Jellyfish "blooms" may endanger fish stocks: UN report

Surges in jellyfish populations due to overfishing may be one of the reason behind the drop in fish stocks observed in the Mediterranean and Black Sea, according to a new UN report.
Overfishing, which removes top predators from the sea, is one of the factors behind jellyfish "blooms", or suddenly increased numbers, the report published by Food and Agriculture Organisation of the United Nations (FAO) said.
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.

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".

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.