Stable-Isotope Differentiation of Mature Ovarian Eggs from Anadromous and Freshwater-Resident Coilia nasus

Abstract

Anadromous and freshwater-resident Coilia nasus frequently share spawning grounds, yet exhibit distinct feeding habits during migration; therefore, we hypothesized that the isotopic profiles of mature eggs from these two ecotypes would differ. In the present study, a total of 102 C. nasus individuals were collected from a spawning ground in Poyang Lake, China; among them, 20 mature females with intact ovaries were selected for further analysis. Their migratory history was confirmed by otolith microchemistry, and carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of mature eggs were analyzed. Otolith microchemical results showed that some of the 20 females exhibited three Sr/Ca stages (1.51 ± 0.46, 4.79 ± 1.00, and 2.09 ± 0.79), identifying them as anadromous, whereas the remaining females showed stable low Sr/Ca ratios (1.49 ± 0.45), indicating freshwater residency. Both δ13C and δ15N values of mature eggs from the anadromous individuals were significantly higher than those from the freshwater-resident individuals (p < 0.01, Mann–Whitney U test). And the results of linear discriminant analysis (LDA) showed no misclassification. These results demonstrated that δ13C and δ15N showed strong discriminatory performance for mature eggs from anadromous and freshwater-resident C. nasus within the present dataset. However, because all analyses were performed on mature eggs collected directly from ovaries, this method has not yet been tested on naturally spawned or fertilized eggs in the field. Therefore, future studies should first validate this method on field-collected fertilized eggs to determine its practical efficacy for locating spawning grounds.