Effect of Stocking Density on the Performance of the Seaweed Ulva Reticulata

The seaweed biofilter Ulva reticulata was grown at two stocking densities (1 and 3 kg m-2) in a low cost integrated system in Zanzibar, Tanzania. The seaweed was stocked in 2 m2 cages made of 1 inch netting material placed at the outflow of fish ponds. Control seaweed was grown at the fish pond inflow channel. At a stocking density of 3 kg m-2, the weight of the seaweed increased by 700 g (fw) during the first week and then an increase of less than 200 g in the fifth week. Total weight increased from 4,500g at stocking to 6,630g during the fifth week. Growth rate was 1.3 % d-1. The seaweed also removed 0.4 g N m-2 d-1 of nitrogen from nutrient-rich fishpond effluent water. At the lower stocking density, of 1 kg m-2, biomass increased by 500 g during the first week increasing to 700 g during the fifth week. Total weight increased from 6,000 g at stocking to 30,000 g during the fifth week. Growth rate averaged 3.9% per day. The seaweed also removed TAN of 6 g N m-2 d-1. The growth rate, biomass yield and nutrient uptake were significantly (ANOVA, P<0.01) higher than those of the controls. The seaweed significantly raised pH and oxygen levels of the fish pond effluent water at both stocking densities. Therefore, both stocking densities lead to efficient performance of the seaweed biofilter Ulva reticulata, but 1 kg m-2 is better than 3 kg m-2 in this system. Higher densities are, however, known to be good in other integrated systems, calling for more work on this integrated system.