Ábrahám Levente: Biomonitoring a Dráva folyó magyarországi szakasza mentén 2000-2004 - Natura Somogyiensis 7. (Kaposvár, 2005)

Héra, Zoltán: On experiences in monitoring molluscs (Mollusca) in the area of Duna-DrávanNational Park - A Duna-Dráva Nemzeti Park térségében végzett puhatestű (Mollusca) monitoring tapasztalatai

30 NATURA SOMOGYIENSIS In the region of DDNP respectively in its narrower area, several molluscs with ability to quickly spread their area are known to be potentially and factually invasive. In order to protect the natural habitats it is extremely important to monitor the presence and spreading of invasive molluscs even if at present we do not have effective means for slowing it down. In 2001 Potamopyrgus antipodarum (Gray, 1843) (Fig. 3.) was found in a gravel pit lake. A wide range of similar habitats stretch along the Dráva, besides this animal is able to settle even in very small surface water bodies. It is a very resistant species which can endure temporary dry habitats lasting for shorter periods. It produces very high individual density level in detritus. Its dispersion would determinally affect the mollusc fauna of natural waters. A new offender of terrestrial ecosystems is Avion lusitanicus (Mabille, 1868), a slug, which has been proved to be present in the country since 1986 (VARGA, BÁNKÚTI and KOVÁCS 1995). In 1996 it appeared in great mass in the inner area of Zákány and the Csurgó area. Since the beginning of monitoring investigation the number of the infested sites has doubled. It spreads in weed assemblages and groves. It requires high humidity and leads a hiding way of life feeding mainly on plants. In the years of higher rainfall its population increases in a noticeable way. At present it does not reach the maximum individual density on any of the sampling sites as it would be expected according to literature data. The most common member of bivalves Dreissena polymorpha (Pallas, 1771). First we came to know about its presence in the holocene from Lake Balaton (1932), since then it has been spreading continuously in lakes and rivers. In the Dráva it has been present since 1997 (VARGA and UHERKOVICH 1998) and has found its way into gravel pits as well. As it leads an immovable way of life it characteristically settles on shells of larger bivalves, but it can also be found on protective paving and vegetal material. According to the surveys it is commonly spread in rivers and lives in masses; its population size has not changed in a traceable manner during investigation. Anodonta woodiana (Lea, 1834) (Fig. 4.) a bivalve species, originates in East-Asia and it was imported into Europe with different fish species (Ctenopharyngodon idella, Hypophthalmichthys molitrix). In 1990 it was already present in the Danubian water system and it is spreading towards the south. It also settled in Lake Balaton and some parts of our larger fishponds. In the Dráva it was identified during water fauna monitoring in 2002. It has been recorded in three of the sampling sites in the river, but also in oxbows and pit lakes. The increase in species area is a fact, the present sampling data do not provide accurate dispersion data. When monitoring water fauna in float debris 20 water snail shells were found, one of them is a protected species. Table 2. contains the average density data of species in the float debris from three sampling sites over a 5 - year period. Due to the characteristic features of sampling methods, species whose shell does not retain gas bubbles sink to the bottom of the bed, so they are not present in the float debris. This causes the striking contradiction which explains why Amphimelania holandri (Pfeiffer, 1828) for example, is of occasional occurance, while in the solid parts of the sampling site its density exceeds 95%. Table 3. contains species found in float debris of gravel pit lakes in the vicinity of Bélavár. Malacologically this area is valueless, investigation in this area was mainly aimed at monitoring the potential dispersion of invasive and protected species. No such occurance was recorded during the years in question. Species frequency in the sampling sites has considerably changed. The main reason is the fast succession in the lake, which strongly modifies snail population size.