The Aral Sea

Ecology

Native fish Despite its former vast size, the Aral Sea had relatively low indigenous biodiversity. However, the Aral Sea basin had an exceptional array of endemic fish subspecies (as well as the three endemic sturgeon species), most of which still survive in the North Aral Sea, but some such as the sturgeons have been extirpated or even driven extinct by the lake's shrinkage. Native fish species of the lake included ship sturgeon (Acipenser nudiventris), all three Pseudoscaphirhynchus sturgeon species, Aral trout (Salmo trutta aralensis), northern pike (Esox lucius), ide (Leuciscus idus oxianus), asp (Aspius aspius iblioides), common rudd (Scardinius erythropthalmus), Turkestan barbel (Luciobarbus capito conocephalus), Aral barbel (L. brachycephalus brachycephalus), common bream (Abramis brama orientalis), white-eyed bream (Ballerus sapa aralensis), Danube bleak (Chalcalburnus chalcoides aralensis), ziege (Pelecus cultratus), crucian carp (Carassius carassius gibelio), common carp (Cyprinus carpio aralensis), Wels catfish (Silurus glanis), Ukrainian stickleback (Pungitius platygaster aralensis), zander (Sander lucioperca), European perch (Perca fluviatilis), and Eurasian ruffe (Gymnocephalus cernuus). All these fish aside from the stickleback lived an anadramous or semi-anadromous lifestyle.

The salinity increase and drying of the lake extirpated the Aral trout, ruffe, Turkestan barbel and all sturgeon species, and dams now block their return and migration routes, with the Aral trout and Syr Darya sturgeon (Pseudoscaphirhynchus fedtschenkoi) possibly extinct due to their restricted range.^[15][17] All other native fish barring the stickleback (which persisted during the lake's shrinkage and salinity increase) were also extirpated, but have returned to the North Aral Sea following its recovery from the 1990s onwards.

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 

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.^[14] 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.

Invertebrates Prior to its shrinkage, the Aral Sea had about 250 species of native aquatic invertebrates, the majority (about 80%) being freshwater species; the rest were marine invertebrates with ties to the Ponto-Caspian and Mediterranean-Atlantic fauna. The dominant species (excluding protozoa) were rotifers, cladocerans, and copepods. Advanced crustaceans (Malacostraca) were represented by a single amphipod species, Dikerogammarus aralensis, an endemic of the Syr Darya basin.

There were several native bivalves in the Aral Sea, including members of the genera Dreissena (including an endemic subspecies of zebra mussel, Dreissena polymorpha aralensis), Hypanis, and the lagoon cockle (Cerastoderma glaucum) (formerly considered distinct species Cerastoderma rhomboides and C. isthmica). Native gastropods included Theodoxus pallasi and a member of Caspiohydrobia.

Many of these invertebrates had their numbers drastically reduced due to the introduced fish species. Later, during an unsuccessful attempt to introduce mullet (Mugil sp.) to the Aral from the Caspian Sea, the rockpool shrimp (Palaemon elegans) was inadvertently introduced to the sea. The shrimp is thought to be responsible for the extirpation of the near-endemic amphipod Dikerogammarus aralensis, which now survives only in the Syr Darya basin. The copepod Calanipeda aquaedulcis was introduced to the Aral to replace the zooplankton species reduced by the herring population, and the North American mud crab Rhithropanopeus harrisii was inadvertently introduced during this attempt as well.

Later, as the lake's salinity increased, many of the freshwater-adapted species disappeared, only leaving behind the marine and saline species. However, the zooplankton population in the North Aral Sea has recovered as salinity has decreased from the 1990s onwards, with extirpated crustacean and rotifer species returning naturally via the Syr Darya River, at the expense of the saltwater species. The cladoceran Moina mongolica, extirpated by the introduced fish species, has also returned. The zebra mussel (Dreissena polymorpha aralensis) has been reintroduced. In contrast, in the South Aral Sea only a few nematodes, rotifers, and parthenogenic brine shrimp (Artemia parthenogenetica) exist. The future prospects for aquatic invertebrates in all remaining Aral Sea fragments depend on their future changes in salinity.