Introduction
Fish polyculture is any production in which at least two fish species are reared in the same rearing system at the same time, with the objective of producing several products with economic value. Such practice is a potentially interesting option for future aquaculture developments by benefiting from the coexistence of taxa and/or interactions among species. Regardless of the systems in which polyculture is applied (i.e. traditionally in ponds or more recently in recirculated aquaculture systems, RAS), it requires compatibility and complementarity occur among the fish species farmed together (Thomas et al., 2021). Thus, fish species to be combined must be able to live in the same production system at the same time, without detrimental interactions (such as predation), benefiting from different resources with little, or, even better, no competition for resources (e.g. trophic, spatial), or developing commensal or mutualistic interactions. These prerequisites for polyculture design were used to identify species to be combined with pikeperch reared in RAS. The aim is to define a strategy to identify the species combinations that best meet the challenges of production and fish welfare for the aquaculture of the future.
Materials and methods
Several polyculture approaches were applied in RAS. Fish combinations were selected on the basis of their compatibility and complementarity. Pikeperch were combined with sterlet, with tench, or with sterlet and tench in a first experimental series and, with common carp, with black-bass, or with common carp and Eurasian perch in a second series. Pikeperch were also reared in monoculture as a reference system. All these experiments were carried out at the Experimental Platform for Aquaculture (UR AFPA, Lorraine University) in France. They were applied in experimental units with a strict control of physico-chemical parameters and a daily fish monitoring. To evaluate polyculture scenarios, several biological traits classified into two categories were measured (Amoussou et al., 2022a): i) production data (survival rate, mean final weight, weight heterogeneity, specific growth rate and Fulton Condition Index), and ii) welfare data (behavioral traits [agonistic and flight] and physiological traits [cortisol, hematocrit, glucose, serotonin and dopamine]). Multi-trait analyses were then conducted to evaluate the fish polyculture scenarios. Finally, a ranking procedure was also developed to select polyculture scenarios (Amoussou et al., 2022b).
Results
Our experimental works reveal that all studied species were impacted by polyculture, but they are not equally affected. From an aquaculture viewpoint, polyculture could result in beneficial or detrimental impacts on fish production and welfare depending on the species. Our results also demonstrate the value of a multi-traits evaluation based on an integrative analysis to better address the impacts of the polyculture. Differences are measured in the expression of some traits related to fish production or welfare. The application of the ranking procedure makes it possible to consider a variety of stakeholders’ expectations, with or without weighting, and thus result in a variable classification of polyculture scenarios.
Discussion and conclusion
Assessment of any polyculture scenarios requires all species to be considered. The objective is to exclude situations in which one species takes advantage of the rearing conditions to the detriment of another. Polyculture only makes sense if all species take advantage of these rearing conditions. A multi-trait assessment is useful for a balanced assessment between production and welfare objectives, with possible adjustment according to the varying and changing expectations of stakeholders.
References
Amoussou N., Lecocq T., Fourrier C., Nivelle R., Fleck C., Fontaine P., Pasquet A., Thomas M., 2022a. 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.
Amoussou, N., Thomas, M., P., Pasquet, A., Lecocq, T., 2022b. Finding the best match: A ranking procedure of fish species combinations for polyculture development. Life, 12(9), 1315.
Thomas M., Pasquet A., Aubin J., Nahon S., Lecocq T., 2020. When more is more: taking advantage of species diversity to move towards sustainable aquaculture. Biological Reviews, 96(1), 767-784.