Stable Isotope Insights Into the Trophic Role of the Invasive Atherina boyeri (Atherinidae) in Central Anatolia, Türkiye

This study aimed to determine the trophic role of the big‐scale sand smelt (BSSS) ( Atherina boyeri ) in Lake Eğirdir, a large freshwater lake in Central Anatolia, Türkiye, using carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope analyses. Although this euryhaline species rarely occurs in freshwater habitats, it has recently expanded into many freshwater systems worldwide, including Türkiye. The species was introduced into Lake Eğirdir approximately 18 years ago and has been associated with the decline of several native fish stocks, primarily due to predation on fish eggs. Stable isotope samples of BSSS (boneless and skinless dorsal muscle) were collected during spring and autumn of 2010 from four stations across the lake. Mean isotope signatures of BSSS ( n = 21) were δ 13 C = −18.73 ± 1.24‰ and δ 15 N = 11.65 ± 0.71, showing little spatial or seasonal variation. The species occupied the second‐highest trophic level in the food web, with a mean trophic position of 3.51 ± 0.19. Diet composition was estimated using three mixing models (IsoSource, SIAR, and Bugalho et al. 2008) to compare methodological outcomes. All models consistently indicated that invertebrates were the dominant dietary source (IsoSource: mean 46%, range 44%–48%; Bugalho et al. 2008: 44%–49%; SIAR: mean 26%, range 12%–39%). When invertebrate prey was partitioned by taxon, crabs and crayfish were found to contribute 40%–60% to the diet of BSSS. Contrary to the stomach contents analysis, stable isotope data indicated very low contribution of zooplankton to the tissues of BSSS. Niche size analysis revealed strong seasonal variation, with SEAc values of 4.16 and 0.29 in spring and autumn, respectively, suggesting that BSSS exploited a broader range of prey when its feeding intensity is higher and a narrower range when feeding intensity is lower. This is the first study in Türkiye to investigate the role of Atherina boyeri in Lake Eğirdir through the understanding of its feeding ecology using stable isotope methods. The long‐standing belief that BSSS severely depletes fish egg populations in Lake Eğirdir is only partially supported by these results, as stable isotope models indicated a relatively minor contribution of fish eggs to its assimilated diet.