Introduction Dietary analyses utilising visual methods to identify stomach and faecal contents have shown that urban red foxes (Vulpes vulpes) in the United Kingdom (UK) consume human-derived (anthropogenic) food to varying degrees. Anthropogenic foods have been implicated in poor health outcomes for synanthropic species that consume them, therefore it is critical to examine the degree of such foods in the UK fox diet. Methods We analysed the stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) of whiskers collected from 93 foxes from across the UK to determine: 1) the viability of stable isotope analysis (SIA) for distinguishing between anthropogenic and natural food consumption in foxes; 2) the degree to which urban foxes consume anthropogenic food compared to rural foxes using a Bayesian mixing model; 3) whether factors such as sex, age and season predict individual levels of consumption of human-derived food in foxes. Main results 1) Urban fox diet was significantly different to rural foxes: urban foxes demonstrated significantly higher δ13C and lower δ15N, a pattern consistent with anthropogenic food consumption. 2) We estimated that food provided either directly or indirectly by humans contributed around 35% of the urban fox diet compared to approximately 6% in the rural fox diet. 3) Across rural and urban foxes combined, there were significant isotopic differences between males and females, with females demonstrating higher δ13C and lower δ15N. 4) No differences in δ13C and δ15N between subadults and adults were observed. 5) Values of δ15N differed significantly between samples from the summer and the winter, where summer had the highest δ15N and winter the lowest. Additionally, winter had the highest δ13C values of the seasons, although this difference was not significant. Conclusion Potential negative outcomes of anthropogenic food consumption are likely to disproportionately impact females than males and urban-dwelling foxes than rural foxes.

Rationale: Increasing demand for fish and seafood means that traceability of marine products is becoming ever more important for consumers, producers and regulators. Highly complex and globalised supply networks create challenges for verifying the claimed catch region. Atlantic cod is one of the most commercially important species in the northeast Atlantic. Several regional fisheries supply cod into the trade network, of which some are at more risk of overexploitation than others. Tools allowing retrospective testing of spatial origin for traded cod products would significantly assist sustainable harvesting of wild fish, reducing incentives for illegal fishing and fraud. Methods: Here we investigate whether stable isotope ratios of carbon, nitrogen and sulfur in muscle tissue can be used to identify the catch region of Atlantic cod ( Gadus morhua). We measured the isotopic composition of muscle tissue from 377 cod from ten known catch regions across the Northeast Atlantic and Northeast Arctic, and then applied three different assignment methods to classify cod to their region of most likely origin. The assignment method developed was subsequently tested using independent known-origin samples. Results: Individual cod could be traced back to their true origin with an average assignment accuracy of 70-79% and over 90% accuracy for certain regions. Assignment success rates comparable to those using genetic techniques were achieved when the same origin regions were selected. However, assignment accuracy estimated from independent samples averaged c25% overall. Conclusion: Stable isotope techniques can provide effective tools to test for origin in Atlantic cod. However not all catch regions are isotopically distinct. Stable isotopes could be used in conjunction with genetic techniques to result in higher assignment accuracy than could be achieved using either method independently. Assignment potential can be estimated from reference datasets, but estimates of realistic assignment accuracy require independently collected data.