Seaweed from Zanzibar is exported to China, Korea, Vietnam, Denmark, Spain, France and the US.
It is used as a base for cosmetics, lotions, toothpaste, medicines and food – in China, for instance, it is eaten as a vegetable.
At its peak, the trade contributed almost $8m (£5m) a year to the Zanzibar economy. Half of that, says Mr Mohammed, went straight into the pockets of the farmers.
But many of these women are once again depending on their husbands.
They are trying to find other ways of making money but it is not easy.
In the village of Paje, women sit around with their children, laughing, cooking and weaving.
Zeinab Hassan Makame, a former seaweed farmer, invites me into her house. Her veranda is now filled with cut plastic bottles growing an array of seedlings – mangos, guavas and limes.
“I have now decided to prepare a tree nursery and weave baskets and mats. The present income is not as much as it used to be, but the beginning is always difficult. I hope to be successful.”
Zanzibar’s government has commissioned more research into the exact reasons for the seaweed mortality and how to counteract it.
Some scientists have suggested moving the farms to deep water where it is cooler. This has had some success in the neighbouring island of Pemba, which is now contributing 80% to Zanzibar’s seaweed exports.
But the problem for the women in Zanzibar is that traditionally they cannot swim.
So, now they must either learn how to swim or say goodbye to seaweed farming.