The role of sandy environment in enhancing growth, cognition and reproduction in cultured Senegalese sole ( <i>Solea senegalensis</i> ) females

Abstract Physical enrichment improves fish welfare and reduces negative traits in captivity. Senegalese sole (Solea senegalensis), a key species in European aquaculture, is often raised in intensive fiberglass tanks, which can lead to adverse traits, particularly for future breeders. This study assessed the long-term effects of using a sand substrate during the on-growing phase on Senegalese sole females' growth, cognition and maturation. Six tanks with juvenile soles were divided into two environments (three fibreglass, three sand) and sampled every six months for 2 years (n = 30 per group). This study measured biometrics, blood, gonad histology and gene expression in the brain and gonad. Females from sand grew faster than females from fibreglass. The histological results showed that the sand group was more mature than the fiberglass group from the second sampling, where the sand group reached previtellogenic oocytes. Sex steroids and luteinizing hormone (Lh) levels were also significantly higher in the sand group after two years. These results in maturation were supported by the fhra and pgr expression in brain and gonads. Females from sand showed higher cognitive capacity according to c-fos, nr4a2 and bdnf expression. These results show that using sand as an enrichment improves growth, cognition and reproduction in Senegalese sole females.