Utilisation of organic matter from Babylon snail (Babylonia areolata) culture sediments by cultured juvenile sandfish (Holothuria scabra)

The ability of sandfish to reduce organic matter in sediments from Babylon snail (Babylonia areolata) pond aquaculture in Vietnam was examined. Sandfish were cultured at low (25 in.. m−2, 43 g.m−2) and high (100 in.. m−2, 171 g.m−2) density in 500 l tanks provided with pond sediment or clean beach sand as a bottom substrate. A treatment that included pond substrate with no sandfish was included as a substrate quality control. Survival, weight gain, absolute growth (AGR) and specific growth (SGR) of sandfish, and total organic matter (TOM) and total nitrogen (STN) contents of the substrate were monitored over 56 days. No additional food was supplied to sandfish. Sandfish survival was similar between treatments (81–100 %). Significant interactions between substrate type and sandfish density were found for sandfish AGR (F1,8 = 7.84, P = 0.023) and SGR (F1,8 = 7.86, P = 0.023). Growth was most heavily correlated with sandfish density. AGR and SGR were greater when sandfish were cultured at low density on pond substrate (0.09 ± 0.03 g.day-1 and 2.59 ± 0.46 %.day-1, respectively). Mean individual sandfish weight gain in this treatment was 4.33 ± 1.31 g, an increase of 87.4 % compared to sandfish cultured on clean sand at the same density (2.31 ± 0.47 g). Growth of sandfish cultured at high density was poor, regardless of substrate type, with weight loss occurring in 4 of 6 high density treatment tanks. Organic matter content of substrates increased throughout the study in all treatments, however accumulation was significantly lower in tanks with sandfish. Sediment total N increased during the experiment but was significantly reduced in treatments with sandfish, compared to no sandfish. Although sandfish are unable to mitigate organic enrichment resulting from Babylonia culture, the results indicate that they derive significant benefit to growth from the enriched substrate.