C:N ratios affect nitrogen removal and production of Nile tilapia Oreochromis niloticus raised in a biofloc system under high density cultivation Jorge A. Pérez-Fuentes a , Martha P. Hernández-Vergara a, ⁎, Carlos I. Pérez-Rostro a , Ira Fogel b a Instituto Tecnológico de Boca del Río, Boca del Río, Veracruz 94290, Mexico b Centro de Investigaciones Biológicas del Noroeste, La Paz, Baja California Sur 23096, Mexico abstract article info Article history: Received 21 July 2015 Received in revised form 5 November 2015 Accepted 6 November 2015 Available online 7 November 2015 Keywords: Alternative aquaculture system Biofloc technology Molasses Zero water exchange Over a six-month trial, growth performance of juvenile tilapia Oreochromis niloticus, raised under biofloc cultiva- tion, was recorded. Five treatments with three replicates and 237 juveniles per replicate at different C:N ratios were tested (10:1, 12.5:1, 15:1, 17.5:1, 20:1). Molasses was added every fourth day as the carbon source. Trials were performed in eighteen 3.14 m 3 circular tanks with constant aeration. Survival in all treatments with biofloc was similar and significantly higher (94.60 ± 2.03%) than in the control treatment (84.96 ± 1.53%). Best weight gain (g/day) occurred in the control treatment, but was influenced by low density. The 10:1 and 15:1 C:N treat- ments had similar weight gains, but not the 12.5:1 and 17.5:1 C:N treatments. Total production in the 10:1 ratio treatment was the highest (18.03 ± 0.25 kg m -3 ), with slightly declining production, in this order: control (17.86 ± 0.16), 15:1 (17.73 ± 0.18), 12.5:1 (17.20 ± 0.28), 17.5:1 (16.44 ± 0.04), and 20:1 (16.28 ± 0.02). Water quality remained similar in all biofloc treatments, but lower concentrations of nitrogen compounds occurred in the 10:1 C:N ratio tanks. These results suggest a 10:1 C:N ratio provides good survival and growth of tilapia with no water exchange. Biofloc technology is a good strategy in areas where alkaline pH is a limiting factor for aquaculture activities because the pH decreases gradually as molasses is added. Statement of Relevance: The success and permanence of aquaculture sector relies on the implementation of sus- tainable and environment-friendly technology, the BFT optimizes energy and resources during production, so our results contribute significantly to the sector. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Fish farming, using biofloc technology, has a distinguished advan- tage over traditional fish farming because it requires low-to-zero water exchange and breaks down nitrogen metabolites and recycles ni- trogen as bacteria biomass, supplying a complementary and inherently nutritious food for the commercial crop. The result is a more efficient and environmentally non-destructive activity (Hargreaves, 2006; Avnimelech, 2007). Effective growth of biofloc microorganisms requires an external carbon source to promote bacterial biomass that leverages waste nitrogenous metabolites in the system. Avnimelech (1999) and Asaduzzaman et al. (2008) recommend a 15:1 to 20:1 C:N ratio for good biofloc cultivation; the high nitrogen excretion present in the systems comes from waste because fish usually excrete 50–70% of the nitrogen present in their diet. With high nitrogen concentration in the water, it is necessary to add a cheap carbon source that is usually a by-product of the food processing industry (Emerenciano et al., 2013). Some researchers suggest that sugar and molasses are assimilated more quickly by bacteria, increasing production of biofloc in less time (Hargreaves, 2013). Goldman et al. (1987) show that marine bacteria absorb more ammonia when there was a 6:1 to 10:1 C:N ratio, with sugar as the carbon source. Some results indicate that the C:N ratio var- ied with the species in the system and the carbon source, but the more efficient ratios were between 15:1 and 20:1, which increased productiv- ity in ponds by 20–45%, and survival by 20–30% (Avnimelech, 2012). Biofloc systems also reduced consumption of commercial feed by 30–37%, decreasing the cost of production by 14% (Panjaitan, 2004; Hanson et al., 2009). Similar results were found by Avnimelech (2012), stating that raising tilapia with biofloc technology can reduce commercial feed by 20% and increase production by 15.5 kg m -3 (Rakocy et al., 2004). Azim and Little (2008) find that the portion of pro- tein in commercial diets is reduced by 24% during tilapia production in biofloc systems, obtaining a gain equivalent to tilapia fed commercial diets with 35% protein. Biofloc technology is a good strategy to reduce production costs, but research about the best C:N ratio for optimal Aquaculture 452 (2016) 247–251 ⁎ Corresponding author at: Laboratory of Genetic Breeding and Aquaculture Production, Instituto Tecnológico de Boca del Río, Km 12, Carretera Veracruz-Córdoba, Boca del Río, Veracruz 94290, Mexico. E-mail address: mphv1@yahoo.com.mx (M.P. Hernández-Vergara). http://dx.doi.org/10.1016/j.aquaculture.2015.11.010 0044-8486/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aquaculture