Overfishing, which removes top predators from the sea, is one of the factors behind jellyfish "blooms", or suddenly increased numbers. 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, from FAO's General Fisheries Commission for the Mediterranean.
Jellyfish "might be the proverbial straw that broke the camel's back" says the Review of Jellyfish Blooms in the Mediterranean and Black Sea.
Normally, only the impact of human fishing activities is taken into account in setting sustainable fishing limits, the report says. But jellyfish can also have a high impact on fish eggs and larvae, either directly or by competing for the same food sources. They should thus be considered in any ecosystems-based approach to fisheries management.
The severe effect jellyfish can have on fish stocks was demonstrated in the early 1980s when Mnemiopsis leidyi, a jellyfish species normally resident on the Atlantic, 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.
In the Adriatic a drop in fish populations was also observed 20-30 years ago with a successive surges of mauve-coloured, Pelagia noctiluca jellyfish that deliver a vicious sting. The combined effect of Pelagia predation on the one hand and human overfishing on the other 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.
Jellyfish have now become persistently abundant in almost all the oceans of the world, leading some experts to speak of "a global regime shift from a fish to a jellyfish ocean" in which jellyfish supplant fish.
Although the reasons for the phenomenon are not fully understood, they may include, besides overfishing:
- Global warming, which enhances species that thrive at tropical latitudes.
- Eutrophication, which increases nutrients in the water.
- Widespread use of sea walls to prevent coastal erosion and the large number of tourist harbours, which make an ideal habitat for those jellyfish who go through a stage as polyps in their early lives.
Measures advocated to prevent or cope with jellyfish blooms include:
- Incorporate jellyfish research into fisheries research.
- Develop jellyfish products for food and medicine. Some jellyfish species are a food source in several countries.
- Among other possibilities, the discovery of an "immortal jellyfish", Turritopsis nutricula, capable of reversing its ageing process, holds out the promise of developing powerful rejuvenation products for humans.
- Establish early warning systems of jellyfish blooms, with protective barriers for aquaculture farms.
- Taking steps to reduce overfishing, greenhouse gas emissions and the causes of eutrophication "would undoubtedly improve environmental quality at large and might, thus, also reduce the present prevalence of jellyfish," the report adds.