A swath of Namibian waters favored by fishing fleets seems to have been taken over by jellyfish – big ones, and lots of them. A new scientific survey found more than four times as much jellyfish (by mass) than fish in a 30,000 square mile stretch of once-prime fishing grounds. And 99% of the jellyfish sample was made up Aequorea forskalea, a once-rare whopper that can be a foot or more across and weigh as much as a wet chihuahua.
Now, I’m just as enthralled by jellies’ strange beauty as the next envious landlubber. But the sudden appearance of prizewinning jellies in Namibian waters seems to have come at the expense of fish stocks – perhaps even as a price of overfishing.
The Benguela current nourishes the Namibian waters with strong upwelling that brings nutrients to the surface and kick-starts binges of productivity by phytoplankton. That starts off a feeding frenzy among all the little copepods and other zooplankton, and the party continues all the way up the food chain until, in normal circumstances, you end up with shimmering schools of fat, happy mackerel, hake, and herring. (Kathleen Wong has a nice article on upwelling in the new Bay Nature.)
Overfishing subtracts, via nets wriggling with fish, a big, visible piece of this scenario. But it adds something too: scads of choice prey in the form of uneaten copepods, larvae and small fish. Enter the jellies. You can almost see them smiling and rubbing their tentacles together.
Research cruises in the 1950s rarely reported large jellies, as Christopher Lynam (Ph.D. student, University of St. Andrews, UK) and his coauthors note in this week’s Current Biology paper. Now, an estimated 12 million metric tons in the authors’ study area are clogging power plant intake pipes, covering fish catches with a layer of stinging slime, and even fouling diamond mining equipment.
The authors suggest a more worrying possibility, too. With scores of jellyfish eating fish food as well as fish larvae, the depleted stocks of market fish may not soon get the chance to rebuild their numbers. I suppose we can only wait and see. And start working on beer batter that makes jelly-guts taste good.
Photo disclaimer: Not being able to run over to Namibia to snap a picture of A. forskalea, I used a stand-in jelly. It’s a purple-striped jelly photographed in Monterey Bay National Marine Sanctuary. From the NOAA Photo Library.