Although large numbers of jellyfish can occur in healthy marine ecosystems, destructive jellyfish blooms are being observed more frequently in the Mediterranean, Black and Caspian Seas, the Gulf of Mexico and the Northeast US coast. There have been particularly severe incidences in the Far East. The Sea of Japan, for example, has seen outbreaks of the giant Nomura jellyfish, each of which can reach 2 metres in diameter and weigh up to 200 kilograms.
Jellyfish blooms have serious consequences. As well as pushing other marine creatures out of an ecosystem, they can cause injury, or even death, to bathers; burst fishing nets; kill fish in fish farms; block coastal power station and industrial inlets; and cause loss of tourism revenues through beach closures. In Europe, the comb jelly Mnemiopsis leidyi, introduced to the Black Sea in the mid-1980s, has had devastating effects. Consuming vast quantities of zooplankton, including fish larvae, an explosion in the population has caused huge damage to the marine ecosystem, including commercial fish stocks.
Partly funded by the EU Ecojel project1, the research suggests this 'jellyfish joyride' is associated with a number of damaging human influences. These include overfishing, high nutrient levels in coastal waters from fertilisers and sewage (which cause algae blooms and eutrophication), ship ballasts moving jellyfish from their natural habitat to new areas, a warming climate and damage to marine habitats. It has also been suggested that acidification of seawater, the result of increased atmospheric carbon, could promote jellyfish infestations, but more research is needed to determine this.
Jellyfish survive harsh conditions much better than many fish species for a number of reasons. They have a varied diet so can take advantage of poor food supplies, a rapid rate of growth,(faster than most small fish), the ability to survive starvation by shrinking in size and then growing again when food becomes more plentiful (whereas fish become weaker) and a tolerance to low-oxygen zones (the result of eutrophication) where few jellyfish predators can survive.
Small filter-feeding fish compete with jellyfish for food and also prey on more vulnerable jellyfish. Continual fishing of these predatory fish could reach a tipping point where jellyfish start to overwhelm the ecosystem. Growing jellyfish populations would, in turn, eat more of the eggs and larvae of these predatory fish. Jellyfish could then form an alternative stable state, where they displace fish and prevent them from returning in large numbers by eating their eggs and larvae.
Jellyfish infestations could invade further habitats and again overwhelm resident predators, creating a situation where the hunted becomes the hunter. Combined with other human stressors, such as eutrophication, together with a warmer climate, marine ecosystems could become an environment more supportive of jellyfish than fish.
The researchers suggest a combination of short- and long-term responses and future research initiatives are needed to manage the problem. Among these are: encouraging people to eat more jellyfish products, currently popular in Asia, and investigating their medicinal properties; recent research has found that jellyfish could suppress arthritis, for example.
Long-term observation programmes and biocontrol measures for jellyfish (jellyfish are under-researched) also need to be established and hard underwater surfaces where polyps (a stage in the lifecycle of some jellyfish) attach need to be cleaned. However, reducing overfishing and eutrophication and supporting measures to reduce global warming are considered to be the most effective long-term measures for controlling jellyfish populations.