The trophic ecology of 12 mesopelagic fish species in the northern Benguela Current ecosystem contributes to species coexistence

Mesopelagic fishes are abundant and play an important role, even if incompletely understood, in the ecology and functioning of the global ocean. Mesopelagic fishes are predominantly carnivorous, and assemblages may comprise species that feed on similar zooplankton resources. Yet, their species diversity remains generally high. This study examined the trophic ecology of 12 of the most common mesopelagic fish species found in the northern Benguela Current ecosystem in an effort to understand interactions between species. Integrating morphometric traits, stomach content analysis, and stable isotope values revealed complex, multidimensional niche partitioning within the mesopelagic assemblage. Though morphological similarities suggested potential competition, systematic gape-size analysis demonstrated a clear scaling relationship with prey-size spectra, providing a mechanical basis for resource partitioning. Stomach content analysis confirmed this partitioning, with species-specific preferences for distinct zooplankton groups. These dietary differences were reflected in stable isotope biplots of carbon and nitrogen, which delineated a clear trophic hierarchy. By combining these three methodologies, our study indicates that coexistence is achieved through a synergy of morphological specialisation and trophic positioning, rather than spatial segregation alone.