Jellyfish Thrive in Overfished Seas

By Robin Meadows

Jellyfish as wide as dinner plates now dominate Namibian waters where fish once teemed, according to a recent report in Current Biology.

“This is a profound ecosystem change, with possible consequences to fish stock recovery,” say Christopher Lynam of the University of St. Andrews in Fife, Scotland, and six coauthors.

Masses of large jellyfish cause all sorts of troubles, from burst trawl nets to clogged coolant intakes at power stations. The ascendance of jellyfish may also bring ecological woes. This is because jellyfish, besides competing for the same food as fish, also eat fish eggs and larvae. Moreover, say the researchers, “Ecosystem shifts from dominance by fish to dominance by jellyfish may be irreversible. ”

Virtually unheard of in the 1950s, two jellyfish species, Aequorea forskalea and Chrysaora hysoscella (about 13 and 27 centimeters in diameter, respectively), have bloomed off Namibia since fish stocks there began collapsing in the 1960s. Landings of sardines (Sardinops sagax) and anchovies (Engraulis encrasicolis) fell from 17 million tons (MT) in the late 1970s to only one MT today. Fewer large fish meant more food was available for jellyfish, which in turn meant more jellyfish.

Now the researchers have estimated that the biomass of jellyfish on the Namibian shelf far outweighs that of commercial fish stocks, at 12.2 vs. 3.6 MT, respectively. These findings are based on trawl net sampling and echosounder surveys over the nearly 34,000 square nautical miles of the shelf from Angola to South Africa. Echosounders beam sound waves downward toward the sea floor; the waves then scatter and bounce back up as they hit groups of creatures in the water column. The sound waves cover a range of frequencies, and jellyfish can be distinguished from fish because they scatter various frequencies to different extents.

Jellyfish biomass has also increased in several other parts of the world, including the Bering Sea and the Yellow Sea, and there is evidence that coastal outbreaks may be linked to warmer oceans.

Lynam, C.P. et al. 2006. Jellyfish overtake fish in a heavily fished ecosystem. Current Biology 16:R492-R493.

photo: Chrysaora hysoscella jellyfish – Yves GLADU /