Jellyfish, seemingly simple organisms belonging to a large class of invertebrates, can be found in all oceans around the world. In recent years, they have attracted global concern as we are witnessing more frequent and massive blooming of jellyfish. There are even warnings that jellyfish might be taking over the oceans from fish. On the other hand, due to their low economic value as compared to fish, there has been little research and many mysteries remain about jellyfish ecology and life cycles. How are we to stop the “jellyfish attack”?
Jellyfish play multiple roles in the marine ecosystem. They have particularly close relationship with fishes. Immature jellyfish prey on phytoplankton, competing with small fish. Adult jellyfish, on the other hand, consume fish larvae and juveniles, while itself being food for larger fish. Researchers of jellyfish blooms have summarised three food chains that describe the competition between jellyfish and fish:
Under natural conditions, the primary food chain would be the first one, dominated by fish; whereas the jellyfish-dominated second and third food chains only occasionally take over. The short-term blooming of jellyfish is a natural phenomenon that helps control the dominant fish species and balance fish diversity. Unfortunately, human activities have interfered with the natural interaction by prolonging the time of jellyfish blooms and increasing jellyfish populations, with disastrous consequences.
Firstly, overfishing by humans has decreased the populations of fish that prey on and compete with jellyfish. The thriving jellyfish further reduce fish populations by consuming huge amounts of fish larvae and juveniles, entering a vicious cycle.
Secondly, large scale carbon dioxide emissions from human activities have altered the global climate. Rising sea temperatures are favorable to the reproduction of tropical and temperate jellyfish species. Overuse of fertilisers in agriculture has also caused eutrophication and blooming of phytoplankton that indirectly provide more food for jellyfish. Furthermore, natural coasts are being increasingly replaced by artificial banks. Their cement surfaces are suitable for jellyfish hydroids to attach and grow into jellyfish.
A solution in delicacies?
Frequent occurrence of jellyfish blooms badly hit the fishery industry. Even fishing vessels can be damaged by the unassuming creatures. In Japan, years ago, a fishing vessel was capsized by a net filling with a massive number of the giant Nemopilema nomurai. If poisonous jellyfish are present, public health is threatened - as is local tourism. Perhaps more profoundly, jellyfish invasions can clog the cooling systems of nuclear power plants and industries in coastal areas that utilise seawater for cooling. Severe accidents may happen if the cooling systems are hampered.
Australian scientists have suggested controlling jellyfish populations by eating them - and exploring more edible jellyfish species. In fact, since ancient times, jellyfish have been on the plate of Chinese cuisine. However, there are problems to this proposal. For one, the oceans have been widely contaminated by microplastics and other sources of pollution - which accumulate in the bodies of jellyfish. If the jellyfish are consumed by humans, the pollutants will enter the human body and pose a threat to our health. Besides, edible jellyfish species account for only a small proportion of the jellyfish population. If selected species are consumed, we may cause unexpected results in the ecosystem.
So far, global scientific data is still inconclusive as to whether jellyfish will overrun the oceans in future. It is a warning, nonetheless, that human factors are at the root of the problem. We must prevent overfishing, reduce carbon dioxide emissions and apply fertilisers appropriately … to minimise our impacts and restore the health of the ocean ecology. This is the right course to resolving the “jellyfish attack”!