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Anthropogenic influences caused depletion and subsequent recovery of marine predators, but ecological consequences of altered predator abundance are not well understood. Although many methods are used to study predator diets, methodological biases and logistical challenges preclude robust sampling schemes. We aimed to compare two non-invasive methods: metabarcoding scat-derived deoxyribonucleic acid and hard parts analysis of scat for the Northwest Atlantic grey seal ( Halichoerus grypus (Fabricius, 1791)), a species that rebounded after near extirpation. We hypothesized that metabarcoding would detect a greater diversity and frequency of prey, and that notable differences in diet will be detected since prior studies. Grey seal scat samples ( N = 247) were collected between 2018 and 2019 from Monomoy Island, Massachusetts, USA. Metabarcoding detected greater prey richness on average, with more frequent detections of clupeids (Clupeidae) and flatfish (Pleuronectiformes), whereas hard parts analysis more frequently detected phycid hakes ( Urophycis spp. Gill, 1863). Combining methods increased detections of 13 prey taxa, with 32 prey taxa identified overall. Skates (Rajidae), flatfish, clupeids, and sand lance ( Ammodytes spp. Linnaeus, 1758) were top-occurring prey. Our study highlights the importance of using multiple methods to characterize generalist predator diets using non-invasive techniques and suggests grey seal diet has changed since the early 2000s. 

Abstract BackgroundEnvironmental DNA (eDNA) is an effective tool for the detection and monitoring of presence or absence of rare and invasive species. These techniques have been extended to quantify biomass in vertebrates, particularly in fish species. However, the efficacy of eDNA techniques to quantify biomass in invertebrate species has rarely been examined. This study tested whether eDNA could be used to determine the biomass of the world-wide invasive green crab,Carcinus maenas. In a controlled laboratory study, the relationship between biomass andC. maenaseDNA concentration was examined in the context of different biotic (activity) and abiotic (temperature) parameters. ResultsWhen incubating different numbers of crabs in sterile saltwater for up to 7 days, a relationship between eDNA concentration and biomass was observed at temperatures of 6.7 ℃ and 18.7 ℃, but not at 12.8 ℃. Additionally, motor activity, aggression level, time of sampling, and features of organismal decay had significant impact on the concentration ofC. maenaseDNA collected. ConclusionsWe show that eDNA concentration did not correlate with biomass, and that biomass, temperature, organismal characteristics, and potentially many more parameters affect shedding and degradation rates for eDNA in this species, thus, impacting the recoverable eDNA concentration. Therefore, eDNA techniques are not likely to provide a reliable signal of biomass in the invasive invertebrate speciesC. maenas.