Prey supply of otters (Lutra lutra) in small rivers in north-eastern Belarus: census method used and preliminary data obtained
A census method used for estimating species diversity, density and biomass of water-dwelling prey of otters (Lutra lutra) in small rivers is described in this paper. The method is based on net equipment mostly consisting of walls, fykes and fastenings. We created two nets that were adapted for size, flow rate and other para meters of small rivers. The preliminary results on species diversity and biomass of water-dwelling prey (fish, crayfish, frogs and rather big aquatic beetles) of otters in small rivers of north-eastern Belarus in the warm seas on obtained by the method described are presented. In summer (June-August) the total biomass of aquatic prey recorded in censuses on 100 m stretches of the Lovat river (northeast Belarus) varied f om 0.1 kg to 4 kg and was on average 1.7 kg. Fish made up the main part (67-89%) of the aquatic prey biomass. In total we caught twenty fish species, but in the sample stretches surveyed in the census only 4 to 11 fish species were recorded. The fish species richness gradually increased with river size and as the tlowing rate decreased. The same trend was found in fish biomass. Crayfish was most abundant in the moderately tlowing parts of the Lovat river. In the river parts with a slow flowing rate they were slightly less abundant, while in fast tlowing waters the least biomass of cray tïsh was registerecl. With an increase in the size of the river and a decreasing flowing rate the biomass of rather big aquatic beetles and edible frogs increased, as was also the trend for the fish community.