Is culture performance of juvenile sandfish, <i>Holothuria scabra</i>, in ocean-based nursery systems influenced by proximity to milkfish (<i>Chanos chanos</i>) farms and <i>hapa</i> net mesh size?

Abstract

The impacts of proximity to milkfish farms (i.e. ‘close’, ‘intermediate’ and ‘distant’) and different hapa net mesh sizes (i.e. 400 μm and 1 mm) on periphyton characteristics and growth performance of sandfish, Holothuria scabra, juveniles (4.6 ± 0.1 mm), were evaluated in a 30-day field experiment in the Guiguiwanen Channel at Bolinao, North-western Philippines. Growth and survival of sandfish juveniles were determined twice, and chlorophyll a and ash-free dry weight (AFDW) of periphyton were monitored every 5 days during the study. Opisthobranchs that recruited inside the hapa nets were identified and counted during monitoring. Survival of sandfish juveniles differed significantly between sites but was not influenced by mesh size by day 15. Survival was significantly higher at the intermediate site by day 15. Juvenile survival was significantly higher in 400 μm mesh hapa nets across all sites by day 30. Result showed that mean length and absolute growth rates (AGR) of sandfish juveniles were significantly influenced by mesh size but not by site at day 15. Juveniles reared in the 1 mm mesh hapa nets were larger and grew faster by day 15. However, by day 30, mean length and AGR of sandfish juveniles were significantly affected by site but not mesh size. Juveniles reared at the close site were significantly larger and grew faster than those reared at the intermediate site while mean length and AGR of juveniles reared at the distant site were not significantly different to those cultured at either the close or intermediate sites. Growth rates of juveniles were lower on day 30 compared to day 15 possibly resulting from reduced food availability, as indicated by lower periphyton AFDW, resulting from inter- and intraspecific competition. A positive correlation was found between AGR and the number of surviving sandfish juveniles by day 15, while no correlation between the two parameters was observed by day 30 indicating that reduced growth rates were not mediated by interspecific stocking density, but most likely resulted from high intraspecific grazing pressure. Chlorophyll a content of periphyton was higher during the second half of the experiment; however, total periphyton biomass (as AFDW) generally decreased overtime. This reduced food availability coincided with the higher number of opisthobranchs in the hapa nets by day 30. Results showed that sandfish juveniles can grow in an environment with varying levels of nutrient enrichment, however, areas close to mariculture activity may support improved growth because of resulting increases in periphyton biomass. Additionally, the use of finer mesh (400 μm) hapa nets positively impacted juvenile survival and would support earlier transfer of juveniles from the hatchery to the ocean reducing husbandry inputs and costs.