Diet spectra of several fish species in South Greenland waters revealed through DNA-metabarcoding: The role of gelatinous zooplankton as prey

Diet studies of top predatory fish are fundamental in understanding predator-prey relationships in marine ecosystems and food web dynamics. In the case of commercially exploited fish species, they can eventually inform fish stock management and fisheries directives. In the Northwest Atlantic, fish die studies have been conducted in Canadian (Flemish Cap, Grand Banks), and Icelandic waters, and are notably absent in Greenlandic waters. Moreover, traditional diet studies have relied on visual stomach content analysis, which are unable to identify heavily degraded tissues and may produce systemic errors underestimating quickly digested prey species, such as gelatinous zooplankton (GZP). To assess the complete prey composition of common fish species in Southern Greenland waters, DNA metabarcoding with a multi-marker approach targeting fragments of both the nuclear 18S rDNA V1-2 and the mitochondrial cytochrome c oxidase I (COI) genes was applied to stomachs of Atlantic cod (Gadus morhua), redfish (Sebastes spp.), wolffish (Anarhichas spp.), greater silver smelt (Argentina silus), plaice (Hippoglossoides platessoides), and haddock (Melanogrammus aeglefinus). A broad range of taxa were identified by both genes; while COI showed a better taxonomic resolution on species-level for most of the metazoans, 18S had advantages in detecting GZP taxa. Spatial variation in the prey composition of G. morhua was identified, for which the diet on the east coast of Greenland was almost exclusively composed of Meganyctiphanes norvegica (krill) and annelids, while the diet on the west coast contained considerably more benthic organisms of the order Ophiurida and Actiniaria. Differences in the diet composition of Sebastes spp. were found between juveniles, which mainly fed on Calanoida or Euphausiacea, and adults, which showed a more diverse diet including ctenophores, cephalopods, and teleosts. Distinct diets were identified for Anarhichas denticulatus and Anarhichas minor. The individuals of A. denticulatus were identified as major GZP predators (Scyphozoa, Ctenophora), whereas A. minor was identified as a benthic predator of echinoderms. A predominance of GZP predation was revealed for Argentina silus, the stomachs of which were recorded as empty during visual assessment, but were shown to contain GZP (Hydrozoa, Scyphozoa, Ctenophora) by up to 100% of the detected reads in the metabarcoding analysis. Spatial variation in the diet composition was shown for Hippoglossoides platessoides along the west coast of Greenland, where the stomachs of individuals from northern and southern stations were dominated by Arthropoda reads and the middle station was dominated by reads assigned to the echinoderm Ophiopholis aculeata. Melanogrammus aeglefinus showed spatial variation on a more restricted spatial scale, for which the diet of individuals at one station was dominated by arthropods whereas that of the individuals collected at a station nearby was dominated by annelids. Overall, the results indicate that GZP predation is prevalent in all fish species studied, albeit in different proportions, and is detectable with both target gene fragments, COI and 18S.