Bulgarian - Swiss Biodiversity Conservation Programme ECOLOGY AND CONSERVATION OF ATANASOVSKO LAKE Collection of Final Scientific Reports of Atanasovsko Lake, Sofia , 1997
THE ICHTHYOFANA OF ATANASOVSKO LAKE D.Biol. Dimitar Popov Institute of Zoology at Blgarian Academy of Sciences
The ichtyofauna of the reserve has not been studied so far probably because of the industrial production of salt. This production began in 1912, and was intensified since 1922. Such an activity gives to the area special character in hydrobiological meaning. Beeng separated into large number of evaporating and crystalizing basins with various salinity (from 0.12 %o to 168 %o) the water surface together with the canals is a complex system of waterbodies wich makes impossible the formation of a stable faunistic ichtiological complex. The great difference in the environmental conditions, and first of all the salinity, act during the different seasons and is an additional obstacle against the formation of rich of species fish fauna. Quantitatively, however, the eurybiont species like the caucasian gobby Knipowitschia caucasica and the speaclebacks Gasterosteus aculeatus and Pungitius platigaster develop in mass. Abbundant development is observed also of the mosquito fish Gambusia affinis holbrooki, introduced in this country in 1924. This species is freshwater, but tolerate salinity up to 7 - 8 %o, and is found in the northern canal along the brine basins. At the north-western part of the reserve there is a late quarry full with freshwater where the golden fish Carassius auratus gibelio is abundant, and the carp Cyprinus carpio develops also in much smaller quantity. All of the species mentioned are able to reproduce in the reserve. In the large evaporating basins with salinity between 5 to 60 %o the caucasian gobby and both species of speacklebeck are permanent inhabitants; besides them mugilid species enter during the summer by the marine water and remain there because the available food. This was the practice in the past, when fishermen traped young fish along the coast and replaced them in the brine basins for fattening. Some 40 t of fish anually were produced in this way in the Pomorie Lake. In the eastern part of the canal that connects the brine basins with the sea, some marine species occure; they are not so important in the faunistic complex of the reserve. The qualitative and the relative quantitative characteristics of the ichtyofauna is shown on table 1. During the two years of study mass mortality of fish was observed in the canal and some of the brine basins, obviously connected with the sudden exchange of the salinity when marine water entered. As an example, in site 10 in July 1996 a spot of dead fish was observed with a lenght of about 100 m along the shore, consisted of gobby and speackleback in ratio about 10 to 1. About 6000 individuals per m2 were numbered. A conclusion could be made tha both species form an abundant stock and are important food recource for the fish eating birds. The same event was observed also in the canal between the sea and the sluice to sites 1 and 2. In this place, however, the mosquito fish was the victim. In the northern part of the canal, where the water is constantly fresh, such mortality was not observed. An approximate estimation of the trophic value of the fish populations could be made knowing the food spectrum of the fish consuming bird species, that are constant or temporal inhabitants of the reserve,, and also the measures of the fish and their habitats. The speackleback and the mosquito fish have similar lenght, up to 50-60 mm, the gobby is smaller and is rather mesobenthic species. The mugilids and the cyprinids are larger and pelagic; so they are the food for large and typical fish consumers as pelicans, cormorants and herons are. In this aspect of the matter, a reccomendation could be made about additional increasing of the fish stock in the larger basins by introducing of young individuals of mugilid spesies, which would improve the food ensurancy of the pelicans, for example. In the freshwater part of the canal grass carp could be introduced to press the higher water vegetation, and on the other hand, would increase the trophic source of the larger fish consuming birds.The quarry in the north-western part of the reserve is also of interest. Ground waters with low salinity (2 %o) collect there. The natural process of formation of the fish fauna by eggs beeng transported on the legs of the birds could be speeded up by artificial
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Management Plan of Atanasovsko Lake
Appendix Three