International Symposium “The Aegean Sea 2000”, 5-7 May 2000, Bodrum-Turkey
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Proceedings of the International Symposium “The Aegean Sea 2000”, 5-7 May 2000, Bodrum-Turkey MEDITERRANEAN - BLACK SEA FAUNAL EXCHANGE Yuvenaly ZAITSEV Odessa Branch, Institute of Biology of Southern Seas 37, Pushkinska Street, 65011 Odessa, Ukraine INTRODUCTION Several times during their geologic history the Sea of Marmara, the Black Sea, the Sea of Azov and the Caspian seas have been connected to the Mediterranean Sea and both the Atlantic Ocean and Indian Ocean (GROVES and HUNT, 1980). On the other hand, prolonged lowering of the sea level has periodically severed the connection between each of these basins because of the shallowness of the connecting channels. The present connection between the Mediterranean and Black seas was established about 6,000 or even 5,000 years ago (ZAITSEVand MAMAEV, 1997). At that time the level of the Black Sea was below that of the Mediterranean, now on the contrary, the Black Sea level is higher and this state is influencing the water exchange through the Bosphorus and Dardanelles. As to water salinity, which depends on evaporations, atmospheric precipitations and river runoff, and which is a very important parameter of each marine habitat, the Aegean Sea is one of the most saline sea in the World Ocean and reached up to 38-39,9 ‰. The salinity of the surface waters of the Sea of Marmara, which is under the influence of the upper current through the Bosphorus from the Black Sea varies between 18‰ and 24‰, at 20 m depth it rises to 30‰ and at 45-50 m to 37‰ (KOCATAS et al., 1993). The Black Sea surface salinity is 17-18‰, except the North-Western part where is 14-16 ‰, due to strong river runoff (ZAITSEVand MAMAEV, 1997). At least, the Sea of Azov is a brackish water sea due to its isolation from the Black Sea and to Don and Kuban rivers runoff. Its salinity varies from 3-5‰ on the North-East to 13-14‰ to the south, nearly the Kertch Strait connecting to the Black Sea (BRONFMAN, 1995). The Black Sea (and the Sea of Azov) biota- the historically established specific diversity of flora and fauna- reflects the geological processes that have influenced the marine ecosystem. Research has shown that there are different elements in the Black Sea fauna comprising groups of taxonomically distinct species sharing a common origin in a past ecological event. These organisms react in a specific way, which varies according to their origin and the marine environmental conditions prevailing in the Black Sea, and consequently occupy different habitats within the sea (BACESCU, 1967, ZAITSEV & MAMAEV, 1997). Pontian relics The species which had once lived in the Neoeuxinian Lake, the predecessor of the modern Black Sea, should be considered among its most ancient inhabitants. The lake-sea came into existence some 18,000-20,000 years ago after the end of the Würm glaciation during which the whole northern part of Europe had been covered by glaciers. Melting water filled the lake-sea and substantially reduced its salinity. This water body was completely isolated from the Mediterranean. It is believed that its salinity was approximately 5-7‰ and that it was inhabited by brackish water organisms. Its fauna included: bivalves such as Dreissena polymorpha (the zebra mussel) and Hypanis; the polychaetes Hypania and Hypaniola; the crustaceans Pontogammarus and Paramysis; and such fish as the kilka, Clupeonella delicatula, many species of gobies (Gobiidae), sturgeons (Acipenseridae) and herrings (Clupeidae). These organisms are generally referred to as the “Pontian relics” and today they can only be found in waters with low salinity. Many of such surviving species inhabit the brackish-water Caspian Sea, which is why they are also sometimes referred to as “Caspian relics”. Some of them (e.g. Huso huso, Acipenser stellatus) can be observed in the brackish-water northern Adriatic Sea. Boreal-Atlantic relics A second group consists of marine thermophobic species originating from temperate and northern seas. They are referred to “Boreal-Atlantic relics” or “cold- water relics”. They include: the ctenophore Pleurobrachia rhodopis; the copepods Calanus helgolandicus (C. ponticus) and Pseudocalanus elongatus, the spiny dogfish, Squalus acanthias; the sprat, Sprattus sprattus phalericus; the flounder, Platichthys flesus luscus; the whiting, Merlangius merlangus euxinus; and the Black Sea trouth Salmo trutta labrax. It is difficult to be certain when and how these cold-water species entered the Black Sea. They may have entered through the rivers during the time of Neoeuxinian Lake, or at a later date during the early stages of the formation of the Bosphorus, when the Mediterranean Sea was colder than is today. At least, the sprat and the whiting are found in the Mediterranean. Whenever cold-water species entered the Black Sea, they constitue the second oldest element in the Black Sea fauna. Mediterranean immigrants The most numerous and the most important element of the Black Sea fauna is the third one. It is composed by Mediterranean origin species. Some 6,000 years ago, the Bosphorus established a connection with the Mediterranean, and via the Mediterranean, with the Atlantic Ocean. Gradually, the salinity of the Black Sea began to rise, and it soon reached a sufficiently high level to support many Mediterranean species. These species are reffered to as the “Mediterranean immigrants” constitue the third, and most populous, element in the Black Sea fauna, comprising up to 80 percent of the total fauna in the Black Sea by species number. Most of them prefer warm and saline waters, and for this reason are predominantly inhabiting the upper layers of the sea, which are not directly affected by the rivers. 2 The penetration of saline waters and Mediterranean settlers into the Black Sea put pressure on the autochthonous Pontian relics. They retreated to the brackish-water areas of the sea and took refuge in limans, estuaries and deltas except euryhaline species, such as the sturgeons and the herrings, which can also be found in saline Black Sea waters. In summer-autumn seasons almost all Mediterranean immigrants can be found througout the sea. Invertebrates and fishs spend the winter either in a state of anabiosis, with a much reduced metabolic rate, either on the seabed, or in the warmest areas of the Black Sea along the shores of southern Crimea, the Caucasus and Anatolian coasts. After completing their winter migrations such fish as the anchovy form in these areas dense shoals suitable for fishing. The most termophilic species such as the mackerel Scomber scombrus, bluefish Pomatomus saltator, and tuna Thunnus thynnus migrate to the Sea of Marmara for the winter. In spring, the overwintering species migrate back to their spawning (except the mackerel, which spawn in the Sea of Marmara) and feeding grounds. Mediterranean immigrants can be found in all major taxa of the Black Sea biota. They include: most sponges, scyphozoan jellyfish, polychaetes, molluscs, crustaceans, all echinoderms, and over 80 percent of fish species. Almost all of them are breeding into the Black Sea and some of Mediterranean settlers have been so good adapting to this marine environment that they have formed local subspecies and even distinct species. This is the case of the Black Sea and the Sea of Azov anhovy (Engraulis encrasicholus ponticus and E. encrasicholus maeoticus), garfish (Belone belone euxini), silverside (Atherina mochon pontica), red mullet (Mullus barbatus ponticus), and turbot (Psetta maeotica). However, not all species inhabiting the Mediterranean Sea have been able to adapt and become naturalised into the Black Sea. Some have been prevented from doing so by low salinity and density of water, some by low water temperatures during winter, and others by the lack of oxigenated deep waters. As a result, some widespread mass Mediterranean taxa, such as the radiolarians, pelagic foraminifers, corals, siphonophores, pteropods, cephalopods and tunicates still do not have a single species in the Black Sea. A good example of a species whose life cycle occurs in two seas is the mackerel. It spawns in the Sea of Marmara because the Black Sea water is not enough dense to enable its pelagic eggs to float. After breeding, in April, the mackerel migrate through the Bosphorus into the Black Sea feeding grounds, mostly in the north-western part of the sea. It feeds on zooplankton, shrimps, anchovy and other small pelagic fish. In August young specimens (four to five months old) of mackerel appear into the Black Sea for feeding on zooplankton. In October-November the mackerel returns for wintering and breeding in the Sea of Marmara. This fish was an important commercial species, their annual catches into the Black Sea reached up to 50,000 tonnes in all riparian countries. After the early 1970‟s, the migration of mackerel from the Sea of Marmara to the Black Sea ceased because of a sharp decline in its population as a result of 3 pollution in its breeding grounds by local sources (KOCATAS et al., 1993). Now single specimens of Scomber scombrus can be found in the Black Sea. There is also an almost complete cessation of migrations of bonito, bluefish, and some other species through the Bosphorus. It‟s quite possible that the chemical pollution is non the only culprit in this situation. Perhaps the strong sound emited by ship‟s propellers in the Bosphorus produce an acoustic barrier in this biological corridor? This is an author‟s supposition which need to be investigated in the context of exchange of fish, marine mammals and may be some invertebrate species between the Mediterranean and the Black Sea through the Turkish straits. The process of the mediterranisation of Black Sea fauna (PUSANOW, 1967) continue and some examples of this can be given. The Mediterranean sardine (Sardina pilchardus) is very close mo becoming fully naturalised in the Black Sea.