Abstract
Water levels in the Watarase Reservoir, central Japan, were altered as part of a water quality improvement program. Specifically, the water level was lowered to near drying-out of the reservoir. This level exceeds the normal lowest watermark, and is hereafter referred to as “drying-out”. Since such treatments may affect the birds in the reservoir, a population survey was conducted in and around the reservoir from January 26th to May 15th, 2004. The survey was conducted in the three main ponds (Watarase Reservoir, Tatara-pond, and Joho-pond) throughout five water-level phases. Based upon levels in the Watarase Reservoir, these were defined as falling, maintaining the lowest level, drying-out, rising, and full. At each survey point, the number of birds present changed in relation to even slight changes in water level. However, covariance analysis indicated these population changes to be non-significant between each point and water level phase. No significant differences were detected between the bird populations at Watarase Reservoir and Tatara-pond over the duration of the study. Although a slight difference was detected between population decreases at Watarase Reservoir and Joho-pond, the decrease at the latter site where no water level lowering took place, was more conspicuous than at the former site. The decline in the population of birds detected in this study reflected the natural cycle of seasonal migration, and was not an effect of the drying-out of the reservoir.
