OPPORTUNISTIC SETTLERS IN THE ARAL SEA

For the first time an idea to acclimatize invertebrates as a food for fishes in the Aral Sea was put forward by Zenkevich in 1932. He and other scientists noted that in the fauna there were not many species of aquatic invertebrates abundant in other water bodies and being valuable food for fishes. It was recognized as necessary to complete faunistically poor biota with commercially valuable fishes and some invertebrates as a food for them. Necessity of introductions was based also on the expected increase of the Aral Sea salinity. As since the most of benthic invertebrates was of fresh and brackish water origin consequences must be fatal. To preserve the Aral Sea importance for fishery after its salinisation it was considered necessary to introduce euryhaline invertebrates and fishes.

There were proposed many potential acclimatizants: bivalves Abra ovata and Mytilaster lineatus, caspian amphipods, shrimps, crab, caspian Mysidacea, Corophiidae, Cumacea, Leander sp., Astacus leptodactilus, polychaetes Nereis succinea, N. diversicolor, Nephthys hombergii and different forms from Azov, Black and Baltic Seas. After preliminary studies it was recommended to introduce mysids (Mesomysis kowalewskyi, Paramysis baeri, Limnomysis benedeni), polychaetes (Nephthys hombergii, Nereis succinea), cumaceas (Shizorinchus bilamellatus, Pterocuma pectinata), amphipods (Corophium nobile, C. curvispinum) and mollusc Monodacna colorata.

In the beginning acclimatization was carried out in spite of recommendations, plans and methods. Before 1954 only fishes were introduced what undermined the forage reserve. Together with the valuable species into the Aral Sea some unwanted have come what has disturb balance in the ecosystem. The first invaders (shrimp Palaemon elegans) accidentally came together with introduced food fishes (mullets).

Further introduction of invertebrates were planned. In 1958–1965 there were introduced bivalves Syndosmya segmentum, Mytilaster lineatus, polychaete Nereis diversicolor (from the Sea of Azov) and mysids (from delta of Don) Mesomysis kowalewskyi, M. intermedia and Paramysis baeri. In 1964–1965 there was unsuccessful attempt to introduce bivalve mollusc Monodacna (Hypanis) colorata.

As for planktonic invertebrates only mediterranean-atlantic copepod Calanipeda aquaedulcis was introduced (in 1965–1966 and in 1970 from the Sea of Azov). This crustacean has settled over all Aral Sea and has occupied place of copepod Arctodiaptomus salinus eaten by acclimatized planktophags. Together with C. aquaedulcis crab Rhithropanopeus harrisii tridentatus appeared accidentally in Aral.

The last attempt of aquatic invertebrates introduction was in 1986–1987. This attempt to acclimatize azov-balck sea mussel Mytilus galloprovincialis was not and cannot be successful because in the Aral Sea this time there were not hard grounds necessary for these molluscs.

Totally during period from 1954 till 1986 into the Aral Sea were introduced 11 species of aquatic invertebrates and 9 of them naturalized.

The first (unsuccessful) attempt to introduce fishes (Acipenser stellatus) was in 1927. Since 1954 till 1960s 18 species were introduced and 15 of them survived. In 1954–1959 Baltic herring was successfully introduced. In 1960–1961 from Chinese waters the grass carp (Ctenopharyngodon idella) and silver carp (Hypophthalmus molithrix) were acclimatized. In unsuccessful attempt to introduce the Caspian mullets (Mugil auratus and M. saliens) in 1954–1965 six species of gobies, silverside (Atherina mochon pontica) and pipefish (Syngnathus nigrolineatus) were brought accidentally. In addition, in the river mouths the black carp (Mylopharhyngodon piceus) and snakehead (Ophyocephalus argus) and in the sea the flatfish (Pleuronectes flesus luscus) were acclimatized.

Planktopags, especially introduced Baltic herring, exhausted the food reserve and destroyed basis of its reproduction. The first was eaten the leading component of zooplankton copepod Arctodiaptomus salinus. Changes occurred also in Cyclopoida and Cladocera. Except of large cladoceran Podonevadne camptonyx other became very rare (Ceriodaphnia reticulata, Moina mongolica) or disappeared (Cercopagis pengoi aralensis).

In spite of some negative consequences of introductions for aboriginal species the results of acclimatization could be appreciated as positive. Productivity of benthos has risen. Significance of acclimatizants, especially marine and euryhaline, in the Aral Sea fauna rose with the salinity increase when the most of aboriginal species of invertebrates became extinct.

Table 1 Alien species in the Aral Sea (Fishes)

Way of introduction: del, deliberately, inc, incidentally, assoc, in association with deliberate acclimatizants
Ecologycal status: N, necton, B, benthos, N/B, nectobenthos, P, plankton
Effect: -, negative, +, positive, 0, none, ?, unknown