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[WINDHOEK] Namibia has dismissed claims by scientists that an "explosion" in jellyfish numbers off the country's coast is due to overfishing. It says the trend is instead due to 'regime change' among marine species.

"This is a normal occurrence," said Moses Maurihungirire, director of resource management at the Ministry of Fisheries and Marine Resources, in an interview with SciDev.Net yesterday (16 August).

"Sometimes you find that a certain species becomes dominant due to migration."

Last month, researchers led by Andrew Brierley of the University of St. Andrews in Scotland published a study in Current Biology that said the mass of jellyfish off Namibia's coast was nearly four times greater than that of fish.

The team suggested that years of overfishing had caused a dramatic decline in fish, which compete with jellyfish for food. With fewer fish around, the jellyfish rapidly increased in number.

"The weight of evidence suggests that the jellyfish population has bloomed on exactly the same time scale as the commercially important fish stocks have collapsed," Brierley told SciDev.Net. "Thus, in our opinion, fishery collapse is a proximate cause of the jellyfish boom."

Brierley's team assessed the amount of jellyfish and fish in an area of more than 30,000 square nautical miles off the Namibian coast.

They say that the jellyfish population in the northern stretch of the study area is now so large that it significantly interferes with fishing operations and industrial water uptake systems.

Maurihungirire denies that Namibian fishing vessels are to blame. He told SciDev.Net that Namibia had the world's best mechanism for monitoring overfishing.

"Each fishing vessel has a scout on board to monitor that it does not exceed its quota," he said.

Fish stocks, including sardines and anchovies, have however been heavily exploited in the region since the 1960s. This year, the Namibian government reduced the total allowable catch for all fish species to allow the stocks to recover.

Maurihungirire agrees that the jellyfish pose a problem as they consume fish eggs and juveniles, especially those of sardines. He says the ministry is investigating whether the jellyfish could be exploited for commercial purposes.

Link to full paper in Current Biology