Is it the blue gravel substrate or only its blue color that improves growth and reduces aggressive behavior of gilthead seabream Sparus aurata?

The positive effects of a blue gravel substrate as a means of environmental enrichment have been previously observed for gilthead seabream Sparus aurata. The aim of the present study was to investigate whether the blue color per se is efficient to produce similar benefits to those observed with the blue substrate. If this hypothesis proves true, then it would be more practical just to paint the bottom of the rearing tanks to this specific color. Triplicated tanks contained the Blue Substrate (BS), the Photo of the Blue Substrate (PBS) or remained plain - Control (C). Fish groups of 18 specimens (mean initial body mass 20.2 ± 0.26 g) were reared in these conditions for 75 days in a recirculating aquaculture system (RAS). According to results, BS fish showed greater final body mass and less aggression than PBS and C fish indicating that the presence of the blue substrate and not its blue color is responsible for the beneficial effects of the blue substrate. In the case of several growth parameters (i.e. specific growth rate, food conversion ratio, total and standard length), the lack of significant difference between BS and PBS fish suggests that the substrate color has also merit. In conclusion, present results confirm that the use of the blue substrate on the tank bottom improves gilthead seabream welfare and provides for growth benefits. It is also shown that it is not enough just to paint the tank bottom in this certain color. The effectiveness of this environmental enrichment, ensuring gilthead seabream welfare, strengthens the use of land-based aquaculture facilities and the reassessment of RAS inclusion in existing legislation concerning organic aquaculture for on-growing production stage.