An evaluation of waterfowl sampling methods for the national census on river and dam lake environments.
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概要
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A national census of river and dam lake environments (NCRE) conducted by the Ministry of Construction of Japan has been underway since 1991. Information on the variation in number and habitat use of waterfowl is important in determining the optimum sampling effort necessary for accurately estimating the population density. However, this variation is not sufficiently considered in the census methods used in the NCRE. In this paper, we report the variation in number and habitat use of waterfowl, especially ducks, over short and long temporal scales (daily and monthly changes, respectively) at Takayama dam lake in central Japan. Between December 1998 and March 1999, we counted wintering waterfowl from a small boat and from the lake shore, and analysed the results in relation to the valid range of census methods adopted by the NCRE. General results of the field observations were : (1) the waterfowl assemblage was stable in January and February ; (2) the waterfowl assemblage was stable on any given day and only slightly varied during five consecutive days ; (3) the locations of ducks on the dam lake remained almost the same during both short and long temporal scales ; and (4) terrestrial observations were not useful for counting mandarin ducks and greenwinged teals resting on the surface of the dam lake. These results suggest the following: (1) the winter waterfowl census in central Japan should be conducted in January or February ; (2) the area surveyed in the census should be large enough to encompass the large home ranges seen in several waterfowl species (e.g. common cormorants, mallards and European wigeons) ; (3) routes for line transect and positions for point counts should be chosen carefully when observers count waterfowl on land ; and (4) boat-based observations are recommended for waterfowl censuses.
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