Extinct fish are species that no longer exist in the world today. This can happen due to various reasons, including habitat loss, overfishing, and climate change. Some well-known examples of extinct fish include the Passenger Pigeon and the Tennessee Valley Authority fish, which were once abundant in their environments. The study of extinct fish helps scientists understand the impact of environmental changes on aquatic ecosystems. By examining fossil records and historical data, researchers can learn about the diversity of fish species that once thrived and the factors that led to their extinction. This knowledge is crucial for conserving current fish populations and their habitats.