Enrichment in a Fish Polyculture: Does it Affect Fish Behaviour and Development of Only One Species or Both? [PERSON]  
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Five categories are recognised for environmental enrichment [5]: physical, sensory, dietary, social and occupational enrichments. However, environmental enrichment may lead to conflictual results according to species and the level of complexity. Some studies have shown that environmental enrichment might have beneficial effects on fish growth [6; 7; 8; 9; 10] but also on neural structures [11], behaviours (aggressiveness, space occupation and dominance) [12; 13; 14; 15; 16] and cognitive abilities [17]. Other studies showed that the introduction of plants in the tanks or aquaria might be used by the fish as territories and lead to intraspecific conflicts for the space [18; 19; 20]. Therefore, physical enrichment could be a coveted resource for some fish species, especially if they are limited in number [19]. The beneficial effects of environmental enrichment on behaviour are thus not obvious and are species- and context-dependent. Moreover, all the studies were conducted for fish monoculture. Polyculture is a fish farming practice defined as the rearing of two or more fish species in the same environment at the same time [21]. It is mostly applied in ponds but can also be applied in more artificial systems such as in RAS in order to optimise the resources (e.g., spatial and trophic) of the farming system [22; 23]. As these rearing systems are very poorly diversified, physical enrichment may help improving the rearing conditions for fish species in polyculture [24]. However, two main problems remain: (i) the choice of the fish species, which may be associated in polyculture and (ii) the type of enrichment devices depending on the fish species requirements. Here, we focused on physical enrichment of the structures for which different types of enrichment may be added to the tanks. Shelters or cover structures are known to reduce aggression, providing caches for subordinates when there is a high intraspecific pressure [25; 26; 27; 28; 29; 30; 31; 32]. Introduction of structural objects can also reduce fin injuries [33; 34; 35; 36] and improve growth, survival [37] and welfare [38; 39; 25]. Shelters can also reduce maintenance and metabolism, as observed in juvenile salmon [40]. However, shelters can have different effects depending on the fish species biology (e.g., cannibalism). Shelters reduced cannibalism in African sharbtooth catfish (_Clarias gariepinus_) [41] but not in juvenile Asian redtail catfish (_Hemiibgrus nemurus_) [42]. There are similar results concerning fish growth: shelters may have positive, negative or no effect on fish growth depending on the size or the species of the fish. A few studies tested several types of physical enrichment simultaneously; the use of a cover and vertical pipes in the same tank increased the biomass gain for juveniles of brown trout (_Salmo trutta_) and rainbow trout (_Oncorhynchus mykiss_) [10], but the physical enrichment of the rearing environment in fish polyculture is another challenge. To the best of our knowledge, there is no study on the influence of enrichment in a polyculture farming system except studies that integrated the role of nutrient-enriched fertilisers on the growth of fish and shrimps reared in polyculture [42]. In this study, we introduced physical enrichments in a RAS polyculture of two fish species: pikeperch (_Sander lucioperca_) and sterlet (_Acipenser ruthenus_). These two species have an economic value due to the high quality of their flesh (see for pikeperch [43] and sterlet [44]). This fish association has been already used in RAS polyculture studies [22; 23; 43] because of their abiotic compatibility and complementarity in the exploitation of the same trophic resource.