Habitat differences and vertical migration patterns of mesopelagic fish: A case study on the trophic niche of Chauliodus sloani in cold seep-adjacent waters

The adaptive strategies of species to differentiated habitats represent a universal ecological principle. This study takes Chauliodus sloani , a dominant mesopelagic fish in cold seep-adjacent waters, as a model to explore the adaptive patterns of its trophic niche in differentiated habitats. In August 2023, C. sloani samples were collected at different depths (75–750 m) and regions (cold seep and non-cold seep areas). Stable carbon and nitrogen isotope techniques were employed to characterize their trophic niche attributes. Results indicated that the δ 13 C value of C. sloani in cold seep-adjacent waters was −19.29 ± 0.31 ‰, δ 15 N was 9.83 ± 0.53 ‰, and the trophic level was 3.37 ± 0.68. Trophic indicators including carbon isotope range (CR), nitrogen isotope range (NR), mean centroid distance (CD), mean nearest neighbor distance (MNND), and standard deviation of MNND (SDNND) were higher in non-cold seep areas than in cold seep areas at the same water depth. This supports that C. sloani in cold seep areas exhibit lower dietary diversity but higher redundancy and evenness, endowing populations with stronger anti-disturbance capabilities. The directional changes in trophic structures further validated this result, confirming more stable trophic structures in cold seep populations. At 750 m depth, C. sloani exhibited minimal overlap in core trophic niche (SEAc) with other depths, demonstrating significant niche differentiation and independent population establishment at the strata. Zooplankton constituted the primary food source of C. sloani , but the contribution ratio of food sources varied significantly with depth. The contribution ratio of cephalopod prey increased with increasing water depth. This research elucidates both the adaptive strategies of C. sloani trophic niches across heterogeneous habitats and verifies distinctive diel vertical migration behaviors in mesopelagic fishes, offering novel insights into deep-sea organism survival mechanisms within complex ecosystems and associated energy transfer pathways. • The diversity of food sources and trophic structure of Chauliodus sloani in the cold spring area were both lower, and the trophic niche was narrower than that in the control area. • The Chauliodus sloani populations of cold seep were more stable and resistant to disturbance compared to control areas. • Trophic structure diversity and trophic niche decreased with depth and trophic structure stability and resistance to disturbance increased with depth of the Chauliodus sloani . • The main carbon source for Chauliodus sloani is zooplankton, and its proportion decreased gradually with depth, with a gradual increased in the proportion of cephalopods contributing. • Chauliodus sloani differ significantly in trophic structure and diet in deep-sea waters, and separate populations may exist.