Whooper Swan population drastically reduced due to damage of lotus vegetation caused by flooding
By Tetsuo Shimada
Herbivorous waterfowl and plants are known to interact in a variety of ways through their foraging behaviour. Whooper Swans Cygnus cygnus are aquatic plant foragers that are widely distributed in various aquatic environments, including the sea coast of Japan. Lake Izunuma-Uchinuma, northern Japan, is one of the main wintering grounds for the swans throughout Japan, where the main food resource for the swans is the lotus Nelumbo nucifera rhizome (Fig. 1a, b).
Fig. 1. Whooper Swans searching on lotus rhizome by up-ending behaviour (a) and foraging dug-out rhizome (b). Photographed by Hiromi Kano.
Extensive flooding in July 2022 resulted in significant damage to the vegetation of the lotus. Lake Izunuma which has an area of 357 ha, was 62% covered by lotus vegetation in October 2021, but only 11% in September 2022 (Fig. 2, 3a).
Fig. 2. Growing conditions of the lotus Nelumbo nucifera in Lake Izunuma-Uchinuma, Japan, in 2022 (a: lotus plants growing steadily on 6 June, b: lotus plants submerged on 17 July, c: lotus plants withered by flooding on 4 August).
The number of Whooper Swans decreased from 2021/2022 to 2022/2023 (November; 2,650 ± 378 (mean ±SD) vs. 1,133 ± 286, December; 3,388 ± 394vs. 374 ± 163, January; 3,112 ± 811 vs. 506 ±131, Fig. 3b).
Fig. 3. Percentage of lotus coverage in Lake Izunuma before the wintering season (a), the number of Whooper Swans in Lake Izunuma-Uchinuma (b).
The ANCOVA results for 2021/2022 showed that the slopes of the regression equations for Lake Izunuma-Uchinuma and Miyagi Prefecture (excluding the lake) were not significantly different (P = 0.45, Fig. 4a). No significant differences were detected among the sites or in the number of days from November 1st (P = 0.94 and P = 0.15, respectively). However, the slopes of those of 2022/2023 were significantly different (P = 0.02, Fig. 4b). There were no significant differences among the sites or the number of days from November 1st (P = 0.93 and P = 0.07, respectively).
Fig. 4. The number of swans were z-standardized and regression equations for Lake Izunuma-Uchinuma and Miyagi Prefecture (excluding the lake) were generated. In 2021/2022, the slope of each equation did not differ from each other (a) but those of 2022/2023 had a significant difference (b).
The number of swans in Lake Izunuma-Uchinuma decreased significantly compared to the populations of swans at other wintering sites, despite the fact that the daily average temperatures at the three sites on the migration route of the swans did not differ significantly between these seasons. The significant decrease in lotus vegetation caused by flooding in July 2022 is considered the cause of the decrease in the number of Whooper Swans wintering on the lake in the 2022/2023 season.
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Reference:
Tetsuo Shimada, Hironobu Tajiri & Kosuke Fukuda. 2025. Lotus vegetation withered by flooding in summer reduced the Whooper Swan (Cygnus cygnus) wintering population in Lake Izunuma-Uchinuma, Japan. Waterbirds 47: 1-7.
