THE jellyfish has no brain, no bones and no teeth.
Although sensitive to light, it lacks eyes. Despite such handicaps, it has among its ranks some of the world’s nastiest creatures.
Most species are harmless but a few can inflict lethal stings on creatures as large as us. Box jellyfish are more dangerous than sharks. Jellyfish ‘blooms’ are reported from time to time off Irish beaches.
In 2005, a lion’s mane invasion made bathing impossible along the east coast. Two years later, thousands of mauve stingers drifted into a fish farm at Glenard, Co Antrim, killing 100,000 salmon. Similar incidents have occurred elsewhere. For the last 10 years, billions of giant jellyfish, some weighing 200kg and two metres in diameter, have disrupted fishing in the Sea of Japan.
Is this a new phenomenon or just one not reported in the past? Do jellyfish numbers fluctuate on a long-term cycle? If there is a population explosion, what is causing it? Is global warming, leading to increased ocean temperatures and changes in currents, a factor?
A new report places the blame elsewhere. The culprit, according to France’s Institut de recherche pour le développement (IRD), is our growing demand for fish.
The Benguela is a cold oxygen-rich current, which flows up the coast of southern Africa from the Cape to Angola. Teeming with phytoplankton, it sustains huge populations of fish. IRD scientists compared jellyfish numbers at two locations along the Benguela, one off Namibia, the other in South African waters. Fishing is poorly controlled by the Namibians and stocks there become exhausted from time to time. When this happens, the boats leave the area only to return when fish populations recover. Off South Africa, 1,000km to the south, the situation is different; the fishery is protected and fish stocks are maintained.
Jellyfish numbers, the researchers found, have remained stable off South Africa, whereas they have increased in the over-fished Namibian waters.
The Windhoek Observer quotes Philippe Cury of the IRD; ‘In the 1960s, the waters off Namibia used to yield a million tonnes of sardines annually. This has been replaced by 12 million tonnes of jellyfish’.
The Ministry of Fisheries in Namibia challenges the claim. “The study is not conclusive because we have yet to develop a quantitative way to measure and record jellyfish populations” a spokesman said.
Jellyfish eat phytoplankton, which is also the food of small fish such as sardines, anchovies and herrings. When the sardine populations are wiped out by fishermen, the IRD scientists argue, competition for food is reduced and jellyfish numbers soar.
Small fish eat the eggs and larvae of jellyfish. With the demise of the sardines, more young jellyfish survive and this reinforces the population explosion. Then the jellyfish turn the tables on their tormentors; they eat the larvae of fish. In the marine environment, hunters can become victims of their prey.
The main jellyfish seekers, tuna and sea turtles, are in trouble. Demand for tuna is increasing and tuna fishing has become a huge industry worldwide. Some species of ‘bluefin’ are so over-fished their populations are close to collapse. Numbers everywhere are down and jellyfish benefit. The plight of sea-turtles is even more critical.
Ironically, a penchant for jellyfish is a factor in their demise. Turtles, such as the giant leather-backs which visit Irish waters annually, mistake discarded plastic bags, drifting in the sea, for jellyfish. When a turtle swallows a bag, its food canal can be blocked and the unfortunate reptile starves to death.
The fourth International Jellyfish Bloom Symposium takes place in Hiroshima two weeks from now. The fishing industry lobby is so powerful, however, that persuading it to take a more holistic approach to its activities is an uphill struggle. Perhaps the solution to the jellyfish problem is in our own hands. We could eat jellyfish, making harvesting viable.
Exploiting them might help restore balance in the sea.
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