Stable isotope study maps trophic overlap between native and non-native fishes in China’s Jiulong River Basin

A July 2024 field survey of fish communities in China’s Jiulong River Basin found that eight non-native species represented 14.89% of the 47 species collected but accounted for 44.55% of abundance and 49.85% of biomass. Carbon and nitrogen stable isotope analysis showed that overlap was greatest in the intermediate omnivorous guild, indicating potential competition between tilapia and several native fishes, although isotopic overlap alone does not demonstrate that competition caused population declines.

The observational study focused on the Jiulong River Basin, the second-largest river system in Fujian Province, which provides water for approximately 12 million people in Longyan, Zhangzhou, and Xiamen. Researchers sampled native and non-native fishes at 16 sites in July 2024 using gillnets in pelagic areas and traps in benthic areas. Benthic invertebrates were collected concurrently to establish an isotopic baseline. The overall catch comprised 761 fish from five orders, 11 families and 38 genera: 39 native species and eight non-native species. A total of 298 fish samples underwent stable isotope analysis.

Tissue preparation involved removing scales and skin, excising muscle above the lateral line, drying samples at 60 °C for 48 hours and grinding them to powder. Carbon and nitrogen isotope ratios were measured with an elemental analyzer coupled to a stable isotope ratio mass spectrometer, with analytical precision better than ±0.3‰ for both elements. The Asian clam, Corbicula fluminea, served as the primary-consumer baseline for trophic-position calculations, using its mean δ15N value of 6.43‰ and an assumed 3.4‰ enrichment between adjacent trophic levels.

Across the sampled fish, δ13C values ranged from −27.27 ± 0.05‰ to −14.88 ± 0.58‰, while δ15N values ranged from 7.54 ± 0.04‰ to 14.15 ± 0.81‰. Estimated fish trophic positions extended from 2.33 to 4.27, producing an estimated food-chain length of about 4.27 trophic positions. Pelteobagrus fulvidraco occupied the highest estimated trophic position, while Sinibrama macrops occupied the lowest. The authors compared this value with other large river systems and described the vertical pathway from primary consumers to top predators as structurally intact, while noting that the comparison does not establish long-term stability.

Isotopic niche width varied substantially among species. Individual tilapia generally occupied broader niches than most native species: the average sample-size-corrected standard ellipse area across the four tilapia species for which the study reported values was 3.06‰², with a standard deviation of 0.79‰². Coptodon zillii had a sample-size-corrected area of 3.47‰², Sarotherodon galilaeus 3.51‰² and Oreochromis aureus 3.40‰². By contrast, the native assemblage as a whole had a larger sample-size-corrected area of 13.08‰² than the non-native assemblage at 4.97‰², while total isotopic niche area was 67.81‰² for native fishes and 19.64‰² for non-native fishes.

Bayesian niche analysis estimated that 63.59% of native fish niche space overlapped with non-native fish niche space, while 99.56% of the non-native niche space was encompassed within the native niche space. Overlap was concentrated among intermediate omnivores. Across all tilapia–native pairings, the average was 50.78%. C. zillii had an average overlap of 60% with native species, including 100% with Pseudogobio vaillanti, 86% with Acrossocheilus fasciatus and 80% with Xenocypris davidi. S. galilaeus averaged 58%, with complete overlap with P. vaillanti and 87% overlap with A. fasciatus. Oreochromis aureus and O. niloticus averaged 44% and 41%, respectively.

Among other non-native fishes, Pterygoplichthys pardalis averaged 52% overlap with native species, including 100% with P. vaillanti, 84% with Carassius auratus and 82% with X. davidi. Clarias batrachus averaged 27%. Separate comparisons between tilapia and fish in lower or upper trophic groups also showed overlap: C. zillii averaged 43% with native fishes in those groups, and O. aureus averaged 33%. Pairwise estimates were generated with 2,500 Monte Carlo simulations and quantified the probability that individuals of one species would fall within the 95% niche region of another.

Habitat distributions differed between the groups. Native fishes were more often associated with riffles, cobble or gravel substrates and clearer water, whereas non-native fishes were concentrated in places with muddy or sandy substrates, turbid and eutrophic conditions, and abundant aquatic vegetation. At several such sites, non-native fish made up more than 65% of relative abundance. The authors associated this pattern with the concentration of tilapia in intermediate trophic positions and proposed that their flexible feeding could create competitive pressure on native benthic omnivores and detritivores, particularly when resources are limited. This remains a potential mechanism inferred from isotope similarity and habitat association, not a measured effect on native abundance or survival.

Based on the observed overlap, the study proposed targeted removal of tilapia in high-density areas during the breeding peak, protection or restoration of benthic microhabitats, and enhanced populations of native fishes. It also recommended long-term monitoring combining environmental DNA, stable isotopes and routine surveys so management responses could be assessed. The study was limited to a single July sampling campaign in one basin, and its isotope-based estimates characterize trophic niches rather than directly observing feeding interactions or quantifying realized competition. The results therefore identify areas where further interaction data and community responses would be needed to determine whether high overlap translates into displacement of native fishes.

Subject of Research: Stable isotope assessment of trophic niches and potential competition between native and non-native fish in the Jiulong River Basin

Article Title: Fish trophic structure and potential competitive interactions in a typical subtropical river

Article References: Feng, S., Wang, J., Liu, W., Hui, Y., Wang, G., Li, J., Pan, X., Wu, C., Lin, L., Wang, X., Wu, Q., Wu, Z., Xiao, Y., Sun, X., Huang, L., & Ji, F. (2026). Fish trophic structure and potential competitive interactions in a typical subtropical river. NeoBiota, 108, 1-20. https://doi.org/10.3897/neobiota.108.187010

Image Credits: AI Generated

DOI: 10.3897/neobiota.108.187010

Keywords: Jiulong River Basin, non-native fish, tilapia, stable isotopes, trophic structure, isotopic niche overlap, resource competition, fish conservation, subtropical river, Fish, trophic, structure