## 2 Material and Methods
### Behavioural Parameters  
The behavioural parameters focused on space occupation and abnormal behaviours of the two species. These behaviours were chosen because space occupation can be directly linked to the presence of physical structures in the tanks and abnormal behaviours are related to stress (e.g., stereotypies or behaviours not expressed in the wild). Therefore, these behaviours are indicators of behavioural changes due to environmental enrichment. These behaviours were extracted from videos that were realised with two cameras (Sony HDR CX550 VE) for each tank. To compare the space occupation by the two fish species, we divided each tank into two zones (Figure 1). The first zone (zone 1) matched with the low light intensity (5 lux) zone defined by the projection of the shadow due to the presence of the plank and the two belt feeders for the three tanks with physical enrichment. This zone 1 is drawn virtually for the tanks without enrichment. The zone 2 was the high light intensity (20 lux) zone. This luminosity was the same everywhere for the tanks without enrichment. Even with the two cameras, all the surface of the tanks was not visually accessible. Only 54% of the total surface of each tank was in the field of the cameras, and this part was equally shared in zones 1 and 2 (Figure 2).  
Figure 1: Top view of the tank with physical enrichment and showing parts of the tank that differ by the luminosity (zone 1–5 Lux vs. zone 2–20 Lux) due to the cover with PVC plank (PVC). The zones are delimited by the dash line. The surface of zone 1 was larger than the surface of the planks because it projects shadow on the bottom of the tank. W.A. = water arrival; F1, F2 = automatic feeders; VP1, VP2, VP3 = vertical pipes.  
We recorded fish behaviours (space occupation and abnormal behaviours) during two periods: first between day 19 and day 22, and second between day 39 and day 42. Each recording sequence lasted 30 min, and there was a sequence in the morning after the feeding phase and another during the afternoon three hours before the dawn when all the feed was eaten. Each day, a couple of tanks with and without enrichment were recorded. The association between the two tanks was realised at random. The six tanks were recorded one time. During a recording session, we stopped the air diffusion to facilitate the fish view and their counting. This short pause of the air diffusion had no consequence on the dissolved oxygen concentrations. The two cameras were synchronised.  
All the 30 min of video recordings were analysed. For that, 20 images were extracted from the video every one minute and a half. On each image, we counted the number of fishes of the two species in each zone (see above for the delimitation of the two zones, shaded or not). These two zones were reported on the screen of the computer and the position of each fish was pointed.  
For the two species, abnormal behaviours were defined as:  
* Particular swimming. Two different behaviours were observed: (i) the fish swam on the back (SB) with the ventral body above, and (ii) the fish swam vertically (SV) with the head above the water surface.
* Jumps (J) were also observed; the fish was out of the water with its entire body above the surface. This behaviour was observed mainly along the wall of the tank.