<P> The Caspian turtle (Mauremys caspica), although found in neighboring areas, is a wholly freshwater species . The zebra mussel is native to the Caspian and Black Sea basins, but has become an invasive species elsewhere, when introduced . The area has given its name to several species, including the Caspian gull and the Caspian tern . The Caspian seal (Pusa caspica) is the only aquatic mammal and is endemic to the Caspian Sea, being one of very few seal species that live in inland waters, but is different from those inhabiting freshwaters due to the hydrological environment of the sea . </P> <P> Archeological studies of Gobustan petroglyphs indicate that there once had been dolphins and porpoises, or a certain species of beaked whales and a whaling scene indicates of large baleen whales likely being present in Caspian Sea at least until when Caspian Sea was a part of ocean system or until Quaternary or much more recent periods such as until the last glacial period or antiquity . Although the rock art on Kichikdash Mountain assumed to be of a dolphin or of a beaked whale, might instead represent the famous beluga sturgeon due to its size (430 cm in length), but fossil records suggest certain ancestors of modern dolphins and whales, such as Macrokentriodon morani (bottlenose dolphins) and Balaenoptera sibbaldina (blue whales) were presumably larger than their present descendants . From the same artworks, auks, like Brunnich's Guillemot could also have been in the sea as well, and the existences of current endemic, oceanic species such as lagoon cockles which was genetically identified to originate in Caspian / Black Seas regions, and these petroglyphs suggest marine inflow between the current Caspian Sea and the Arctic Ocean or North Sea, or the Black Sea . </P> <P> The sea's basin (including associated waters such as rivers) has 160 native species and subspecies of fish in more than 60 genera . About 62% of the species and subspecies are endemic, as are 4--6 genera (depending on taxonomic treatment). The lake proper has 115 natives, including 73 endemics (63.5%). Among the more than 50 genera in the lake proper, 3--4 are endemic: Anatirostrum, Caspiomyzon, Chasar (often included in Ponticola) and Hyrcanogobius . By far the most numerous families in the lake proper are gobies (35 species and subspecies), cyprinids (32) and clupeids (22). Two particularly rich genera are Alosa with 18 endemic species / subspecies and Benthophilus with 16 endemic species . Other examples of endemics are four species of Clupeonella, Gobio volgensis, two Rutilus, three Sabanejewia, Stenodus leucichthys, two Salmo, two Mesogobius and three Neogobius . Most non-endemic natives are either shared with the Black Sea basin or widespread Palearctic species such as crucian carp, Prussian carp, common carp, common bream, common bleak, asp, white bream, sunbleak, common dace, common roach, common rudd, European chub, sichel, tench, European weatherfish, wels catfish, northern pike, burbot, European perch and zander . Almost 30 non-indigenous, introduced fish species have been reported from the Caspian Sea, but only a few have become established . </P> <P> Six sturgeon species, the Russian, bastard, Persian, sterlet, starry and beluga, are native to the Caspian Sea . The last of these is arguably the largest freshwater fish in the world . The sturgeon yield roe (eggs) that are processed into caviar . Overfishing has depleted a number of the historic fisheries . In recent years, overfishing has threatened the sturgeon population to the point that environmentalists advocate banning sturgeon fishing completely until the population recovers . The high price of sturgeon caviar, however, allows fishermen to afford bribes to ensure the authorities look the other way, making regulations in many locations ineffective . Caviar harvesting further endangers the fish stocks, since it targets reproductive females . </P>

Why is the caspian sea a sea and not a lake