Since the beginning of the 2000s, jellyfish have overrun the world's seas; the Mediterranean, Japan and Black seas. Is this a periodic phenomenon or something caused by global warming? These questions still haven't been answered. A recent research completed by the IRD and its partners reveals overfishing to be the major cause.
An endless rein
Since the numbers of the jellyfish predators have decreased due to overfishing, jellyfish are taking advantage of the small fish being overfished, like sardines, herring and anchovies. Jellyfish feed on zooplankton. In the areas that are being overfished, the jellyfish are thriving. Plus the small fish also feed on jellyfish eggs and larvae. Therefore under normal circumstances the small fish would be regulating the jellyfish population.
Examples as proof
To show the chief role played by overfishing, resources correlated two ecosystems that belonged to the same ocean current in Benguela, which flows along the south coast of Africa. The first ecosystem was located in Namibia where fish stock management is not contained. Here the fish aren't able to reproduce in time so jellyfish dominate this area. The second ecosystem is 1000 km south, off the coast of South Africa. Here fishing has been restrained for 60 years and jellyfish populations have not increased.
Businesses are affected
Under the waves, the links in the food chain are flexible. Prey sometimes feed off their predators. For example jellyfish eat larval fish. In the 1960s in Namibia, 10 million sardines made way for 12 million jellyfish. Jellyfish are a bother for tourists. The sting from their poisonous filament, though rarely fatal, puts many economic activities at risk. This is especially true in development countries.
Some fast facts on jellyfish
- Jellyfish don't have brains, hearts or teeth
- Jellyfish are made up of 98% water, which is more than the adult human
- The majority of jellyfish are carnivorous