Growth performance of the sea cucumber Holothuria scabra and the seaweed Eucheuma denticulatum: integrated mariculture and effects on sediment organic characteristics

Deposit-feeding sea cucumbers play a key role in marine ecosystems through bioturbation, burrowing and feeding on organic matter in marine sediments. Many deposit-feeding holothurians have therefore been recommended for integrated multitrophic aquaculture systems (IMTA). We set up an integrated mariculture system of sea cucumber Holothuria scabra and seaweed Eucheuma denticulatum in Bweleo, Unguja Island of Zanzibar, Tanzania, to investigate the effect of stocking density on the growth and survival of culture species, total organic matter (TOM) and total organic carbon (TOC) content in the sediment. Treatments that included a fixed stocking density (500 g, ca. 200 g m(-2)) of E. denticulatum and 4 sea cucumber stocking densities (monoculture, low, medium and high density; 0, 150 +/- 5, 236 +/- 24, 345 +/- 48 g m-2, mean +/- SD) of medium-sized H. scabra (114 +/- 37 g) were established. Stocking density of H. scabra did not influence survival of either species. Seaweed cultured under high stocking density of H. scabra had a higher specific growth rate of 2.33% d(-1) than that cultured at the medium or low densities or without sea cucumbers. Sea cucumbers cultured at low stocking density had a higher mean growth rate of 0.80 g d(-1) compared to those cultured at medium or high densities. TOM and TOC in sediments decreased over the experimental period at medium sea cucumber stocking density, while at low and high stocking densities, organic matter accumulated. The study demonstrates that the integration of E. denticulatum and H. scabra at 200 g m(-2) enhances seaweed growth and can reduce organic matter content in the sediments.