Cousin Island is recognised as one of the most significant hawksbill turtle nesting sites in the western Indian Ocean. Female turtles come ashore between October and February to lay their eggs in the beach sand, returning multiple times in a season. The conservation programme has monitored and protected these nests since the island was made a reserve in 1968, producing a long-term dataset that documents the slow recovery of this critically endangered species.
Green turtles also nest on the beaches, though in smaller numbers. The surrounding waters form a marine protected area, and the reefs around the island have some of the highest fish biomass recorded in any of the Seychelles marine protected areas. Snorkelling is possible during visits, conditions permitting.