Copyright @ 2020, Indonesian Aquaculture Journal, p-ISSN 0215-0883; e-ISSN 2502-6577 51 Indonesian Aquaculture Journal, 15 (2), 2020, 51-58 # Correspondence: Institute for Mariculture Research and Fisheries Extension. Jl. Br. Gondol Ds. Penyabangan Kec. Gerokgak Kab. Buleleng, Kotak Pos 140, Singaraja 81101, Bali, Indonesia. Tel. + 62 362 92272 E-mail: yasmien_na@ yahoo.com Available online at: http://ejournal-balitbang.kkp.go.id/index.php/iaj OPTIMUM DENSITY OF Nannochloropsis sp. FOR M ASS LARVAL REARING OF CORAL TROUT, Plectropomus leopardus (Lacepède, 1802) Yasmina Nirmala Asih # , Sudewi, Afifah Nasukha, Daniar Kusumawati, Ketut Mahardika, Ahmad Muzaki, and I Nyoman Adiasmara Giri Institute for Mariculture Research and Fisheries Extension Jl. Br. Gondol Ds. Penyabangan Kec. Gerokgak Kab. Buleleng, Kotak Pos 140, Singaraja 81101, Bali, Indonesia (Received:November 29, 2019; Final revised: August 3, 2020; Accepted: August 10, 2020) ABSTRACT Coral trout, Plectropomus leopardus (Lacepède, 1802)is a visual feeder. Turbidity caused by phytoplankton or clay particle in the water will affect the visual foraging of coral trout larvae. Addition of Nannochloropsis sp. has been included in standard operational procedure for marine fish larval rearing as green water. However, the density of Nannochlorposis sp. in coral trout larval rearing system has not been evaluated. This study aimed to evaluate the optimal of Nannochloropsis sp. required for rearing of coral trout larvae. Nannochloropsis sp. was given to two days old larvae (D-2), with the densities of 2 x 10 5 , 4 x 10 5 , and 6 x 10 5 cell/mL. After 50 days rearing period (D-50), evaluation on the average size and total harvest were recorded. The results showed that the density of 2 x 10 5 cell/mL Nannochloropsis sp. was the best in survival rate (2.35 ± 1.05%) than other densities, but they were not significantly different (P> 0.05) than those of 4 x 10 5 cell/mL (1.67 ± 0.70%) and 6 x 10 5 cell/mL (1.26 ± 1.05%). The lower densities, 2 x 10 5 and 4 x 10 5 cell/ mL, were dominated by more than 50% of > 2.7 cm sized juvenile. Histological analysis of fish eyes supported that the two lower densities produced dominant cone shape as the receptor cells in the retina observed. From an economical aspect, addition of 2 x 10 5 cells/mL resulted on the higher profit, hence optimum density of Nannochloropsis sp. added in coral trout larval rearing on a mass scale was 2 x 10 5 cells/mL. KEYWORDS: coral trout; larval rearing; Nannochloropsis sp.; turbidity INTRODUCTION Appropriate feeding and water managements have pivotal roles in the larval rearing of coral trout, Plectropomus leopardus . The first critical stage was at the first feeding when the egg yolk had been absorbed completely, thus larvae should obtain the energy source out of their bodies to survive. The first feed- ing time for coral trout larvae is on day-3 when their egg yolks have been absorbed (Qu et al ., 2012) and the mouth gap has opened completely (Sudewi et al ., 2020). It was reported by Kusumawati et al. (2019) that feeding of live feed containing high fatty acid resulted in better survival for coral trout larvae. High fatty acid source could be obtained from Nannochloropsis sp. (Rebollosso-Fuenters et al ., 2011), which is also utilized as green water in the larval rearing. Coral trout is a visual feeder fish (Yoseda et al ., 2008), which deeply relies on their vision to capture the prey. The right amount of light intensity is re- quired for the fish to enable distinguishing the prey and its surrounding. Research results of Andamari et al . (2007) regarding light intensities (0-1,200 lux) for coral trout larval rearing showed range of light in- tensity from 900 to 1,200 resulted the highest sur- vival by the end of larval rearing. This result was accordance with Yoseda et al . (2008) who reported that coral trout at 5 days after hatching (DAH) had better survival when rearing in levels of light inten- sity 1,000-3,000 lux than of 0-500 lux. Light entering into water will be absorbed or scattered by particles (Sandstrom, 1999; Utne-Palm, 2002; Wellington et al ., 2010), known as turbidity. Level of turbidity affects the vision of fish and its behavior in foraging (Cutts & Batty, 2005; Utne-Palm, 2002). According to Cutts & Batty (2005), level of