11:45 AM - 12:00 PM
[ACG40-11] Dietary analysis of ducks flying to the Ariake Bay in winter based on carbon and nitrogen stable isotope ratios and fatty acid composition
Keywords:tidal flat, food haibt, ducks
In shallow water areas including tidal flats, wataerfowls prey on various species of animals and algae, occupy the position of top predators in the food web, and have a significant impact on the marine ecosystem. While waterfowls have an impact on the ecosystem, from a fisheries perspective, waterfowls are sometimes considered as predator for fishery products. In the Ariake Bay, the largest tidal flats in Japan, short neck clam and hard clam are harvested, and cultivation of nori is also a major hisheries. However, the catch of clam has been drastically decreasing since 1977, and there is no recovery even by 2022. In recent years, there has also been reports of feeding damage to the nori. The possibility of feeding damage by ducks in the Ariake Bay has been pointed out as a possible cause of such fishery damage. However, evidence of the feeding damage by ducks on clams and nori is limited to sightings of ducks on tidal flats and near nori nets, observation of predatory behaviour on video, and evaluation based on the stomach contents of captured dukcs. Although these methods can evaluate foraging behaviour at a certain point in time, they do not represent a food history that is representative aof a period of several months. Although the analysis based on carbon and nitrogen isotope ratios is useful for the evaluation of feeding history, there are no reports on duck in the Ariake Bay.
In this study, we analyze the feeding habits of ducks on the mouth of the Mirodikawa River facing the Ariake Bay using carbon and nitrogen stable isotope ratios. In addition, potential food resources defined based on the stomach contents are collected and their carbon and nitrogen stable isotope ratios are measured. Based on these results, we aim to evaluate the feeding habits of ducks flying to the mouth of the Midorikawa River, Japan.
The survey was conducted by Kawaguchi Fisheries Cooperative Association in collaboration with the Jonan Hunting Club, participating in a project to exterminate ducks from December 2021 to March 2022 (five times). A total of 41 ducks were sampled, representing a total of seven species (mallard, spot-billed duck, wigeon, pintail, teal, pochard, and tufted duck). Ducks were dissected immediately after collection, and thoracic muscles, plasma, stomach contents, and intestinal contents were separated. Benthic organisms and nori were collected from the tidal flats, reeds from salt marshes in the Midorikawa River estuary, and seed rice from post-harvest paddy fields in Kumamoto City, Kumamoto Prefectural, Japan. Samples of duck muscle, gut contents and food resources were measured for carbon and nitrogen content and their stable isotope ratios using an elemental analyzer and an isotope ratio mass spectrometer.
The stable carbon isotope ratios (d13C) of mallard, which had the highest number of ducks samples, varied widely from -25.6‰ to -17.6‰. In contrast, spot-billed ducks had d13C values ranging from -25.2‰ to -20.8‰, overlapping the moderate to low d13C range of mallard, while wigeon, teal and Euurasian wigeon had d13C values ranging from -22.6‰ to -17.69‰, overlapping the moderate to high d13C of marllads. When potential food resources were divided into marine (d13C: -21 – -14‰, d15N: 7–12‰) and terrestrial (d13C: –27.9‰, d15N: 6.6‰) sources, we found that there were individuals that fed on terrestrial sorces, individuals that fed on marine sources, and individuals that were in between, indicating that individual differences were large. The spot-billed ducks shoed low d13C, indicating that they are likely to feed exclusively on terrestrial plants. These results suggest that ducks flying to this region do not necessarily feed exclusively marine food resources.
In this study, we analyze the feeding habits of ducks on the mouth of the Mirodikawa River facing the Ariake Bay using carbon and nitrogen stable isotope ratios. In addition, potential food resources defined based on the stomach contents are collected and their carbon and nitrogen stable isotope ratios are measured. Based on these results, we aim to evaluate the feeding habits of ducks flying to the mouth of the Midorikawa River, Japan.
The survey was conducted by Kawaguchi Fisheries Cooperative Association in collaboration with the Jonan Hunting Club, participating in a project to exterminate ducks from December 2021 to March 2022 (five times). A total of 41 ducks were sampled, representing a total of seven species (mallard, spot-billed duck, wigeon, pintail, teal, pochard, and tufted duck). Ducks were dissected immediately after collection, and thoracic muscles, plasma, stomach contents, and intestinal contents were separated. Benthic organisms and nori were collected from the tidal flats, reeds from salt marshes in the Midorikawa River estuary, and seed rice from post-harvest paddy fields in Kumamoto City, Kumamoto Prefectural, Japan. Samples of duck muscle, gut contents and food resources were measured for carbon and nitrogen content and their stable isotope ratios using an elemental analyzer and an isotope ratio mass spectrometer.
The stable carbon isotope ratios (d13C) of mallard, which had the highest number of ducks samples, varied widely from -25.6‰ to -17.6‰. In contrast, spot-billed ducks had d13C values ranging from -25.2‰ to -20.8‰, overlapping the moderate to low d13C range of mallard, while wigeon, teal and Euurasian wigeon had d13C values ranging from -22.6‰ to -17.69‰, overlapping the moderate to high d13C of marllads. When potential food resources were divided into marine (d13C: -21 – -14‰, d15N: 7–12‰) and terrestrial (d13C: –27.9‰, d15N: 6.6‰) sources, we found that there were individuals that fed on terrestrial sorces, individuals that fed on marine sources, and individuals that were in between, indicating that individual differences were large. The spot-billed ducks shoed low d13C, indicating that they are likely to feed exclusively on terrestrial plants. These results suggest that ducks flying to this region do not necessarily feed exclusively marine food resources.