Nature abhors a vacuum. Wipe out one creature, and another will move in. Mammals leveraged this principle when the reign of dinosaurs ended.

Now in a smaller way, jellyfish are taking over.

In a region off the west coast of Africa in the Atlantic Ocean, heavy fishing in recent decades has depleted fish stocks while leading to increased numbers of jellyfish.

Now with some hard data in hand, scientists are calling it a jellyfish explosion.

In fact the jellyfish are so numerous in the study area that they now represent more biomass than all the fish combined. Their numbers, ironically, are beginning to "significantly interfere with fishing operations," the researchers report in the July 12 issue of the journal Current Biology.

Andrew Brierley of the University of St. Andrews and colleagues used echosounders and trawl nets to find out what's in the water along the Namibian shelf, between the borders of Angola and South Africa. Sardines and anchovies were once abundant.

The total biomass of jellyfish in the region is now estimated to be more than three times that of fish.

Similar increases in jellyfish biomass are occurring in many locations around the world, the scientists note. Overfishing and climate change might both contribute to the phenomenon. Jellyfish have few predators, the scientists say, so if fish are depleted and nutrients are available, the jellyfish do quite well.

Meanwhile, a study last year found that jellyfish are invading the globe by hitchhiking from ocean to ocean aboard ships.