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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 RNA viruses are infamous for their ability to cross species barriers, posing threats to global health and security. Influenza A virus (IAV) is naturally found in avian hosts but periodically spills over into marine wildlife. IAV outbreaks occur in the Northwest Atlantic, but grey seals (Halichoerus grypus) appear to be less susceptible to IAV compared to other species. The subclinical nature of IAV infection in addition to life history factors suggest grey seals are a potential wild reservoir host for IAV. We investigated differential gene expression among grey seals naturally exposed to IAV to elucidate genetic mechanisms involved in grey seal disease resistance. RNA sequencing was conducted on blood samples (N = 31) collected from grey seal pups in Massachusetts, US between 2014 and 2019. Samples were grouped for analysis based on presence/absence of viral RNA and antibodies. In the presence of IAV RNA, we observed widespread down‐regulation of genes, including immune genes, potentially as a result of IAV‐induced host shutoff. Immune down‐regulation occurred in acute stage of IAV infection (+ viral RNA, − antibodies), followed by up‐regulation of protein production in peak stage (+ viral RNA, + antibodies), possibly as a result of increased viral replication. Evidence of an activated immune response was observed in late stage of infection (− viral RNA, + antibodies) with up‐regulated adaptive immunity genes. We hypothesize that the combination of down‐ and up‐regulated immune gene expression may prevent overstimulation of the immune response, acting as an adaptation in grey seals to resist IAV‐associated mortality.