Trophic niche and potential food sources of fish driven by habitat heterogeneity in Zhoushan Fishing Ground

Zhoushan Fishing Ground is characterized by environmental complexity and high habitat heterogeneity. Investigating how these conditions drive the trophic niches and food sources of fish communities is essential for elucidating the energy flow dynamics and food web stability of the ecosystem, thereby providing a scientific basis for the conservation of fishery resources. Samples were collected from bottom trawl surveys conducted in the fishing ground during spring and autumn of 2024. The study area was divided into nearshore and offshore zones along the 123°E longitudinal boundary. Stable isotope analysis was employed to examine trophic structure and identify potential food sources. Analytical methods included the Stable Isotope Bayesian Ellipses model (SIBER) for quantifying trophic niche metrics and the Stable Isotope Mixing Model (simmr) for estimating the proportional contributions of basal organic matter sources to fish consumers. The results showed that in the nearshore zone, fishδ13C values ranged from -21.37‰ to -13.91‰,δ¹⁵N values from 8.22‰ to 13.65‰. In the offshore zone,δ13C values ranged from –20.85‰ to –15.71‰,δ¹⁵N values from 7.01‰ to 14.30‰. These differences were primarily driven by environmental heterogeneity. Trophic niche analysis revealed that nearshore fish exhibited higher isotopic metrics, such as theδ¹³C range (CR) and total convex hull area (TA), indicating greater dietary diversity and a broader trophic niche breadth. Moreover, the size-corrected standard ellipse area (SEAc) of nearshore fish largely overlapped with that of offshore fish, suggesting that nearshore populations utilize a wider spectrum of basal resources. This expanded niche space implies not only greater ecological plasticity but also enhanced resilience to environmental perturbations, as access to diverse resources buffers against fluctuations in any single food source. Particulate organic matter (POM) was the primary carbon source for nearshore fish (contributing 36.2%), reflecting the substantial role of terrestrial detritus and sedimentary organic matter in supporting nearshore fish production. In contrast, phytoplankton was the dominant food source for offshore fish (contributing 33.2%), confirming a shift from mixed terrestrial–marine sources nearshore to predominantly marine-based production offshore. These spatial patterns in resource utilization illustrate how environmental gradients shape energy flow through the ecosystem. In conclusion, habitat heterogeneity in the Zhoushan Fishing Ground significantly influences the trophic niche characteristics of fish with diverse feeding habits. The findings indicate that nearshore fish populations possess greater ecological adaptability. By elucidating the spatial differences in fish trophic niches and their underlying drivers, this study provides a scientific foundation for developing targeted fisheries management strategies and enhancing biodiversity conservation in the region. Specifically, management measures should prioritize the preservation of habitat connectivity and the protection of diverse basal resources-particularly in nearshore areas-to maintain the functional integrity of the food web and support sustainable fisheries.