Abstract
Grazing pressure by animals can change the distribution and biomass of seagrass. We examined grazing pressure by conducting transect surveys and acquiring aerial images by drone before and after the arrival of migratory birds along the Asadokoro tide flats, Aomori Prefecture, Japan. The distribution and biomass of the seagrass Zostera japonica decreased sharply between October and November 2018, which was when migrating waterfowl arrived. We hypothesized that grazing pressure by migrating birds such as the Anatidae, including whooper swans (Cygnus cygnus) and brent geese (Branta bernicla), had a major effect on the decline in Z. japonica in late October. Shortly after the Anatidae arrived, most of the Z. japonica in the shallows disappeared, including the belowground parts. The abundance of Z. japonica in this area was insufficient to support wintering swans. Swans likely need food other than Z. japonica for overwintering.
About the authors
Fumiyuki Sato graduated from the Department of Life and Environmental Science, Faculty of Engineering, Hachinohe Institute of Technology. Currently, he works for an environmental consulting company. During his undergraduate studies, he belonged to the Marine Ecosystem Laboratory. In cooperation with Hiranai Town, Aomori Prefecture he conducted ecological surveys of migratory birds and seagrasses on the Asadokoro tide flat during the fiscal year 2018.
Satoshi Tanaka is the chairman of the Hiranai Society for Swan Protection. For many years, he has been engaged in monitoring swans on the Asadokoro tide flat, which is designated as a National Special Natural Monument known as “The swans of Kominato and their migration grounds”. In cooperation with Hiranai town, he and members of the society organize observation meetings and provide environmental education to elementary school students.
Shinji Kirihara is a professor at Institute of Regional Innovation, Hirosaki University. At the Fisheries Research Agency in Aomori Prefecture, in collaboration with local fishermen, fisheries cooperatives and municipalities he develops ecological research on aquatic organisms, such as abalone, sea urchin and sea cucumber, develops aquaculture technology, and spreads the results. In 2006, he was awarded the Marinalg International Award for his research results on changes in biomass of macroalgal species associated with changes in water temperature.
Yoshiyuki Tanaka is a professor at Department of Life and Environmental Science, Faculty of Engineering, Hachinohe Institute of Technology. He has worked on community ecology of seagrasses for a long time. He is a committee member of the long-term seagrass monitoring program of Biodiversity Center of Japan, Ministry of the Environment. He is also a member of the Marine Environment Committee, the Oceanographic Society of Japan. In 2006, he received the Poster Presentation Award of the Ecological Society of Japan (biogeochemical cycle category) for his research on seagrass communities using stable isotope analysis.
Acknowledgments
We thank the committee members of Hiranai-town for providing us with the opportunity to conduct this study. We also thank Takeshi Mitsuya, Masaki Ogata, and other members of the “Hiranai Society for Swan Protection”, who cooperated with the survey and provided valuable information. Dr. Takashi Nakamura and Dr. Masaya Yoshikai gave us helpful comments on the operation of the drone. Ayumu Nakamura, Seishiro Koda and Ryuhei Yoshida gave us constructive comments. We also thank Prof. Matthew J. Dring and two anonymous reviewers for their constructive comments.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: None declared.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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