Fisherman sand

I call them “sand fishermen” because that is what I saw. Every day, and every 6 hours, for 24 hours, they go aboard, they go up the river Niger, they separate their boats and dive to “fish” sand and pebbles of the river bed.
Once each piragua is full, they are all connected with a rope and, using a huge diesel engine, they are pulled back to a location where others empty the boats and the sand is sold to make cement or other construction materials.
Very little money, and a physically challenging job, carried out mostly by youngsters, but everyone knows that Africa and Africans are used to hard labours.