Jellyfish, Phyllorhiza punctata. (Photo Credit: Nick Hobgood)
Overfishing causes jellyfish abundance
Tuesday, May 07, 2013, 07:10 (GMT + 9)
A new study conducted by Institut de Recherche pour le Développement (IRD) researchers and its partners reveals that overfishing is the main factor behind the abundance of jellyfish in all of the planet’s oceans.
The findings were published in the Bulletin of Marine Science.
The overfishing of jellyfish predator species, such as tuna, are leaving jellyfish with less to worry about. Simultaneously, they are primarily benefitting from the overfishing of small pelagic fish, as they all feed off zooplankton and the fewer small pelagic fish there are, the more food is left for jellyfish and the more they can thrive.
Furthermore, because small fish eat the eggs and larvae of jellyfish, when these fish are not overexploited, their feeding habits help regulate the population of jellyfish. In contrast, when their numbers are low or nonexistent, there is nothing left to stop these gelatinous creatures from multiplying unlimitedly.
With the goal of proving the major role played by overfishing in relation to jellyfish, the researchers compared two ecosystems belonging to the same ocean current, the Benguela, which flows along the south of Africa. The first ecosystem they examined is located off the coast of Namibia, where fish management measures are lax.
These stocks are barely allowed any time to bounce back before fishing activities are given the green light to start up again. In these coastal waters, jellyfish are currently taking over.
The second ecosystem is located 1,000 km south of the first, in the waters off the coast of South Africa. This ecosystem consists of tightly controlled fisheries -- and here the jellyfish population has remained in check.
What is happening in affected areas is that jellyfish devour larval fish, something which prevents the renewal of fishery resources. As such, this invasive species threatens fisheries: in Namibia, some 10 million tonnes of sardines in the 1960s made way for 12 million tonnes of jellyfish.
This research highlights how necessary it is to take an ecosystemic approach, rather than a narrow one, towards the exploitation of the sea. The implementation of management measures must take into consideration all levels of the trophic network, not just jellyfish or a certain fishery but the whole ecosystem and how these species affect each other.
Scientists warn that this is the only way to prevent us from running out of fish and eating jellyfish in its place in the near future.
By Natalia Real