The waters around Castelnou are home to aquatic life that enhances the ecosystem's richness. Amphibians like the common midwife toad, that emits an unusual almost electronic sound has been heard at Mas d’en Llinas and a rare tree frog, enjoys a refuge in the outhouse next to the pool, these and the agile frog find their breeding grounds in local ponds. The European pond turtle, a species of high conservation concern, can also be spotted in waters around Castelnou.
Conservation efforts have been vital in preserving Castelnou's delicate balance. The commune's dedication to protecting its biodiversity is evident in the establishment of protected areas and wildlife corridors. Local initiatives focus on reducing human impact and promoting sustainable agricultural practices to safeguard the region's unique flora and fauna.