Development of a non-lethal stomach content analysis method for freshwater eels: An empirical evaluation of the tube method for Anguilla marmorata.

Understanding the feeding habits of predatory fish is essential for unravelling food web structures and implementing conservation strategies in riverine ecosystems. However, conventional lethal stomach content analysis methods are not necessarily appropriate for long-term dietary studies, particularly for threatened species, as they require large sample sizes due to the inability to repeatedly analyse stomach contents from the same individuals. This study aimed to develop and validate a non-lethal stomach content analysis method using tubes for the giant mottled eel Anguilla marmorata, a representative tropical anguillid species. A total of 205 eels were collected from nine rivers on Amami-Ohshima Island, Japan. Among 102 eels, including stomach contents, the tube method achieved an average removal efficiency of 76.5% (biomass content) and a detection rate of 92.4% for prey categories, effectively capturing dietary composition without significant bias. The most important food items were prawns (mainly Macrobrachium), crabs and fish, but aquatic insects, terrestrial invertebrates and a few snails were also eaten. Although crabs were less efficiently removed due to their body size or hard exoskeletons, supplementary use of forceps allowed complete collection of their stomach contents. The removal efficiency was not significantly influenced by eel size, stomach fullness or prey type, suggesting broad applicability of this method. Our findings demonstrate that the tube method, combined with forceps when necessary, offers a powerful non-lethal tool for investigating individual-level feeding ecology of anguillid eels, enabling long-term dietary monitoring and supporting conservation of declining populations. This method will advance ecological understanding and sustainable management of anguillid eels and their freshwater habitats, and this is especially true for tropical eels whose feeding ecology has rarely been studied.