Waist deep in sparkling blue water off the white beaches of the Indian Ocean spice island of Zanzibar, seaweed farmer Mtumwa Vuai Ameir gently ties seedlings to wood poles.

Seaweed farmed on the Tanzanian archipelago is one of Zanzibar's key exports -- used for food, cosmetics and medicines in Asia, Europe and North America -- but now the vital industry is struggling with warmer waters killing the crops.

"We are desperate, and some farmers have been discouraged and abandoned the work," said Ameir, who has been a seaweed farmer for over 20 years.

She works alongside her daughter and husband in the small village of Muungoni, some 42 kilometres (26 miles) southwest of Zanzibar town.

As crop yields decline, cheaper production and transport costs in Asia are also challenging Zanzibar's position as the world's third-biggest producer of spinosum seaweed.

Over 23,000 farmers grow and harvest the seaweed -- around 80 percent of them women -- according to government statistics.

But tens of thousands more depend indirectly on an industry that provides a key income for families with few other means to earn a living.

- Medicine, cosmetics, food -

Seaweed from Zanzibar is exported to China, Korea, Vietnam, Denmark, Spain, France and the US. It is used as an ingredient base for cosmetics, lotions and toothpaste, as well as in medicines. It is also eaten as a vegetable.

Farmers say that reduction in demand from abroad and subsequent falling prices has made turning a profit a challenge.

Seaweed from Zanzibar is used as an ingredient base for cosmetics, lotions and toothpaste, as well as in medicines Marco Longari, AFP/File

"Seaweed is now cheaper in Asia, compared to our price, therefore we must drop prices to maintain our buyers," said Arif Mazrui, who runs Zanque Aqua Farms, a seaweed business, blaming price fluctuation in the world market.

"We have no control over the price, we have to adjust our prices to keep our buyers. It is unfortunate that while we adjust our prices to compete with Indonesia and the Philippines, the farmers are the great losers."

In recent years, Zanzibar has exported some 16,000 tonnes of seaweed a year, according to government statistics.

But levels are declining -- in the first three months of 2016, levels were less than half the amount produced during the same period a year earlier.

Prices too are falling: the price of spinosum seaweed was previously around 700 Tanzanian shillings ($0.31/0.28 euros) per kilogramme (2.2 pounds), but is now less than half, selling for 300 shillings.

The price for cottonii, another type, has tumbled from around 1100 shillings to 700 shillings.

But the plants also face a threat from disease as well as poor weather, which have both caused production levels to drop in Zanzibar.

Warmer waters -- due to climate change or other causes -- is a major factor in the decline in seaweed growth.

- Hotter water, lower yields -

Narriman Jidawi, from the Institute of Marine Sciences at the University of Dar es Salaam, said research into the production decline was under way.

Started at the beginning of the 1990's seaweed farms have become a source of revenue for thousands of women on the Island of Zanzibar Marco Longari, AFP/File

"When it is too hot... seaweed (does) not grow very well, so a lot of women have stopped actually cultivating," Jidawi said.

The university's marine scientists and environmentalists are encouraging seaweed farmers to try and grow their crop in deeper, cooler waters in a bid to minimise infection, after tests showed the seaweed fared better there.

Farming in deeper water, however, is harder to do.

Seaweed grows best in water temperatures of around 25 to 30 Celsius (77-86 Fahrenheit), but temperatures are now rising above 31 C (88 F), which is unfavourable for seaweed growth.

"The seaweed business is now a challenge -- both farmers and exporters are frustrated," Mazrui said. "But we are encouraging them to continue production with hope that the price will rise again in the near future."

The government is worried, and trying to find solutions.

Zanzibar's President Ali Mohamed Shein -- who won a second term in office in March after a controversial re-run of elections the opposition claimed it had won -- used his inaugural speech to parliament to address the seaweed issue.

Improving seaweed production was among his priorities, he said, promising to improve equipment for farmers and to work to boost the market.

Hashim Moumin, head of aquaculture at the ministry of livestock and fisheries, said they were promoting "seaweed processing light industries" as an alternative to relying on exports of raw material.

"We invite investors to establish industries that will use seaweed as material," Moumin said.

For the farmers, they must struggle on with little choice.

"Despite low prices and poor production, I am still reluctant to quit this hard job, because I need to earn money," said Ameir.