"Because fish and jellyfish essentially compete for similar food resources, a dramatic decline in fish populations could theoretically contribute to a substantial increase in the abundance of jellyfish," said Andrew Brierley, head of the pelagic ecology research group at the University of Saint Andrews.

"This type of shift has been predicted as a consequence of 'fishing down the food web'."

Jellyfish have been increasing in the Benguela Current off Namibia for some time but a lack of hard data on their numbers has until now hampered understanding of their likely impact on the ecosystem.

Researchers from the University of St Andrews in Scotland carried out an in-depth quantitative analysis after striking changes were reported in numbers of the large jellyfish species Chrysaora hysoscella (Compass jellyfish) and Aequorea forskalea (Medusa).

The team's findings appeared Tuesday in the journal Current Biology.

"We estimate the total biomass of jellyfish in the region to be 12.2 million metric tons, most of which is due to A.forskalea, while the biomass of fish accounts for only 3.6 million metric tons," Brierley said.

"In the past this region has offered abundant fish stocks, thanks largely to the fact that it is served by cool, nutrient rich upwelling waters," he added, referring to the Benguela Current, which originates in the Southern Ocean.

Fish stocks, including sardines and anchovies, have been heavily exploited there since the 1960s and their numbers have fallen significantly as a result, he said.

The jellyfish population in the northern stretch of the Benguela Current not only outnumbers fish in the region but is now so large that it significantly interferes with fishing operations and industrial water uptake systems, the Scottish researchers found.

The team used scientific echosounders to sample jellyfish and fish in an area of more than 30,000 nautical square miles along the Namibian shelf, between the borders of Angola and South Africa.

Brierley said there could be other factors contributing to the increase in jellyfish there.

"Jellyfish biomass has risen in numerous locations worldwide, possibly as a consequence of fishing, but it is possible that climatic changes could also contribute to jellyfish population increases."

"Also, jellyfish have few predators. Once jellyfish become established it may be very difficult to revert to previous fish domination because jellyfish are predatory on fish eggs and juveniles."

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