After flounder zone we come to the special cave systems with endemic species. Among 150 fish species present in Croatian rivers and lakes, as many as 50 are endemic. Such abundance is the result of geographical position which includes the rivers in the Black Sea and Adriatic Basin. Many endemic species are related to the karst underground habitats, which makes them extremely sensitive and vulnerable to extinction. Therefore, most of them are protected by legislation on the European and national level and their survival depends directly on the activities of man or, in other words, on active protection and preservation of surface water and groundwater.
As many as forty endemic species are present in the Adriatic Basin, which is the result of specific ecological and habitat conditions. Most of these are stenoendemic which reside in a smaller and a very limited area.
So, for example, in Croatian karst rivers such as Jadova we can find Jadova loach, in Zrmanja Zrmanja chub, and in Neretva marble trout. Marble trout is a very interesting species from the trout family, similar to huchen in appearance, but of marbled colour pattern and without black spots on the body. It is extremely rare and endangered migratory species. It can grow up to 140 centimeters and weigh up to 30 kilograms.
Around ten endemic species are found in the water streams of the Black Sea Basin. They include some species which are among the world’s most endangered species, such as Croatian dace and karst dace. Interestingly, both Croatian dace and karst dace were considered the same species, but clear morphological differences were confirmed by molecular analyses which showed that these are two separate species.