Effects of shading on periphyton characteristics and performance of sandfish, Holothuria scabra Jaeger 1833, juveniles

Abstract

Mass production of sea cucumber (sandfish; Holothuria scabra) juveniles is carried out in periphyton-based ocean nursery systems. However, detailed studies on the importance of periphyton as a food source for post-settled juveniles, and on factors influencing biofilm development and composition are scarce. Such information is critical in the optimization of both culture systems and husbandry procedures, and to improve production of juveniles. In this study, the effects of relative food abundance and shading on the feeding activity of the juveniles (~5 mm) under laboratory conditions were determined. The effects of shading (ambient vs. shaded) on periphyton development, and on the growth performance of post-settled juveniles, were also determined in a 30-day field experiment. Food consumption in the ‘high’ food and ambient light treatment was significantly higher (2.4 ± 0.1 mg day−1) than that of juveniles reared in other treatments, including ‘low’ food and shaded culture vessels. Juveniles reared in low food conditions covered significantly greater areas when feeding (2.3 ± 0.28 cm2 day−1) than juveniles reared in high food conditions (0.41 ± 0.20 cm2 day−1). Average length and absolute growth rate (AGR) of juveniles in the shaded treatment were significantly higher by day 15, but differences between light treatments were not significant by day 30. Interestingly, juvenile survival was significantly higher in the shaded (56.9 ± 20.7%) treatment compared to the ambient light (34.4 ± 22.9%) treatment. The chlorophyll a content of periphyton was significantly higher by day 30 than at day 5 in both treatments. Total periphyton biomass (AFDW) was higher in the ambient light treatment than in the shaded treatment, but both generally decreased overtime. Results suggest that food quality is better during the early colonization of periphyton as indicated by the higher absolute growth rates of juveniles during the first 15 days of the experiment. This study generated new information on factors influencing the success of hapa net-based nursery culture of sandfish juveniles and provides a basis for improved use and management of such systems.