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

174 NATURA SOMOGYIENSIS Fig. 5.: Trophic niche breadth (В) of otters in SW Hungary (2000-2004) collected was taken relative to the unit length of the route section. Calculation of density ranking involved correction on the basis of duration and frequency of sampling. The data were recorded in an Excel spreadsheet, and the SPSS 10.0 program (1999) was used for data processing. Results Diet composition and food niche breadth Various fish species composed the main proportion of the diet of the otter in the Drava, in some of its backwaters, in streams with constant flow rate, and in the fishponds (Table 2). In addition to these fish species which formed the principal food source, a significant role was also played by secondary food sources, i.e. amphibians and birds, along certain stretches of the Drava (at Vízvár), and in the gravel pit pool close to the Drava (Somogyudvarhely). Consumption of mammals and invertebrates not normally characteristic of the otter diet occurred frequent in the habitats alongside the Drava which periodically dried up, e.g. the Lankóci alder forest, the canals and some streams. For more detail see LANSZKI et al. 2001, LANSZKI 2002, LANSZKI and MOLNÁR 2003. The food niche of the otter (Fig. 5) proved narrow in the areas where fish were its primary food source. The niche was moderately broad in areas where, in addition to fish, secondary food taxa (amphibians and birds) also occurred frequently in its diet. The areas characterised by a broad food niche were exposed to periodically arising disadvantages (i.e., running dry, large fluctuations in water level, and fishing/human disturbance). The scale of these fluctuations in habitat conditions was indicated by deviation values relative to the mean (s.e.).