1991, Phillips and Gregg 2003). Simple linear mixing models

can be used to resolve diet solutions by Euclidean distances between δ13C and δ15N in biplot space but these models are constrained by the number of isotopes (n) used, limiting the number of sources that can be solved to n + 1. Phillips (2001) established a theoretical framework for more complex models that allow for a greater number of sources to selleck inhibitor be considered, known as IsoSource (Phillips and Gregg 2003). However, as the number of sources used in a mixture increases, so too does the uncertainty in the source combinations. To counter this issue, a Bayesian framework (MixSIR) was adopted which permitted any number of sources to be considered, providing probabilistic

distributions of percentage source contributions (Moore and Semmens 2008). Similarly, Stable Isotope Analysis in R, or SIAR models explicitly recognize uncertainty from a number of sources, but include diet-tissue fractionation and incorporate them into model parameter estimates (Parnell et al. 2008, R Development Core Team 2011). This approach, has allowed robust dietary solutions to be derived in Proteases inhibitor several vertebrate species, such as humpback whales using skin biopsies as the consumer mixture and putative prey items as sources (e.g., Witteveen et al. 2011). Spatial and temporal variation in isotopic baseline of the marine environment (i.e., in phytoplankton tissues) is considerable, ultimately driven by sea temperature, water chemistry, day length, plankton species composition, plankton biomass and carbon and nitrogen uptake regime (Goericke and Fry 1994, Hofmann et al. 2000, Jennings and Warr 2003, Tagliabue and Bopp 2008). Prey should be sampled at a scale (both spatial and temporal) relevant to the predator and tissue examined, such that confounding effects of both spatial and temporal variation can be minimized. However, the potential bias associated with source turnover for contributions depend on the adequacy of the selected sources (i.e., putative prey), which should be based

on empirical evidence (Phillips et al. 2005, Ward et al. 2011). Euphausiidae (hereafter referred to as krill) are key species in marine food webs, supporting biomass of pelagic predators including baleen whales Suplatast tosilate (Verity et al. 2002). The most abundant species found in the Celtic Sea (CS) are Meganyctiphanes norvegica and Nyctiphanes couchii, whose distributions are generally confined to continental slopes and shelf waters respectively (Lindley 1982). In the North Atlantic, stomach content analysis carried out at whaling stations (Brodie et al. 1978, Fairley 1981), supported by modeling spatial associations, confirm that some fin whales feed chiefly on M. norvegica, capelin (Mallotus villotus) and herring (Clupea harengus) (Piatt et al. 1989, Skern-Mauritzen et al. 2011).