Introduction
This study is part of the POLYRAS project (Interreg GR), which aims to optimize the sustainable production of locally important fish species, such as pikeperch, through polyculture in recirculating aquaculture systems (RAS). Polyculture, a traditional practice in pond aquaculture, aims to exploit species complementarity to improve feed utilization and overall productivity (Thomas et al., 2020). Applied in RAS, polyculture may enhance feed efficiency, increase production and support animal welfare (Kolowski et al., 2014; Thomas et al., 2020; Penka et al., 2021; Amoussou et al., 2022).
This study evaluates, in a recirculating aquaculture system (RAS), whether co-culturing pikeperch (Sander lucioperca) with European catfish (Silurus glanis), a fast-growing bottom-feeding species, can offer some benefits compared to the monoculture of each species. Specifically, we investigated whether polyculture could maintain or enhance the growth of both pikeperch and catfish, improve overall tank productivity, reduce feed waste, and support species welfare. Results on growth performances and preliminary results on fish welfare are presented in this study.
Material and Methods
The experiment took place over a 174-days experimental period in seven 1 m3 tanks in a RAS supplied with water at 23 °C under a 12L:12D photoperiod. Three experimental rearing conditions were tested: pikeperch monoculture, European catfish monoculture, and biculture of both species (ratio 1:1). The initial stocking density was 8 kg.m-3. Tanks in monoculture were run in duplicate and those in polyculture in triplicate. Initial mean body weights were 167 g for pikeperch and 182 g for catfish. Pikeperch were fed with floating pellets (Le Gouessant Percid Grower®, 52% proteins, 15% lipids) to satiation, while catfish received sinking pellets (Coppens Silurus Pro®, 46% proteins, 19% lipids) at a restricted rate of 1.5% of biomass. Growth was assessed monthly by individually weighing 20 fish per species and tank, and total biomass was assessed at the start, mid-point, and at the end of the experiment. Fin condition in pikeperch, as well as fin and barbel condition in European catfish, were evaluated (adapted from Policar et al., 2016) to assess fish welfare.
After a normality and homogeneity test of variances (Shapiro–Wilk and Bartlett tests), an ANOVA with Tukey's post-hoc or Kruskal–Wallis with pairwise Wilcoxon tests were applied as required (P < 0.05).
Results
Survival rates were up to 95% for all batches. Final mean body weights were similar between mono- and polyculture for pikeperch (respectively 612 ± 152g ; CV=24.1% and 602 ± 145g ; CV= 23.8%) and catfish (respectively 1186 ± 303g ; CV = 25.1% and 1218 ± 335g ; CV = 27.1%) with no significant differences (P > 0.05). Specific growth rates (SGR) were similar between mono- and polyculture for pikeperch (respectively 0.71 and 0.72) and catfish (respectively 1.05 and 1.08). Food conversion ratio (FCR) in polyculture (0.87) was intermediate between those observed in pikeperch monocultures (1.01) and catfish monoculture (0.82). Period-specific SGR analysis highlighted higher growth in polyculture under specific conditions.
After the first three months of experiment, pikeperch reared in monoculture exhibited significantly better fin condition than those in polyculture (P < 0.05), but this difference was no longer observed at the end of the experiment. In contrast, no difference in fin and barbel condition was detected for catfish after three months, whereas at the end of the experiment, individuals reared in polyculture showed significantly better condition than those in monoculture (P < 0.05).
Conclusions and perspectives
Our results indicate that pikeperch–catfish biculture in RAS can sustain favorable growth performances without compromising survival rate and welfare of both species. To further evaluate the potential of the system, isotopic analyses are ongoing to identify the source of the ingested feed and confirm the hypothesis of the valorisation of uneaten feed by catfish. Additionally, cortisol levels in pikeperch scales will be measured to monitor physiological stress (adapted from Carbajal et al., 2018). Based on our observations, we suggest that the performance of this biculture may be enhanced through implementation of both cohort management and increased initial stocking density. Feed valorization and a comprehensive assessment of fish welfare will be addressed in future work based on the present results.
Acknowledgment
This work was funded by Interreg Great Region and European Regional Development Fund (ERDF).
References
Amoussou, N., Lecocq, T., Fourrier, C., Nivelle, R., Fleck, C., Fontaine, P., ... & Thomas, M. (2022). A multi-trait evaluation framework to assess the consequences of polyculture in fish production: An application for pikeperch in recirculated aquaculture systems. Aquaculture Reports, 27, 101349.
Carbajal, A., Monclús, L., Tallo-Parra, O., Sabes-Alsina, M., Vinyoles Cartanyà, D., & López Béjar, M. (2018). Cortisol detection in fish scales by enzyme immunoassay: biochemical and methodological validation. Journal of Applied Ichthyology, 34(4), 967–970. Kozłowski, M., Szczepkowski, M., Wunderlich, K., Szczepkowska, B., & Piotrowska, I. (2014). Polyculture of juvenile pikeperch (Sander lucioperca (L.)) and sterlet (Acipenser ruthenus L.) in a recirculating system. Fisheries & Aquatic Life, 22(4), 237–242. Pěnka, T., Malinovskyi, O., Křišťan, J., Imentai, A., Policar, T. (2021). Effect of density and mixed culture with pikeperch (Sander lucioperca) on effectivity of largemouth bass (Micropterus salmoides) intensive culture. Czech Journal of Animal Science, 66, 428–440. Policar, T., Blecha, M., Křišťan, J., Mráz, J., Velíšek, J., Stará, A., ... & Samarin, A. M. (2016). Comparison of production efficiency and quality of differently cultured pikeperch (Sander lucioperca L.) juveniles as a valuable product for ongrowing culture. Aquaculture International, 24, 1607–1626. Thomas, M., Lecocq, T., Abregal, C., Nahon, S., Aubin, J., Jaeger, C., ... & Pasquet, A. (2020). The effects of polyculture on behaviour and production of pikeperch in recirculation systems. Aquaculture Reports, 17, 100333. Thomas, M., Pasquet, A., Aubin, J., Nahon, S., & Lecocq, T. (2021). When more is more: taking advantage of species diversity to move towards sustainable aquaculture. Biological reviews, 96(2), 767-784.