Formerly the fourth largest lake in the world with an area of 68,000 km

Introduced fish

Other salt-tolerant fish species were intentionally or inadvertently introduced during the 1960s when hydropower and irrigation projects reduced the flow of fresh water thereby increasing salinity. These include the Baltic herring (Clupea harengus membras), big-scale sand smelt (Atherina boyeri caspia), black-striped pipefish (Syngnatus abaster caspius), Caucasian dwarf goby (Knipowitschia caucasica), monkey goby (Neogobius fluviatilis), round goby (N. melanostomus), Syrman goby (N. syrman), bighead goby (Ponticola kessleri), tubenose goby (Proterorchinus marmoratus), grass carp (Ctenopharyngodon idella), silver carp (Hypophtalmichthys molitrix), bighead carp (H. nobilis), black carp (Mylopharyngodon piceus) and northern snakehead (Channa argus warpachowski).

The herring, sand smelt, and gobies were the first planktivorous fish in the lake, leading to a collapse of the lake's zooplankton population. This in turn caused a collapse of the herring and sand smelt population from which neither species has recovered. All introduced species aside from the carp, snakehead, and (possibly) pipefish survived the lake's shrinkage and salinity increase, and during this time the European flounder (Platichthys flesus) was introduced to revive fisheries. The extirpated species (aside from possibly the pipefish) returned to the North Aral Sea following its recovery. Herring, sand smelt, gobies and flounder persisted in the South Aral Sea until increasing salinity extirpated all but the gobies.