Introduction
Recirculating Aquaculture Systems (RAS) reuse the maximum volume of water for the cultivation of aquatic organisms, maintaining water quality through various filtration methods. A RAS comprise production units equipped with automated feeding systems and sensors to control water chemical parameters such as pH and oxygen. It may also include mechanical and biological filtration systems, as well as UV-treatment for pathogen removal. Biological filtration includes the removal of nitrogen compounds as measured by e.g. Total Ammonia Nitrogen (TAN) from the water through nitrification performed by nitrifying bacteria as a biofilm on a carrier substrate with a large surface. The maturation of biofilters is the period when the nitrifying bacteria colonize the substrate. This process is a crucial step in the startup of a RAS system (Timmons et al ., 2018; Pulkkinen, 2020; Balami, 2021; Burut-Archanai et al ., 2021). A biofilter is considered mature when the concentration of TAN stabilizes below 0.7 mg/l (Rojas & Pedersen, 2016). As an outcome of the Cuban/Norwegian project “Production of marine fingerlings in Cuba”, a RAS have been installed at the Mariel Mariculture Station of the Centro de Investigaciones Pesqueras (CIP). This study describes the maturation process that is observed in the biofilter of the larval rearing area of the Mariel Station, Artemisa Province, Cuba.
Materials and methods
The maturation process of the RAS biofilter for tilapia production, were initiated the medio February 2025. Sediments from culture tanks of tilapia within the culture units of the area, were applied as fertilisers . During maturation, t emperature and salinity were measured daily. The pH and TAN were measured once a week applying the commercial kit Colombo Aquatest (Colombo, The Netherlands), with a lover level of quantification for NH3 of 0.5 mg/L. Nitrite, nitrate and phosphate were also measured on a weekly basis by the same kit, and with quantification limits of 0.5, 10.0 and 0.5 mg/L, respectively. In addition, water samples were collected from the culture units and biofilter carriers twice a month, for the subsequent identification of the bacterial community and its succession.
Furthermore, a laboratory experiment was conducted simulating RAS conditions with the aim of identifying the salinity range in which the biofilter maturation process occurs most efficiently. This experiment aimed to optimize the biofilter maturation process that will be carried out in the breeding and weaning areas of the RAS system in Mariel Mariculture Station.
In the laboratory experiment, three ranges of salinity were evaluated, each in hree replicates: Treatment 1 (T1): 30-40 ppm; Treatment 2 (T2): 20-30 ppm; Treatment 3 (T3): 10-20 ppm. Each replicate consisted of 10 L aquarium where biofilter particles were added and that were daily fertilized with water from siphoning two 60 L tanks with 20 tilapia (weighing 2-4 g each). The experiment began on the 6th of February and ended on the 12th of March 2025. During the first week, 100 mL of combined siphoning water from both fish tanks was added to all replicates. The volume was increased to 200 mL in week 2, and since week 3, 300 mL were added. Temperature and salinity were measured daily. Nitrite, nitrate, phosphate, ammonium and pH were measured once a week. In the last week of the experiment, water samples were taken from all the replicates and from the fish tanks, and biofilter carriers were sampled from all replicates to determine significant differences between the bacterial communities. The analysis of bacterial communities will be conducted in two parts, one through classical taxonomy at the lab of CIP in Cuba and the other through DNA sequencing at the Institute of Marine Research (IMR), Bergen, Norway.
Results
The TAN values were less than 0.5 mg/L during the entire period the values were measured. This happened in the Mariel Station and during the experiment conducted in the lab. Nitrite values went from 0.5 mg/L during four weeks at the Station to a spike in week 5, presenting 2 mg/L during three consecutive weeks. Something similar occurred with n itrate, going from 20 mg/L to 50 mg/L in the same period. With the aim of keeping the welfare of the fish, the density of the culture was decreased, and the n itrite and nitrate values were restored in week 8.
Conclusions
A TAN value lower than 0.7 mg/L, are the main indicator of a mature biofilter. Given that the values of these compounds were less than 0.5 mg/L from the beginning in both experiments, at the Station and the lab, the nitrite and nitrate were taken as the main indicators. The increase in the values of both compounds during week 5 until week 7 in the Station may be due to the development of the nitrifying bacteria that carry out the first step of the nitrification process (ammonium oxidation). Nonetheless, the results of the bacterial community identification from the samples that were taken during the entire experiment, will allow a better understanding of the functioning of the biofilter and to give definitive conclusions about the experiment. The above is also the case in the experiment conducted in the lab, where the bacterial community analysis to be conducted is necessary to determine if there were significant differences between the salinities examined.
References
Balami, S. (2021). Recirculation Aquaculture Systems: components, advantages, and drawbacks. 104-109.
Burut-Archanai , S., Ubertino , D., Chumtong , P., Wuttichai , M. (2021). Dynamics of Microbial Community During Nitrification Biofilter Acclimation with Low and High Ammonia. Mar Biotechnol, 671-681.
Pulkkinen, J. (2020). Microbiology of Biological Filters in Recirculating Aquaculture Systems. University of Jyväskylä: Finland.
Rojas, T. P., Pedersen, L. F. (2016) Bacterial activity dynamics in the water phase during start-up of recirculating aquaculture systems. Aq. Eng.
Timmons, M.B., Guerdat, T., Vinci, B.J. (2018) Recirculating aquaculture. Ithaca Publishing Company LLC, Ithaca.