Illustration of a European sea sturgeon (Acipenser sturio) on the beach. This species of sturgeon was once abundant in Europe and could be found in coastal habitats all over the continent. However, they were heavily hunted for their caviar, which resulted in a sharp decline in their population from the second half of the 19th century. Today, the European sea sturgeon is classified as critically endangered by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. The species spend most of their lives in saltwater but migrate to spawn in freshwater rivers. They can grow up to 6 metres in length and weigh up to 1000 kilograms.