This manual provides a clear and well-structured introduction to aquatic biodiversity and conservation, specially designed for undergraduate students. It explains the connection between ecological processes like geochemical cycles and population dynamics with the health of aquatic ecosystems. The manual covers important topics such as the types and importance of biodiversity, major threats, and practical conservation methods. It explores key aquatic environments including estuaries, mangroves, coral reefs, and wetlands, and discusses the conservation needs of aquatic animals like mammals, birds, reptiles, and amphibians. With a focus on sustainable fishing practices, conservation laws, and global agreements, the manual combines scientific understanding with real-world applications to promote responsible use and protection of aquatic resources.'