Multi-Tissue Stable Isotope Analysis Reveals the Feeding Ecology of Dominant Shark Bycatch Species in the Northern South China Sea

Understanding the feeding mechanisms and interspecific coexistence of sharks is crucial for effective conservation. This study conducted stable isotope analysis on muscle and liver samples from 449 individuals of eight common bycatch shark species collected via bottom trawling in the northern South China Sea (NSCS). Results revealed significant differences in δ13C and δ15N values among species and tissue types. Scoliodon laticaudus exhibited the highest trophic position (TPmuscle = 4.60 ± 0.33; TPliver = 4.53 ± 0.29), while Apristurus platyrhynchus had the lowest (TPmuscle = 2.97 ± 0.44; TPliver = 2.75 ± 0.53). Muscle and liver isotopic signals were consistent, but δ13C differences indicated distinct carbon sources, with Carcharhinus sorrah linked to deep-sea organic matter and S. laticaudus to coastal inputs. Significant correlations between δ13C/δ15N and body length in A. platyrhynchus and Cephaloscyllium fasciatum suggest ontogenetic shifts in diet and habitat toward deeper waters. Trophic niche analysis using corrected standard ellipse area (SEAc) showed Halaelurus burgeri with the widest trophic niche (SEAc > 1.7‰2), reflecting a broad diet, while C. fasciatum had the narrowest (SEAc < 0.3‰2), indicating specialized feeding. Additionally, H. burgeri and C. sarawakensis exhibited significant niche differentiation, reducing interspecific competition, whereas C. fasciatum and Squalus megalops showed high niche overlap, suggesting intense resource competition. The narrower liver niche of C. sarawakensis may reflect recent habitat constriction due to bottom trawling. This study elucidates the feeding ecology and habitat resource utilization of NSCS sharks, providing a scientific basis for effective conservation strategies for shark populations in the region.