Borneo Journal of Marine Science and Aquaculture Volume: 03 (1) | July 2019, 13 – 17 13 RNA/DNA ratio in milkfish (Chanos Chanos) larvae reared at different stocking densities Rukisah Saleh 1 , Sitti Raehanah Muhamad Shaleh* 2 , Saleem Mustafa 2 and Sujjat Al-Azad 2 1 Faculty of Fisheries and Marine Science, Borneo Tarakan University, Jalan Amal Lama No. 1, Tarakan Kalimantan Utara, Indonesia 2 Borneo Marine Research Institute, Universiti Malaysia Sabah, Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia * Corresponding author: sittirae@ums.edu.my Abstract Stocking density can induce stress in fish in an aquaculture system if not handled properly, and the chronic stress may lead to mortality. Several studies have reported that the capability to deal with a range of stocking densities differs among fish species and maturity stage. Hence, fish larvae may have different resilience to stress from the adult fish. Milkfish larvae were reared in hatchery for 50 days using a recirculating culture system at four different stocking densities (8,12,16 and 20 larvae/liter). The growth performance was not significantly different (P>0.05) except at stocking density of 20 larvae/liter. The highest survival rate (88.04%) was recorded in the system with 8 larvae/liter while the lowest (55.44%) in the culture tank where stocking rate was 20 larvae/liter. The stocking density also influenced the RNA / DNA ratio of the milkfish larvae. The RNA/DNA ratio showed a pattern that was identical with that of sigmoid growth where stocking rate of 8, 12, and 16 larvae/liter gained weight until 30 days of rearing. Highest RNA/DNA ratio was recorded at 16 larvae/liter (2.85±0.004), while the lowest was at 20 larvae/liter (2.25±0.217). Food availability might play a limiting factor that leads to the lower RNA/DNA ratio of larvae reared at a high density due to competition. Keywords: Milkfish, Sustainable aquaculture, Hatchery rearing, RAS tank, Growth performance ----------------------------------------------------------------------------------------------------------------------------------------------- Introduction Milkfish culture in Indonesia is a widespread activity. A major factor in its growing popularity is that it thrives on ecosystem resources where benthic algae provide food for the milkfish. However, the ever-increasing demand for fish has generated interest in high-stocking density culture. This would require more operational inputs. Intensive and semi intensive systems are always associated with high stocking density (Kholifah et al., 2012) and requirement for supplementary feeds. A high-density carries the risk of stress and this can result in reduced growth and heavy mortality. Aquaculture industry should find a balance between the level of stocking and fish well-being for efficient and sustainable production. Several studies have reported that the capability to deal with a range of stocking density differs among fish species and maturity stage, and this factor should be considered in a culture system (Baldwin, 2010). This study was motivated by the need to determine the suitable stocking rate for rearing of milkfish larvae in the hatchery. RNA/DNA ratio has been used as the condition indicator to assess the effect of environmental factors and nutritional on growth and robustness of aquaculture animals (Mustafa et al., 1991; Gwak and Tanaka, 2001; Melissa and Mustafa, 2004). Assessment of growth and nutritional condition at different stocking levels could provide information of considerable significance in successful culture of milkfish. Changes in growth can be caused not only by lack of enough food, but also by sub-optimal environmental conditions or other factor like stocking density. The utility of this ratio is based on the fact that cellular DNA is relatively constant while changes in RNA content are aligned with protein biosynthesis. Therefore, the ratio of RNA and DNA can serve as an index of protein biosynthesis at a given time (Raae et al., 1988). Individuals in favorable condition are generally characterized by higher RNA/DNA ratio. In addition, RNA/DNA ratios also correlates possitively to changes in feeding conditions (Satterwhite, 2007). Therefore, RNA/DNA ratio can be used as an index of growth potential for fish in captivity (Buckley et al., 1999). Materials and Methods RNA/DNA ratios in milkfish larvae cultured at different stocking rates were analyzed. Twenty- day old milkfish larvae were obtained from local farmers in Tarakan, Indonesia. The experimental trials were carried out in fiberglass tanks, each of 80 L capacity, and connected to a system of water recirculation. Coral rubble and Aquamat TM were used as substrates or media for the filtration system. Experiment was conducted in triplicate sets with different stocking densities: 8, 12, 16 and 20 larvae/liter. Feed were given twice daily at 08:00 and 17:00. The feed was in the form of Bintang SB 001 containing 11% moisture, 3% fiber, 44% protein and 12% fat. To maintain water quality, uneaten feed and feces were siphoned out daily. Water temperature, DO and pH of each tank were also measured daily prior to feeding.