It is generally believed that about 90 percent of all the world's sturgeons swim in the Caspian Sea. Seven species or subspecies of sturgeon populate the Caspian, giving it the greatest diversity of sturgeon in the world.

The Caspian Sea is an endorheic, or terminal lake, meaning that its water does not reach the ocean. Because of this, minerals build up in the water as it evaporates and the lake usually becomes saline. Endorheic lakes are usually more sensitive to pollution than waterbodies that drain to the ocean.

Severe overfishing has decimated fish species and caused sharp declines in catches. Between 1920 and 1940, the most common commercial species were the Caspian lamprey, Volga shad, Caspian trout, and Caspian inconnu. The total catch of these species was about 80 thousand tons. All these species are now included in the Red Books of the Republic of Kazakhstan, Russian Federation, and other Caspian states. In the Islamic Republic of Iran over-fishing of Caspian trout, bream, and zander, along with the damage of their habitats and spawning grounds, has resulted in an almost complete loss of these species. The Caspian zander disappeared due to massive catches in Azerbaijan and Turkmenistan.

During the last twenty years, the sturgeon catch has declined by 88 percent. 90% of the Beluga spawning grounds are gone due to dams. Poaching may take up about 12 times the volume of the official catch.

More research is needed on the impact of invasive species on the Caspian. The comb jelly Mnemiopsis leidyi has recently been discovered.