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.

在非本地苦鱼入侵后，濒临灭绝的苦鱼物种 Acheilognathustypus的数量下降，强调了了解苦鱼之间种间相互作用的必要性。在这项研究中，在一个小池塘中对斑鳝鱼、引进的非本地苦鱼（Acheilognathus rhombeus和Rhodeus ocellatus ocellatus）和其他杂食性鱼类（Oryzias latipes、Pseudorasbora parva和Rhinogobius sp.）的饮食习性进行了调查。在日本，使用肠道内容物和稳定同位素分析（δ ¹³ C 和 δ ¹⁵ N）。分析显示，微藻，特别是具有附生或底栖生命形式的硅藻，在三种苦味鱼的肠道内容物中占主导地位，这三种苦味鱼表现出较高的饮食重叠度（Pianka 指数范围为 0.89-0.98）。尽管苦鱼物种表现出相似的 δ ¹³ C（平均范围 -18.4 至 -16.1‰）和 δ ¹⁵ N（12.1 至 12.6‰）值，但其他杂食性鱼类表现出不同的同位素特征：较低的 δ ¹³ C 和较高的 δ ¹⁵ N 值。三种苦鱼物种之间的同位素生态位重叠有限（0-2.4%）。此外，A.typus和A.rhombeus在校正后的标准椭圆区域中没有显示重叠。这些发现表明，这三种苦味鱼可能以相似的食物为食，主要是微藻，具有不同的稳定同位素比率，使它们能够在可能发生竞争的小池塘范围内划分觅食地点。