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Beluga whales (Delphinapterus leucas) are sentinel species for Arctic environmental change; however, whether belugas are resilient to anticipated changes in prey dynamics remains uncertain, partly due to a lack of dietary information. Mixing models are promising tools to evaluate the diets of marine mammals, using dietary tracers such as fatty acid signatures. Additionally, captive feeding trials are useful to test the effectiveness of these tools for application to wild populations. In our study, we compared the ability of two models, quantitative fatty acid signature analysis in R (QFASAR) and Fatty Acid Source Tracking Algorithm in R (FASTAR) to reconstruct the diets of captive beluga whales that had been fed a long-term diet consisting mostly of Pacific herring (Clupea harengus pallasi), and in smaller proportions, capelin (Mallotus villosus) and squid (Doryteuthis opalescens). To account for fatty acid metabolism, we compared a suite of calibration coefficients derived from captive feeding trials of American mink (Mustela vison), harp seal (Pagophilus groenlandicus), harbour seal (Phoca vitulina richardsi), and grey seal (Halichoerus grypus) fed monotypic diets. For both methods, diets estimated using calibration coefficients derived from feeding trials of mink fed herring and/or seal oil were able to correctly identify herring as the dominant prey source of belugas. In addition, there was higher precision among diet estimates using these calibration coefficients in comparison to those generated using calibration coefficients derived from feeding trials of pinnipeds or mink fed poultry. QFASAR more accurately approximated the diet proportions of the whales than FASTAR, possibly because of the greater robustness to errors in the calibration coefficients. Based on QFASAR, diet estimates best reflect the whales' true dietary proportions consumed approximately two weeks to one month prior to sampling, similar to timeframes seen in previous studies of harbour seals. We recommend caution when using these models with prey that have high within-species variability in fatty acid signatures or small sample sizes, as well as prey with similar fatty acid signatures. The application of mixing models is promising to identify the prey of wild beluga whales and other marine mammals. • Calibration coefficients based on mink fed marine diets best estimated beluga diet. • Models identified herring as main prey using mink-derived calibration coefficients. • Diet estimates reflect diets consumed around 2 weeks to 1 month prior to sampling. • QFASAR better approximated beluga diets than FASTAR using fatty acid signatures. [ABSTRACT FROM AUTHOR]

Individual specialization ( IS), where individuals within populations irrespective of age, sex, and body size are either specialized or generalized in terms of resource use, has implications on ecological niches and food web structure. Niche size and degree of IS of near-top trophic-level marine predators have been little studied in polar regions or with latitude. We quantified the large-scale latitudinal variation of population- and individual-level niche size and IS in ringed seals ( Pusa hispida) and beluga whales ( Delphinapterus leucas) using stable carbon and nitrogen isotope analysis on 379 paired ringed seal liver and muscle samples and 124 paired beluga skin and muscle samples from eight locations ranging from the low to high Arctic. We characterized both within- and between-individual variation in predator niche size at each location as well as accounting for spatial differences in the isotopic ranges of potential prey. Total isotopic niche width ( TINW) for populations of ringed seals and beluga decreased with increasing latitude. Higher TINW values were associated with greater ecological opportunity (i.e., prey diversity) in the prey fish community which mainly consists of Capelin ( Mallotus villosus) and Sand lance ( Ammodytes sp.) at lower latitudes and Arctic cod ( Boreogadus saida) at high latitudes. In beluga, their dietary consistency between tissues also known as the within-individual component ( WIC) increased in a near 1:1 ratio with TINW (slope = 0.84), suggesting dietary generalization, whereas the slope (0.18) of WIC relative to TINW in ringed seals indicated a high degree of individual specialization in ringed seal populations with higher TINWs. Our findings highlight the differences in TINW and level of IS for ringed seals and beluga relative to latitude as a likely response to large-scale spatial variation in ecological opportunity, suggesting species-specific variation in dietary plasticity to spatial differences in prey resources and environmental conditions in a rapidly changing ecosystem. [ABSTRACT FROM AUTHOR]