Introduction
Biofloc technology (BFT) is an alternative aquaculture approach that relies on microbial aggregates to improve water quality and potentially provides an additional nutritious food source for cultured fish. Although BFT is usually applied under tropical conditions, typically between 25 and 30°C, its functioning at lower temperatures remains poorly understood. Nevertheless, the development of BFT under temperate continental conditions could offer promising opportunities for the diversification of inland aquaculture in Europe. The present study investigated the effect of four rearing temperatures (15, 20, 25 and 30°C) on biofloc dynamics, nutrient dynamics and fish performance using common carp (Cyprinus carpio) as the culture model.
Materiel and methods
Experiment was conducted on juvenile common carp (initial mean body of 15 g) in 100l tanks in triplicate at an initial stocking density of 5 kg m���3. The experiment consisted of three successive phases. A first 4-week stabilization period was carried out in order to allow the stabilization of the biofloc which was inoculated from a stock solution at 23°C. Floc is considered stable when floc volume exceeded 15 mL L���1, TAN concentration remained below 2 mg L���1 and nitrite concentration below 1 mg L���1. A second 3-week period was applied to gradually reach the experimental temperature (15, 20, 25 or 30°C) . Then the 12-week experimental phase was conducted. Feed ratios (15°C : 1.5%, 20°C : 1.5%, 25°C : 3%, 30°C : 3.5%) were adjusted daily according to tank biomass and water temperature.
Water quality and biofloc development were daily or biweekly monitored by measuring temperature, dissolved oxygen, floc volume, pH, conductivity, alkalinity, TAN, nitrite, nitrate and suspended solids. Fish feed, molasses, sodium bicarbonate and water renewal were quantified. Total carp biomass was measured four times: at stocking (before the stabilization phase), at the start of the experimental phase (T0), after 6 weeks, and after 12 weeks (end of the experiment). Individual weight and length of 20 fish per tank were recorded every two weeks.
Carbon and nitrogen budgets were then established to determine how the nutrients introduced through feed and molasses were partitioned within the system, to identify where carbon and nitrogen accumulated, and to assess whether carp consumed biofloc particles in addition to commercial feed. Total carbon and nitrogen were measured by isotopic analysis (δ13C and δ15N) in feed, fish muscle, and in both the liquid and solid fractions of the biofloc collected from Imhoff cones after 30 minutes of decantation.
Parallely, microbial community associated with the biofloc (rotifers, ciliates, ���) was identified and quantified. Floc particle size were characterized microscopically through measurements of particle area and perimeter.
Results
Preliminary results showed that the specific growth rate (SGR) increased with temperature, reaching 0.90% day-1 at 15°C, 0.84% day-1 at 20°C, 1.00% day-1 at 25°C and 1.14% day-1 at 30°C. In contrast, feed conversion ratio (FCR) was not improve with increasing temperature. The best feed conversion ratio was observed at 15°C (1.34), whereas higher values were recorded at 20°C (1.58), 25°C (2.09) and 30°C (1.76). However, the results obtained at 20°C and 25°C should be interpreted with caution, as they were based on a single replicate and two replicates, respectively. Two tanks at 20°C and one tank at 25°C were excluded from the analysis due to a bacterial disease affecting the carp and altering the experimental results.
We observed that biofloc-related parameters were strongly influenced by temperature. Mean floc volume remained lower at 15°C (15 mL L���1) compared to other temperature with mean values ranging from 21 to 32 mL L���1 (Table 1). TAN concentrations remained relatively stable among treatments, ranging from 2.0 to 3.2 mg L���1. Nitrite concentrations were consistently higher in the 15°C tanks throughout the experiment (mean value: 1.0 mg L���1), compared with only 0.1–0.4 mg L���1 at higher temperatures. Moreover, nitrate concentrations increased with temperature, from a mean value of 384.3 mg L���1 at 15°C to 557.0, 600.2 and 616.8 mg L���1 at 20, 25 and 30°C, respectively.
Additional analyses are currently in progress to further characterize the functioning of the system. Carbon and nitrogen contents in fish muscle samples are still under investigation in order to complete the nutrient budgets and better understand the contribution of biofloc to fish nutrition. Likewise, the identification and quantification of microorganisms associated with the biofloc, including rotifers, ciliates and other microbial groups are ongoing. Measurements of floc particle area and perimeter are also underway and should provide further insight into how temperature affects biofloc structure and composition.
Overall, our results show that biofloc technology (BFT) remained efficient at temperatures lower than those usually reported for this aquaculture rearing system (25–30°C), while maintaining stable water quality and acting effectively as a biological treatment system. Common carp exhibited satisfactory growth performance under all tested conditions, demonstrating good adaptability to BFT across the evaluated thermal range. These findings suggest that BFT could represent a promising diversification strategy for inland aquaculture in continental Europe, particularly under temperate conditions.
FCR
SGR (%/day)
Floc volume (ml/L)
TAN (mg/L)
Nitrite (mg/L)
Nitrate (mg/L)
15°C
0.90
1.34
15
2.2
1.0
384.3
20°C
0.84*
1.58*
21*
2.0*
0.1*
557.0*
25°C
1.00**
2.09**
32**
3.2**
0.4**
600.2**
30°C
1.14
1.76
26
2.3
0.2
616.8
Table 1 : mean FCR, SGR, biofloc volume and water quality Nitrogen parameters during the 12-week experimental phase at 15, 20, 25 and 30°C. Values are means per treatment; asterisks indicate reduced replication (* single replicate, ** two replicates)