The combined effect of Pelagia predation and human overfishing played a large part in reducing reproductive adult fish "to a threshold that made recovery of fish populations less effective".
"In the past, the system could cope with episodes of jellyfish abundance , but in the case of the early 1980s blooms, the system went in another direction and is still not back to "normal" in pre-Pelagia years," the report states.
Usually, only the impact of human fishing activities is taken into account in setting sustainable fishing limits, but the report says the high impact jellyfish can also have on fish eggs and larvae, either directly or by competing for the same food sources, means they should also considered in any ecosystems-based approach to fisheries management.
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