## 4 Discussion  
Our results showed that (i) there was no effect of physical enrichment on the survival, growth and morphological development of the two species, (ii) space occupation differed between the modalities (with and without enrichment) for the pikeperch and the sterile and (iii) there was no influence of enrichment on abnormal behaviours exhibited by the sterile. Mortality rates should be considered differently according to the two species. They corresponded to dead fish or fish that were not found at the end of the experiment for pikeperch. The loss of individuals could be linked to cases of cannibalism, a common behaviour for this species [53] and observed twice in our experiment. For the sterile, the mortality rates were mainly explained by fish reaching the limit point defined in the study (FCF less than 0.27), which refers to lean fish and are thus out of the experimental protocol. Pikeperch switched its space occupation under the cover places due to the presence of PVC planks, which also corresponded to the part of the tank where the pipes modified the flow. The pikeperch most likely reacted to the presence of cover and pipes according to their preferred habitat [48]. Sterlet also changed their space occupation; they did not use the cover and were more numerous in the part of the tank without enrichment (zone 2). The spatial distribution of the two species was controlled by both the application of enrichment causing a low luminosity (zone 1) and the placement of the feeders in this shaded area. We did not observe aggressiveness between the fish of the two species. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & & Enrichment (E) & & & \\ \cline{2-7} Species & Wi (g) & FCFi & Wf (g) & FCFf & BMI (g) & BMI (g) & BG (\%) & SGR (\%/day) \\ \hline Sterlet & 133.1 \(\pm\) 21.2 & 0.40 \(\pm\) 0.00 & 324.7 \(\pm\) 65.2 & 0.4 \(\pm\) 0.0 & 3993.2 \(\pm\) 17.6 & 7472.2 \(\pm\) 674.3 & 87 \(\pm\) 17.5 & 0.99 \(\pm\) 0.08 \\ \hline Pikeperch & 142.0 \(\pm\) 38.1 & 0.7 \(\pm\) 0.1 & 381.9 \(\pm\) 88.4 & 0.8 \(\pm\) 0.1 & 6373.6 \(\pm\) 39.7 & 14763.5 \(\pm\) 303.1 & 131.6 \(\pm\) 3.4 & 1.10 \(\pm\) 0.03 \\ \hline \multicolumn{7}{c}{No Enrichment (NE)} \\ \cline{2-7} Species & Wi (g) & FCFi & Wf (g) & FCFf & BMI (g) & BMI (g) & BG (\%) & SGR (\%/day) \\ \hline Sterlet & 132.6 \(\pm\) 21.1 & 0.38 \(\pm\) 0.04 & 337.1 \(\pm\) 51.4 & 0.46 \(\pm\) 0.05 & 3979.0 \(\pm\) 11.7 & 7434.5 \(\pm\) 941.6 & 87 \(\pm\) 23.2 & 1.04 \(\pm\) 0.05 \\ \hline Pikeperch & 143.4 \(\pm\) 44.6 & 0.7 \(\pm\) 0.1 & 369.7 \(\pm\) 94.0 & 0.8 \(\pm\) 0.1 & 6452.4 \(\pm\) 92.5 & 14412.0 \(\pm\) 617.0 & 123.3 \(\pm\) 7.8 & 1.05 \(\pm\) 0.03 \\ \hline \multicolumn{7}{c}{Wi: initial weight; FCFi: Fulton condition factor at day 0; Wf: final weight; FCFf: Fulton condition factor at day 90; BMI and BMF: initial and final Biomass, respectively; BG: biomass gain; SGR: specific growth rate.} \\ \end{tabular}
\end{table} 
Table 1: Fish growth data (mean \(\pm\) standard deviation) for the two species, sterile (_Acienser ruthenus_) and pikeperch (_Sander lucioperca_), reared with or without enrichment (E and NE, respectively). In monoculture, several studies showed that physical enrichment of the rearing environment can affect the fish behaviour. In general, environmental enrichment promotes behavioural flexibility in animals reared under captive environments and can positively affect neural plasticity and cognitive abilities [17; 54]; for example, Zebrafish in bare tanks can be more exploratory (cognitive items) [55]. Some results on the effect of environmental enrichment on aggressive behaviours are more contested [29]. For example, structural enrichment reduced aggression levels in zebrafish [18; 29; 31] and in salmon [56], but another study [57] reported that zebrafish in enriched environments were more aggressive (in the mirror test) than control. For structural enrichment, the presence of pipes suspended vertically in the tank modified the water flow in the tank. Different studies showed that such devices can affect fish development.