Multiple Approaches to the Trophic Role of Mesopelagic Fish around the Iberian Peninsula

SIMPLE SUMMARY: Mesopelagic fish form the largest fish biomass in the oceans. They are essential intermediates in marine food webs, having an important role as zooplankton consumers and prey for top predators such as pelagic fish or marine mammals, as well as in connecting the surface and more impoverished oceanic depths, since many mesopelagic species perform diel vertical migrations. The collapse of most commercial fisheries has led to the consideration of the future commercial exploitation of these fish, which is highly debated. Unfortunately, we have limited information about their feeding strategies and contribution to the marine carbon pump of mesopelagic fish, due to the difficulty of investigating their habitat, the twilight zone. This study analyzed the diets of mesopelagic fish species around the Iberian Peninsula (Atlantic and Mediterranean areas) in terms of numerical prey abundance and carbon weight, which allowed for their assignment to specific feeding guilds. We found that, despite their great diversity, mesopelagic fish food resource use is partitioned by being spatially segregated, using different feeding strategists (generalist, facultative feeding behavior, and specialized diets), or as a result of their dietary shift from juvenile to adult stages, rather than having different food compositions. The assessment of the trophic role of mesopelagic species will provide baseline data regarding the impacts that their potential exploitation might have on the populations of top predators that also play a key role in regulating marine ecosystems. ABSTRACT: Myctophids, commonly vertical migrators, and partial and non-migrant stomiiforms constitute most of the mesopelagic biomass, and transport organic matter throughout the food web from the surface to the ocean’s depths. Both the diet and trophic structure of twenty-nine species of mesopelagic fish collected around the Iberian Peninsula were studied through the analysis of stomach contents, quantifying food items with a high taxonomic resolution. The investigation covered oligotrophic to productive habitats, with sampling stations distributed in five discrete zones of the western Mediterranean Sea and the northeastern Atlantic Ocean. The geographic environmental conditions, migratory behavior, and species-specific body sizes allowed for the identification of some major feeding patterns for these fish communities. The trophic niche of migrant myctophids showed a high overlap, with copepods as the primary prey category. The diet composition of generalist myctophids (e.g., Ceratoscopelus maderensis and Hygophum benoiti) reflected the distinct zooplanktonic communities between zones. Large stomiiforms (Chauliodus spp., Sigmops elongatus) preferred feeding on micronekton, while the smallest stomiiforms (e.g., Argyropelecus spp., Cyclothone spp., Vinciguerria spp.) preyed on copepods and ostracods. Given the relevance of the mesopelagic fish communities for commercially exploited species and, therefore, for maintaining the sustainability of the fisheries’ activity in the zones analyzed, the information provided in the present study is essential for a better understanding of the biology and ecology of these species.