Stocks of the tiny kilka fish have been devastated
Hundreds of fishermen on the Caspian Sea risk losing their livelihood because of an invasion of a marine organism that is depleting the fishes' food.
The tiny creature, the Mnemiopsis, has become known as the Caspian Monster.
The invading organism, which is classified as a ctenophore, has a voracious appetite and alarming reproduction rate. It has caused the number of kilka, small fish the size of anchovies, to drop by at least 50% in just a few months.
The famous sturgeon of the Caspian Sea, which produces caviar, has not been affected.
Falling stocks
Late one evening, a BBC crew made its way out into the choppy waters of the Caspian Sea. It was almost dark, and more than 100 fishing boats were setting out into the open waters to catch the kilka.
"We used to get three to six tonnes every night," said one fisherman, Mahmood Ghorbani. "Now, we're lucky if we get half a tonne."
The problem is simple: the main diet of the kilka is plankton - but the Mnemiopsis is getting to it first.
Debt clearance
"Really it's a disaster for our fishermen and I hope that we will be able to do something in the future, to control these organisms," another man said.
With kilka stocks falling, some of the fishermen have already been laid off; others have not been paid for months.
Some of boat owners have had to sell their craft to pay off their debts. Fishing companies are also in trouble, said Rahim Jamshidi, the managing director of the Shahed Company.
"If it goes on like this, it'll be misery for us," he added.
"We can't pay the fishermen, we can't pay for fuel. Nobody will give us credit any more. We just don't know what to do. We're in deep trouble."
Sea infested
At night, with their spotlights on, the fishermen can see that the sea is infested with the little "gelatinous monsters", which are no larger than five centimetres (two inches) in length.
The Mnemiopsis can double its size in just one day. It reaches maturity within two weeks, and then produces 8,000 offspring every day.
The organism spread from the Black Sea, transported into the Caspian in the ballast water of ships. The Mnemiopsis is originally thought to have come from Florida.
The Mnemiopsis thrives in the Caspian because it has no predators.
Possible solution
Ahmet Kideys, a Turkish expert, believes there is an answer to its ravages.
In the Black Sea, fish stocks crashed by more than 80% because of the presence of the ctenophore. But now they are starting to recover, and the Mnemiopsis has been almost wiped out - not by humans, but by another, very similar, but slightly larger gelatinous plankton called Beroe.
"At the moment the answer is Beroe, because we know it has been proven 100% at the Black Sea," said Mr Kideys.
"The good thing about Beroe is that when its food, the Mnemiopsis, is finished, you cannot find Beroe any more. That's really an excellent solution."
Beroe reached the Black Sea the same way Mnemiopsis did - hitchhiking in ships' bilges.
Now, the question is whether it should be deliberately introduced to the Caspian, to bring Mnemiopsis under control.
Mr Kideys believes it is a safe solution, but whether it will work is not yet certain, because nobody knows if Beroe can survive in the less saline Caspian waters.
All five Caspian states would also have to agree on the solution. But it is the only solution on offer. And for the stricken fishermen of the Caspian, it cannot come soon enough.