Trophic ecology of mesopelagic fishes in the northern and southern Benguela Upwelling Systems revealed through stable isotope patterns

ABSTRACT: Mesopelagic fishes are important components of marine food webs as trophic links between primary consumers and higher trophic levels, but their trophic ecology is poorly understood. We used stable isotopes (δ¹³C and δ¹⁵N) to compare the trophic position, trophic enrichment factor, and isotopic niche among feeding guilds of mesopelagic fishes (including myctophids, stomiids) during different seasons in the northern and southern Benguela Upwelling Systems. These subsystems are influenced by differing water masses with various physical and biological properties, which can lead to changes in the marine food web. We aimed to determine if and how the trophic ecology of mesopelagic fishes differs between seasons and subsystems. When separating fishes into guilds based on dietary preferences (zooplanktivores vs. piscivores) and migration patterns (migrating vs. non-migrating), the trophic position was highest during winter in the south for non-migrating piscivores (4.5), but the trophic positions of piscivores (3.2-4.4) were not always higher than those of zooplanktivores (3.1-4.5). This may indicate different isotopic enrichment between species or a very generalist diet from zooplanktivores such as myctophids and their prey. Trophic enrichment factors for δ¹⁵N among guilds ranged from 3.8 to 7.5‰ between the baseline and zooplanktivores and from -0.8 to 1.6‰ between zooplanktivores and piscivores. Previously, mesopelagic fishes have been incorporated into food web models in the Benguela Upwelling Systems, but only as a single entity. Elucidating the complex seasonal and regional trophodynamics of various mesopelagic feeding guilds may contribute to more accurate assessments of trophic transfer efficiencies and higher-resolution models in these systems.