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From the dark depths of the open oceans vast blooms of jellyfish are descending unforseen upon coastal waters worldwide, reports Adam Anson for TheFishSite. As the frequency of these invasions increases, the threat to both tourism and fisheries alike becomes evermore evident, but what has triggered this increase and what can be done to stop it?

In a scenario that seems almost too surreal to be believed, postcard-perfect coastlines are replaced with an encroaching mass of gelatinous bodies between one sunrise and the next. These 'swarms', which can cover areas up to hundreds of square miles, stretch across horizons. In their tentacled trail the delicate balance of age-old ecosystems can be upturned and toppled almost immediately. Worryingly, global studies now suggest that these invasions are becoming ever more frequent.


Photo: Rebecca J. Wagnett, NRC Postdoctorial Research Associate
National Science Foundation

Although any accurate assessment of the worldwide economic toll these incidents are taking has not been calculated - as a consequence of broken fishing nets, killed fish, egg consumption and clogged machinery - jellyfish are believed to have accounted for the loss of hundreds of millions - if not billions - of US dollars since the 1980's. But it is the individual incidents alone that belie the true impact of these invasions.

A good illustration of the force of these invasions dates back to the early 1990's when a voracious, invasive jellyfish-like creature known as the comb jelly was introduced into the Black Sea. After just eight years they dominated it. According to a recent report by the National Science Foundation, by 1990, the total biomass of the Black Sea’s comb jellies totaled around 900 million tonnes — more than ten times the weight of the total annual fish catch from all of the world’s oceans.

The report, Jellyfish Gone Wild, claims that over one thousand of these fist-sized comb jellies filled each cubic meter of water in Black Sea jelly blooms. As the jellyfish spread destruction in the Black Sea it moved out into the Azov and Caspian Seas resulting in a massive crash in anchovy populations.


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"500 million Nomurai jellyfish–which may each weigh up to 450 pounds and sport a bell up to seven feet in diameter, floated into the Sea of Japan daily during recent summer Nomurai blooms"
Extract from Jellyfish Gone Wild

In the Black sea alone it is estimated that US $350 million in losses to the areas fishing and tourism industries resulted from the invasion of the comb jelly, whilst losses from the ongoing comb jelly invasion of the Caspian Sea are expected to exceed those from the Black Sea invasion.

A different kind of jellyfish altogether caused havoc in Japan. According to the report, 500 million Nomurai jellyfish – which may each weigh up to 450 pounds and sport a bell up to seven feet in diameter, floated into the Sea of Japan daily during recent summer Nomurai blooms. Resulting losses to fishermen in just one Japanese prefecture have, thus far, totaled at least US$20 million.

In Australia US$10 million dollars of damage was done to the shrimp industry when invading jellyfish struck in the year 2000, and on the other side of the globe another devastating invasion washed up on British shores. In 2007 an extraordinarily large swarm of jellyfish left 100,000 farmed salmon dead off the coast of Northern Ireland in a completely unexpected attack. Reports at the time claimed that stock worth £1million were suffocated in their cages by the swarm, which is estimated to have covered 25 square kilometers of sea and been up to 10 metres thick.

Now the Bering Sea has been encircled by a huge mass of jellyfish. A full invasion could be catastrophic for the areas fisheries that currently produce fifty per cent of America's fish and shellfish total numbers. Chesapeake's Bay's once productive waters have also fallen foul of an invasion in recent years.

A Growing, Breeding, Multiplying Problem

Jellyfish grow and multiply at an incredible rate. Many double their size every day that conditions are suitable and others begin reproducing just days after their birth. The self-fertilizing hermaphrodite comb jellyfish can release 8,000 eggs into the water per day. As far as scientist know these huge blooms have been happening for an extremely long time. Jellyfish are thought to have inhabited the world up to 500 million years ago.

However, there has been a lot of evidence to suggest that the frequency of these invasions are now increasing on a global scale. Scientists believe that something has triggered the jellyfish to multiply in greater numbers and many claim that global warming and manmade pollutions are contributing to this trend.

Scientists believe that the birth of jellyfish swarms are suddenly triggered when a certain change of climate indicates optimum conditions. These signals may be given of by a change in water salinity, oxygen content, currents or temperature. Although there is no proof of the theory, human pollution of both the oceans and the atmosphere above may be signalling vast jellyfish blooms.

Pollution that runs into the oceans can create a habitat called a 'dead zone'. In these toxic zones very little life survives, however, with the elimination of competitors the hardy jellyfish can strive in enormous numbers. Currently more than 400 vast marine Dead Zones have been identified worldwide. Their combined ocean coverage totals 100,000 square miles.

According to Jellyfish Gone Wild, the number of global Dead Zones has doubled about every 10 years since the 1960s. "During the summer of 2008, the Gulf of Mexico’s Dead Zone covered about 8,000 square miles, about the size of Massachusetts", says the report. It adds that 38,600 square miles of the North Atlantic have been periodically covered by blooms of salps, a jellyfish-like creature.

Unfortunately, jellyfish invasions have proven very difficult to identify and control, due to their mysterious nature and their ability to survive in waters both extremely hot and cold the world over. Furthermore, research into swarms has been limited and under funded. New tools are currently in development to help identify the individual impacts of overlapping environmental stresses and DNA analyses.

However, amidst the tales of woe some benefits have been found. Tentacles of Large jellyfish are providing hiding places for young pollock pursued by predators in the Bering Sea; and in 2008 a jellyfish invasion in China turned out to be a bonus for local shrimpers, who sold the gelatinous mass on the Asian market where it is considered a tasty delicacy.

January 2009

the Fish Site Editor

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