According to official data the main trends in changes of climate indices are the following (Hydrometeorological conditions… 2009):
- increase of average annual air temperature at coastal sea station for the last 30 years within 0.42-0.55 оС per each ten years.
- 1 оС increase of average annual water temperature for the last 50 years
- rise not only of the Black Sea but also of the Sea of Azov with an average rate of 2.11 cm/year
- decrease of salinity of the Azov Sea: 2.28 g/l for the southern coast and 2.67 g/l for the northern coast for the last 30 years.
As main ecosystem indices there were selected:
- Structure of ichtyocoenoses. For this index it is used the percentage of freshwater species out of the number of all fish species recorded in a monitoring plot. The percentage of warm-loving species or saline water species can be also used as indices.
- Number of fish species of different ecological groups. Monitoring of this index is based on the study of relative or absolute number of fresh water species. For the Azov Sea, the Prussian carp was selected (Carassius gibelio). In other regions of the Black Sea basin the number of warm-loving species or species of Mediterranean complex can be used for monitoring.
During the 20th century ichtyocoenoses of the Azov Sea were subject to considerable transformation because of changes in hydrological and hydrochemical regime. The latter were induced both by climatic factors and different kinds of anthropogenic activity in the basin. Climate processes has an impact on wind activity, temperature, continental runoff and, as a consequence, sea salinity (fig. 1).
In a period of natural hydrological regime (1931-1952) the ichthyofauna of the Azov Sea was represented by 79 species.
In 1952 the runoff of the main river of the Black Sea basin – the Don River – was regulated. It coincided with a period of dry years and resulted in increase of average sea salinity, which reached 13.8 ‰ in 1975-1976. Under these conditions several dozens of the Black Sea fishes penetrated in the Azov Sea, enriching ichthyofauna of water bodies in the studied region. In a period of a maximum sea salinity (1975-1976) in the Azov Sea, in addition to traditional seasonal invasive species the invaders included such rare for the basin species as Symphodus cinereus, Parablennius sanguinolentus, Parablennius incognitus, Pomatoschistus marmoratus, Gobius niger, Diplodus puntazzo, Gaidropsarus mediterraneus. However, these species, both rare and new, were mostly recorded in the southern part of the Azov Sea and only Gobius niger, two years after its penetration into the sea, became rather common not only in southern but also in northern parts of the Azov sea (Obytichnyi and Berdianskyi Bays). In this period ichtyocoenoses included up to 140-150 species.
After 1976, in spite that the volume of taking out the river runoff still remained at the same level, the Azov Sea came into phase of freshening due to changes in atmospheric processes. This was added by a gradual increase of the river runoff in the Azov Sea basin.
|Fig. 2. Dynamics of the river runoff in the Azov Sea basin|
Some increase of the runoff during the last decade can be seen even on the example of small rivers (Molochnaya River, fig. 3)
|Fig. 3. Dynamics of the runoff of Molochnaya River.|
Salinity of the Azov Sea in the next 12 years decreased to 10-11% i.e. to the values observed during natural regime of the water body. It still remains at this level by the present moment (Gargopa, 2003).
According to the current data, ichthyofauna of the Azov Sea (including Kerch Strait, limans, Don and Kuban river deltas) includes 95 species of Cyclostomata and fish.
Due to a current trend of the sea freshening there were recorded finds of such freshwater fish species which had never occurred earlier in marine waters, namely the tench, rudd, Wels catfish, European perch.
In the result of analysis it was found out correlation between the number of typically freshwater fish species and salinity. The correlation value is 0.74 (fig.4).
Fig. 4. Relationship between the percentage of typically freshwater fish species and salinity in the Azov Sea
Thus, according to field studies of 2010-2012 there was a small increasing trend of typically freshwater species in the study region. While in 2010 the percentage of this fish group was 10.1% of the total number of species, in 2011 it was 11.6%. This increase was because of registration of the pumpkinseed in 2011 in waters of Utliukskyi Liman. Intensity of its distribution is shown in Fig.5.
Fig. 5. Chronology of distribution of the pumpkinseed (Lipomis gibbosus) in water bodies of Ukraine
Also, it should be also noted increase in numbers of species already inhabiting the Azov Sea. The most prominent example is the Prussian carp. A decreasing trend of salinity (up to 9-10‰ averagely for the sea), which has been observed since 1998) favoured to significant extension of this species range. In addition, due to discharges of a great amount of fresh water from canals of the North-Crimean irrigation system, some parts of Eastern Syvash was freshened to 4-9‰. Such essential changes in ecological conditions of water bodies have lead to wide distribution of the species in the Azov Sea, its limans and bays. Nowadays the Prussian carp occurs along the whole coastline of the Azov Sea, in the channel of Molochnyi Liman, in all parts of Utliukskyi Liman, in freshened areas of Syvash.
According to field studies in 2010-2012 the index of occurrence of the Prussian carp in catches remained relatively stable. Thus, in 2010 this species was found in 34.42% of catches, in 2011 – in 12.19%, and in 2012 – in 11.42%.
The site near Kerch Strait in the Azov Sea is characterized by wedging-out of salt Black Sea waters which mean unfavourable conditions for dwelling of freshwater fish species. In this marine site the Prussian carp occurs only sporadically during a period of northern and eastern winds inducing wind-driven tides. However, this site is more frequently penetrated by invasive fish species.
In the Azov Sea 27 invasion fish species of 19 families are recorded. Analysing the structure of invasive species in the sea it should be noted that the reason of invasion for 50% of them are hydroecological changes in the basin, and for 43% - fish-breeding arrangements. Only 7% penetrated in the sea because of building of hydro-constructions.
Intensity of invasion into the sea was linked with several factors, main of them were intentional invasion of the species, increase or decrease of the sea salinity (fig.6).
It should be mentioned the fist invasion period, distinguished on basis of intensification of works on breeding the Asian fish in rivers, storage lakes and ponds of the Azov Sea basin. Artificial increase of these fish numbers has lead to their casual penetration from fish-breeding water bodies to rivers and further to the sea. Already in the 1950s-60s in the sea were recorded Hypophthalmichthys molitrix, Aristichthys nobilis, Ctenopharyngodo idella, Mylopharyngodon piceus, Carassius gibelio.
Fig. 6. Intensity of invasion in the Azov Sea under changes of salinity
In a period of increasing salinity the Black Sea species penetrated into the Azov Sea: Scorpaena porсus, Scophthalmus rhombus.
Nowadays, there is a clear trend of appearance of freshwater species new for the sea ichthyofauna. Fist of all, they are Eudontomyzon mariae, Rhodeus amarus, Pseudorasbora parva, etc. It allowed to distinguish the third period of invasion into the Azov Sea.
Therefore, the 2 periods are characterized by invasion of freshwater invasive species instead of marine ones. Such trend of replacing one species group by another may be repeated for the Azov Sea if there is increase of salinity in future (according to one of scenarios of global climate changes).