Until the early 2000s, the seaweed industry was a backbone of Zanzibar’s local economy, employing 20,000 women farmers, lifting their standard of living and social status. But the seaweed industry has been battered by rising temperatures, says Vaterlaus, threatening the livelihoods of thousands of farmers in Zanzibar.
A 2021 study by researchers at the University of York in the UK found seaweed yields and quality had dropped drastically in the area due to rising temperatures, stronger winds and erratic rainfall.
Despite early gains, the production of seaweed fell by 47% between 2002 and 2012 due to climate change, disease and the decrease of the number of farmers due to low prices, the researchers concluded.
“I found the prices of seaweed are low and the people don’t earn good money,” says Vaterlaus.
In his bid to help cash-strapped seaweed farmers in Jambiani, Vaterlaus introduced the idea and method of growing sponges to the area.
Seaweed is highly vulnerable to climate change, but sponges can tolerate warmer temperatures, allowing them to thrive in hot conditions, Vaterlaus adds.
“During the hot season, it is hard to produce seaweed but sponge farming is still possible,” he says.
Aziza Said, a marine biologist at the University of Dodoma in Tanzania, agrees that sponges are more resilient to hotter temperatures, adding that they also require less maintenance and fetch a higher market price than seaweed.
By providing an alternative to fishing, sponges also reduce pressure on natural resources and protect the environment, Said says. And they enrich the sea bed by spitting out fatty and amino acids for other organisms to absorb, she adds.
Research has also shown that the spongy creatures play an important role themselves in combatting climate change. Sea sponges exist in all oceans around the world and make up 20% of the global silicon biological sink. Their skeletons break down into microscopic pieces of silicon, which helps control the carbon cycle in the ocean and reduces the greenhouse effect, experts say. Dissolved silicon is critical for the growth of diatoms, tiny organisms which absorb large amounts of CO2 in the ocean using photosynthesis.
According to Said, diatoms grow well when there’s a large enough supply of dissolved silicon in seawater.
“When diatoms die, their shells sink to the ocean floor, effectively absorbing carbon in the form of organic matter and silica,” she says.
Sea sponges also effectively filter sea water and reduce marine pollution, according to another study. A single sponge can pump thousands of litres of water per day through a maze of channels and pores that trap impurities and organic substances, the researchers note.
According to another study, up to 24,000 litres (5,300 gallons) of sea water can be pumped through a 1kg (2.2lb) sponge in a single day.