Introduction
Biofloc technology (BFT) offers new opportunities for intensive freshwater aquaculture by improving water quality through microbial flocculation and nutrient recycling1. It also allows for reduced water consumption, better feed conversion, and the potential reuse of microbial biomass as a nutritional resource. While widely used for tropical species like tilapia (O. niloticus) and shrimp, its application to temperate freshwater fish is limited and relatively unknow. The POLYRAS project (Interreg GR) aims to diversify and develop polyculture, which is widely used in pond systems, in three sustainable aquaculture systems: Recirculating Aquaculture Systems (RAS), aquaponics systems, and BFT. Within this cooperative project, our study focuses specifically on the Biofloc technology.
This work explores the possibility to rear new temperate freshwater species in polyculture in BFT mainly focusing on survival rate and growth performances. The six selected species, black bass (Micropterus salmoides), Russian sturgeon (Acipenser gueldenstaedtii), pikeperch (Sander lucioperca), perch (Perca fluviatilis), catfish (Silurus glanis), and tench (Tinca tinca), were chosen based on their economic potential in the Greater Region and adaptation to different polyculture scenarios, as identified in the TOFF database2. As most of these species have not been previously reared in BFT systems, this preliminary study was designed to screen and identify new candidate species for BFT aquaculture, evaluating their growth and investigating how each species may influence microbial dynamics in the system.
Material and Methods
The experiment was performed in three individual 250 L tanks maintained in BFT and one 250 L control tank in clear water (RAS). Fish were stocked at an initial density of 6 kg/m³, with the number of individuals adjusted according to species size. Each species was successively reared for a period of three weeks and fed to satiation throughout the experimental period.
Growth performances were assessed through morphometric measurements (body weight and total or fork length) and food conversion ratio (FCR) assessment. Water quality was monitored daily (t°, O2, floc volume) or twice a week (pH, conductivity, TAN, and alkalinity) to ensure stable environmental conditions.
At the end of the experiment, water samples from BFT tanks were analyzed to characterize microbial communities. Zooplankton and phytoplankton were counted in 1 mL subsamples, and floc particle size and bacterial density were measured.
Results and discussion
The performance of black bass in BFT is promising, with 100% survival, as in the RAS control, and a positive growth (3 g), slightly lower than in RAS (5.9 g). In contrast, sturgeon showed variable survival (50–100%) and marked weight loss in BFT (−16.5 to −61.4 g), while 100% survival and significant weight gain (44.3 g) were recorded in RAS. With stable water quality parameters, no specific issue could explain these results, suggesting sturgeon may not be suitable for BFT systems. Preliminary trials with perch and catfish were stopped after one week due to signs of stress, such as reduced feeding and abnormal behavior. These findings highlight both the potential and the limitations of BFT for temperate freshwater species (Table 1).
Pikeperch demonstrated good growth performance across all BFT tanks (9.3 g). Survival was affected by elevated ammonia concentrations linked to a temporary instability of the biofloc. Under more stable conditions, pikeperch could be successfully reared in BFT. Tench demonstrated excellent adaptability to BFT system, with 100% survival (Table 1). Individual weight gain ranged from 34.7 to 51.1 g in the BFT tanks, slightly surpassing the 34.5 g average in the RAS control. The stability of physicochemical parameters throughout the trial supported these good performances (Table 1).
The analysis of planktonic communities suggests that species reared in BFT systems may influence the abundance and composition of phytoplankton and zooplankton, with noticeable differences in ciliate, and phytoplankton counts (Table 2). Measurements of floc particle size and bacterial abundance are still in progress and should further clarify the interactions between fish species and microbial dynamics in BFT systems.
These preliminary results highlight black bass, pikeperch and tench as promising candidates for BFT culture.
References :
1. Avnimelech, D. Y. Tilapia Production Using Biofloc Technology. (2011).
2. Lecocq, T. et al. Stronger together: A workflow to design new fish polycultures. Rev. Aquac. 16, 1374–1394 (2024).