Surges in jellyfish populations may be one reason for a drop in fish stocks observed in the Mediterranean and Black Sea, according to a new report “Review of Jellyfish Blooms in the Mediterranean and Black Sea” published by FAO .

While on the other hand overfishing, which removes top predators from the sea, is one of the factors behind jellyfish surges. 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," the report states. 

According to the specific report, only the impact of human fishing activities is normaly  taken into account in setting sustainable fishing limits. Jellyfish can also have a high impact on fish eggs and larvae though. Therefore it is proposed to be considered in any ecosystems-based approach to fisheries management.

Overwhelming impact
The enormous effect jellyfish can result on fish stocks was demonstrated in the early 1980s. Mnemiopsis leidyi, a jellyfish species normally resident on the Atlantic, was then accidentally introduced into the Black Sea.  The impact on fish populations affected greatly the fisheries. 

"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 ocean
Jellyfish have lately become persistently abundant almost in all oceans of the world. So there are experts speaking of  "a global regime shift from a fish to a jellyfish ocean" in which fish are replaced from jellyfish. 

The drivers are not fully understood, but they may include also:

• 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.

The review also underlines the importance of monitoring and the ability to forecast blooms and highlights the “Citizen Science” method as an alternative to evaluate the presence and abundance of medusa species. (Article’s Original Source) 

PERSEUS jellyfish spotting webpage presents more information about the "Citizen Science" method, short descriptions and photos of medusa species commonly seen in the Mediterranean and Black Sea and offers easy access for participation to everyone in these areas.