| Summary: | peer reviewed We present here an innovative use of stable isotopes ratios of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S) combined with mercury (THg) and selenium (Se) concentrations in seal body tissues as ecological tracers of harbour and grey seal foraging activities and exposure to Hg. SIBER (Stable Isotope Bayesian Ellipes) allows the use of stable isotope ratios, [Se] and [Hg] to build geometric spaces (or “isospaces”). These can be used as proxies of species ecological niches commonly referred to “isotopic niches” . Our objectives are (1) to evaluate potential temporal changes in foraging patterns and exposure of grey seals and harbour seals (2) to highlight the trophic plasticity of these two seal species in a changing environment. A multi-tracer approach ([THg] and [Se]; 13C, 15N, 34S values) was applied to harbour seals and grey seals (blood; German and Scottish coasts, n=174 and n=80, respectively) during different time periods (from 1900 to 2017). Additionally, the same tracers were analysed in harbour seals (hair; from 1930 to 2017) from the Natural History Museum of Stockholm (n=10), free-ranging and stranded harbour seals from the North Sea (German and Belgian coasts, n=40 and n=6, respectively). SIBER package in R was used to explore the variation in the isotopic niche (δ13C vs. δ15N and δ15N vs. δ34S) and the ecological niches (δ13C vs. T-Hg, T-Hg vs. δ15N, δ34S vs. T-Hg, δ13C vs. Se, Se vs. δ15N and δ34S vs Se). Our multi-tracer approach revealed: (1) an increase of the size isotopic and ecological niche size in agreement with a more offshore foraging distribution, and an increase of benthic resources (more flatfish and less sandeels) for Scottish grey seals over time (Fig.1). In contrast, we calculated a smaller isotopic niche size for harbour seals over time likely linked to an increase of phytoplankton biomass, supporting zooplankton for planktivorous fish species such as clupeids. (2) Long-term changes having affected the North Sea are reflected in hair collected from harbour ...
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