2025 Volume 19 Pages 17-32
Middendorf’s Bean geese (Anser fabalis middendorffii) winter in Katano-kamoike, a Ramsar site in Kaga City, Ishikawa Prefecture, where they forage on rhizomes of Wild rice (Zizania latifolia) and fruits of Water chestnut (Trapa natans var. quadrispinosa). Water chestnut is an annual floating-leaved plant species that germinates in April and grows rapidly after June. The leaves, located at the outer edge of the rosette, rise above the water surface and fully expand by the end of July. Blooming begins in early July and peaks in August, and after local people drain the pond water, the plants deteriorate by the end of August. I collected Water chestnut fruits at the end of August or the beginning of September for nine years, between 2009 and 2022. I recorded the density, size, and dry weight of the fruits, as well as the rosetta density, and investigated the effects of weather conditions on them. Significant negative correlations were found between the density of Water chestnut fruits and rainfall in July. The dry weight of the fruits showed significant positive correlations with the daily average temperature and with the sum of the daily maximum temperature for June to July. The size of Water chestnut fruits had a significant positive correlation with the monthly maximum temperature in June and the sum of temperatures from June to July. Significant negative correlations were observed between the density of rosettas and the monthly minimum temperature in August and the cumulative temperatures for July to August. The number of Middendorf’s Bean geese recorded during autumn and winter had a significant negative correlation with the size of Water chestnut fruits. Based on previous studies and the phenology of Water chestnuts in Katano-kamoike, air temperatures affect the timing of bloom, the timing when rosettas are fully grown, and the rate of starch synthesis in Water chestnuts, which may result in increased fruit density, size, and weight. Precipitation likely affects the density of Water chestnut fruits by inhibiting fruit growth due to rain clouds blocking sunlight. The number of Middendorf’s Bean geese was only negatively correlated with the size of the fruits, which may reflect the geese's preference for smaller-sized Water chestnut fruits. For the conservation of Middendorf’s Bean geese, further studies on the relationship between weather conditions and other food resources, as well as research on suitable food species for wintering Middendorf’s Bean geese, are important.
