The impact of heavy fishing in the Atlantic Ocean has resulted in a ‘jellyfish explosion’ off Namibia. Scientists at the University of St Andrews have found that the weight of jellyfish in Benguela now by far exceeds that of the once prolific commercially important fish stock in the area.
Though an increasing abundance of jellyfish in the area had been noted for some time, a lack of hard data on jellyfish abundance has until now hampered understanding of the likely ecosystem impact of the jellyfish explosion.
The St Andrews’ researchers, led by Dr Andrew Brierley, carried out an in-depth quantitative analysis after striking changes in numbers of the large jellyfish species Chrysaora hysoscella and Aequorea forskalea had become more commonly reported.
Dr Brierley, leader of the University’s Pelagic Ecology Research Group, said: “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.”
“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 The fish stocks, including sardines and anchovies, have been heavily exploited since the 1960’s, and have been strongly depleted in the process. 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. This type of shift has been predicted as a consequence of ‘fishing down the food web’.
The increased jellyfish population in northern Benguela now 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 team used scientific echosounders to sample jellyfish and fish in an area of over 30,000 nautical miles along the Namibian shelf, between the borders of Angola and South Africa.
Dr Brierley continued: “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.”
The study is published by the 12th July issue of the journal Current Biology, out today.
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