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

Lanszki József: Otter monitoring between 2000 and 2004 in the Drava region (Hungary) - A vidra monitorozás 2000 és 2004 között a Dráva mentén

176 NATURA SOMOGYIENSIS Population density The indirect line transect method based on the number of spraints collected over the five years of the study (Fig. 6) was used for the purpose of monitoring otter density ranking for each habitat (Fig. 7) and to follow changes in this ranking. It would not be possible to draw distinct boundaries between the areas of high, medium and low otter density shown in Fig. 7. The highest otter density data were recorded in the habitats in which human influence was minimal (in areas protected by enhanced conservation management: in the backwater and on the Drava at Bélavár, in the Barcs, Középrigóci ponds, and in places where the primary aim was fish production, such as Fonó fish pond). Medium otter density was recorded in the rapidly flowing, open stretches of the Drava (i.e., in Ortilos and Vízvár), the backwaters alongside the Drava (Bares and Babócsa), the ponds under conservation management (Boronka and Petesmalom) and the streams with constant flow rate. Otter density proved low in the habitats which ran dry periodically (the Lankóci alder forest, the Dombó canal and the Korcsina canal), the streams with low flow rate (Tetves stream) and the ponds where there was intensive fishing (Somogyudvarhely). The results of the molecular genetic analysis have supported the otter observations, showing continuous otter presence along the River Drava between Őrtilos and Bares (LANSZKI 2002 and unpublished data). The mean distance of 3.3 km between positive sites was less than a night movement of otters (ERLINGE 1968, JENKINS 1980). The genetic structure showed clear and close relation between populations living in different river sections and backwater habitats. Discussion In this study fish of various species constituted the main food source for the otter in the Drava, its backwaters and fishponds, where there was an abundant supply of fish. Low frequency of occurrence (below approx. 50-60%) of the principal fish prey indicated that were periods when these fish species were low, or that the quantities available fluctuated greatly from season to season. At such times secondary food sources became more significant. In the habitats prevalent in Hungary these sources were most frequently amphibians and birds. Frequent consumption of other taxa not characteristic in the diet of otters (e.g. mammals and invertebrates) highlighted a lack of commonly available (i.e., primary and secondary) food sources. Such disadvantages arose in habitats which periodically ran dry and where the food supply was sparse. When fish supply is plentiful the food niche of the otter is generally narrow, as the decisive proportion of its diet consists of various species of fish. The generally moderate density of otters recorded on the Drava (one individual per 67 kilometres) may have been related primarily to strong river current and substantial fluctuation in water level, and, where the water level was low, may also have been due to the ri vérbank, which was too steep for the otters. Otters are discouraged by steep banks or those with unfavourable degrees of plant cover (i.e., where vegetation is too dense or bare) which prevent them getting into and out of the water (KEMENES and DEMETER 1995, KRUUK 1995). Water entry places play an important role in social behaviour, e.g. in play, grooming and the marking of territory. Deterioration of these areas (e.g. removal of vegetation on the banks) has a negative effect, as does substantial change in water level. Otters only occasionally mark territory on the stones surrounding the banks