Introduction
In recirculating aquaculture systems (RAS), water quality depends on the continuous removal and transformation of feed residues and fish waste. System stability is highly sensitive to waste accumulation, making effective mechanical and biological filtration essential to control suspended solids, nitrogen compounds, and chemical oxygen demand. Faecal integrity strongly affects removal efficiency: well-formed faeces are easily captured, whereas fragmented faeces break into fine particles that are harder to remove. These increase turbidity and organic load, leading to higher oxygen demand, reduced biofilter performance, and overall deterioration of water quality. This may lead to a decrease in fish performance.
Lignocellulose, derived from soft wood, is known to take positive influence on faecal integrity in companion animals and livestock. This study aims to investigate the effect of lignocellulose on fish performance, faecal integrity and water quality parameters in rainbow trout (Oncorhynchus mykiss) reared in RAS.
Materials and Methods
Rainbow trout with an average weight of 57 g were used for this trial. The animals were acclimatized to rearing conditions (2 weeks) and subsequently homogeneously allocated (240 - 250 animals) to one of 12 identical Ecotrons (RAS units) with 2.000 L tanks. Each group was fed with one of 3 iso-nitrogenous, iso-lipidic and iso-energetic diets containing graded inclusion levels of lignocellulose at 0, 1.0 and 2.0 %. In total 2920 animals were initially stocked and four ecotron replicates were used per diet.
After six weeks zootechnical performance data and faeces (striping 6 fish per unit) were collected. Water oxygen concentration and pH were continuously monitored and standard quality parameters (NO2- and NH4+) including chemical oxygen demand (with small scale tube method; ST-COD) were measured once a week The faecal score was conducted using a 3-level scoring scale with observational faecal consistency: 1: no structure (diarrhea), 2: some structure/firmness and 3: solid faecal structure.
Results
The inclusion of lignocellulose in the diets of rainbow trout did not significantly influence zootechnical parameters. The evaluation of faecal integrity and water quality showed significant influence of lignocellulose compared to the control group (see table).
Table
Results of ST-COD and Faecal Scoring
Group / Parameter
ST-COD average
(mg O2/L)
SD
Faecal Score average
SD
Control
12.2a
1.72
1.9a
0.39
1 % Lignocellulose
8.0b
2.96
2.8b
0.08
2 % Lignocellulose
6.1b
0.89
2.6ab
0.25 a,b: Different superscripts in a column indicate significant differences. COD: (p<0.01); Faecal Score (p<0.05); 1: no structure (diarrhea), 2: some structure/firmness, 3: solid faecal structure. SD: Standard deviation.
Discussion
Lignocellulose, an insoluble dietary fiber composed of cellulose, hemicellulose, and lignin, plays an important role to manage faecal consistency in companion animals i.e. dogs (Abd El-Wahab et al. 2022) and livestock animals i.e. broiler chicken (Farran et al. 2013). Lignocellulose supports gut functionality by stimulating peristalsis and improving intestinal transit. Due to its high water binding capacity, it absorbs and retains free water in the gut, increasing digesta bulk and promoting the formation of firmer, more cohesive faeces.
This feeding trial in rainbow trout (Oncorhynchus mykiss) with inclusion of 1 % and 2 % lignocellulose in the diet compared to a control group shows the efficiency to promote faecal integrity in a new species and environment. In RAS faecal consistency strongly affects waste removal efficiency. The stability of the system is highly dependent on effective removal of faeces from the fish tanks and culture water to control water quality. The significant improvement of the faecal score in this trial between control and 1% lignocellulose inclusion is accompanied by the decrease of ST-COD, a water quality parameter, for lignocellulose enriched diets up to 2%. ST-COD is considered a parameter directly linked to waste removel efficiency.
Further investigations are needed (other fish species, other breeding systems, other sources of lignocellulose) to understand the mode of action and confirm the interest of dietary lignocellulose supplementation in aquaculture.
Acknowledgment
Funded by the European Commission under grant agreement No 101131121, AQUASERV project
References
Abd El-Wahab, A.; Chuppava, B.; Siebert, D.-C.; Visscher, C.; Kamphues, J. Digestibility of a Lignocellulose Supplemented Diet and Fecal Quality in Beagle Dogs. Animals 2022, 12, 1965. https:// doi.org/10.3390/ani12151965
Farran M.; Pietsch M.; Chabrillat T. Effect of lignocellulose on the litter quality and the ready to cook carcass yield of male broilers. Actes des 10��mes Journ��es de la Recherche Avicole et Palmip��des �� Foie Gras du 26 au 28 mars, 2013, La Rochelle, France, pp. 917-921. 2013