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

170 NATURA SOMOGYIENSIS The freshwater otter has a secretive habit and is active at night; therefore spraints and tracks indicating the presence of the species (REUTHER et al. 2000) give the primary base for confirmation of its occurrence and presence and for monitoring of changes in occurrence by means of systematic questionnaire-based surveys. Determination of the size of an otter population often poses practical problems. Genetic analysis (e.g. DALLAS et al. 1999) is the method primarily suitable for application for this purpose, which were also started along the Drava River in 2002. Relative density of otters can also be measured on the basis of the number of spraints and tracks indicating the presence of the species in relation to the unit length of the route used; this allows comparison of areas studied by identical methods. The line transect method (e.g. ROBSON and HUMPHREY 1985, MASON and MACDONALD 1986, REID et al. 1987, PRIGIONI et al. 1995) is suitable for this, but can also entail error. Primary considerations are the frequency of sample collection and the determining and fixing of the route, together with the importance of extending the study for as long as possible. Territory marking by the otter (i.e., frequency of defecation) is influenced by social behaviour related to breeding, and also by season (e.g. CONROY and FRENCH 1985, KRUUK and CONROY 1987). The objective of this study, initiated in 2000, was to monitor the otter population living beside the Drava, to assess the basic situation in sampling areas and to examine possible factors influencing distribution. Material and methods The study was carried out in SW Hungary. The main data recorded for the habitats studied alongside the Drava, which served as sampling areas, are shown in Table 1. This study included not only the above areas but also several aquatic habitats in the BalatonDrava ecology network. The main stem of the Drava River has a steep riparian region, characterised by Central European slow river floodplain woods, composed of willows and poplar. Backwaters on lower relief are covered by Central European slow river floodplain woods and ash-alder woods, while those on higher relief are surrounded by ash-oak-alder forests (IVÁNYI and LEHMANN 2002, JUHÁSZ 2004). The Drava is a high regime watercourse river, the earlysummer and autumn flood marks and winter-end low-water marks are characteristics (the difference is approximately seven metres). The river remains "near natural", unpolluted, with meandering courses and many old river-beds. Diet composition was determined by spraint (faecal) analysis. Sampling commencing in January 2000 alongside the Drava was performed every six weeks at first, then every four weeks from May 2002 to June 2004 and after every six weeks along a standard route (Table 1). The feeding habits of the otter were examined by spraint analysis. The data obtained related to samples for winter and spring and summer and autumn collectively. Diet composition was determined through microscope examination, on the basis of characteristics of feather, bone, scales, pharyngeal teeth, chitin shell, teeth and hair (LANSZKI et al. 1999, 2001) Diet composition and food niche breadth were calculated on the basis of the relative frequency of occurrence of items in the spraints, this in turn being based on minimum number present per sample. Food niche breadth for each period was calculated by the Levins index (KREBS 1989). The taxa used were: mammals, birds, reptiles and amphibians, fish, invertebrates and plants. To calculate relative otter density the number of spraints (both the total and fresh)