Resource partitioning among endangered and non-native bitterling fishes in a small pond: insights from stable isotope and gut content analyses

The population decline of the endangered bitterling species, Acheilognathus typus, following non-native bitterling invasion underscores the need to understand interspecific interactions among bitterling fishes. In this study, the dietary habits of A. typus, alongside introduced non-native bitterling species (Acheilognathus rhombeus and Rhodeus ocellatus ocellatus) and other omnivorous fishes (Oryzias latipes, Pseudorasbora parva, and Rhinogobius sp.), were investigated in a small pond in Japan using gut content and stable isotope analyses (δ¹³C and δ¹⁵N). Analysis revealed that microalgae, particularly diatoms with periphytic or benthic life forms, dominated the gut contents of the three bitterling species, which exhibited high dietary overlaps (Pianka’s index range 0.89–0.98). Although bitterling species displayed similar δ¹³C (mean range −18.4 to −16.1‰) and δ¹⁵N (12.1 to 12.6‰) values, the other omnivorous fishes exhibited distinct isotopic signatures: lower δ¹³C and higher δ¹⁵N values. Isotopic niche overlaps among the three bitterling species were limited (0–2.4%). Furthermore, A. typus and A. rhombeus showed no overlap in the corrected standard ellipse area. These findings suggest that the three bitterling species may forage on similar food items, primarily microalgae, with differing stable isotope ratios, enabling them to partition foraging sites within the confines of a small pond where competition may arise.