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The Patagonian longfin squid Doryteuthis gahi has an annual life cycle with two seasonal cohorts (autumn and spring spawners). Previous studies estimated the trophic level of adult squid by stable isotope analysis of their muscle or gladius tissues, but few studies compared different sizes and spawning cohorts. In this study, mixed models were used to compare carbon (δ13C) and nitrogen (δ15N) stable isotope signatures of muscle and gladius tissues of the autumn and spring spawning cohorts and squid of different sizes. A published isoscape model provided δ15N baselines to calculate the trophic level of D. gahi. Sampled herbivorous prey such as Euphausiacea, planktonic juveniles Munida gregaria and salps Ihlea supported the trophic level estimates. The autumn spawning cohort had higher δ13C values than the spring spawning cohort. δ15N values increased with increasing dorsal mantle length, suggesting an ontogenetic change towards larger prey. Muscle tissue was enriched in 15N but slightly depleted in 13C compared to gladius tissue. Adult squid had an estimated trophic level of ∼3.2, with no significant differences between spawning cohorts. Stable isotope values followed a seasonal trend, with lower δ15N values and lower δ13C value between July and September. Large individuals of the spring spawning cohort were found to have the least niche overlap with other groups of D. gahi. Our study of the trophic levels and ecology of D. gahi can provide important information on its role in the food chain and can build the basis of studies on interaction with other species, and the impact of environmental changes on their populations. This information can inform the development of management strategies aimed at preserving the balance of the ecosystem and ensuring its sustainable use. • Adult squid had an estimated trophic level of ∼3.2, with no significant differences between spawning cohorts. • Euphausiacea, juveniles M. gregaria and salps Ihlea supported the trophic level estimates of D. gahi based on isoscapes. • Stable isotope values followed a seasonal trend, with lower δ15N values and lower δ13C value between July and September. • Small individuals of the spring spawning cohort were found to have the most niche overlap with other groups of D. gahi. • Large individuals of the autumn spawning cohort were found to have the least niche overlap with other groups of D. gahi. [ABSTRACT FROM AUTHOR]

Understanding the drivers and effects of exposure to contaminants such as mercury (Hg) and organochlorine compounds (OCs) in Antarctic wildlife is still limited. Yet, Hg and OCs have known physiological and fitness effects in animals, with consequences on their populations. Here we measured total Hg (a proxy of methyl-Hg) in blood cells and feathers, and 12 OCs (seven polychlorinated biphenyls, PCBs, and five organochlorine pesticides, OCPs) in plasma of 30 breeding female Antarctic petrels Thalassoica antarctica from one of the largest colonies in Antarctica (Svarthamaren, Dronning Maud Land). This colony is declining and there is poor documentation on the potential role played by contaminants on individual physiology and fitness. Carbon (δ13C) and nitrogen (δ15N) stable isotope values measured in the females' blood cells and feathers served as proxies of their feeding ecology during the pre-laying (austral spring) and moulting (winter) periods, respectively. We document feather Hg concentrations (mean ± SD, 2.41 ± 0.83 μg g−1 dry weight, dw) for the first time in this species. Blood cell Hg concentrations (1.38 ± 0.43 μg g−1 dw) were almost twice as high as those reported in a recent study, and increased with pre-laying trophic position (blood cell δ15N). Moulting trophic ecology did not predict blood Hg concentrations. PCB concentrations were very low (Σ 7 PCBs, 0.35 ± 0.31 ng g−1 wet weight, ww). Among OCPs, HCB (1.02 ± 0.36 ng g−1 ww) and p, p' -DDE (1.02 ± 1.49 ng g−1 ww) residues were comparable to those of ecologically-similar polar seabirds, while Mirex residues (0.72 ± 0.35 ng g−1 ww) were higher. PCB and OCP concentrations showed no clear relationship with pre-laying or moulting feeding ecology, indicating that other factors overcome dietary drivers. OC residues were inversely related to body condition, suggesting stronger release of OCs into the circulation of egg-laying females upon depletion of their lipid reserves. Egg volume, hatching success, chick body condition and survival were not related to maternal Hg or OC concentrations. Legacy contaminant exposure does not seem to represent a threat for the breeding fraction of this population over the short term. Yet, exposure to contaminants, especially Mirex, and other concurring environmental stressors should be monitored over the long-term in this declining population. • We quantified Hg and OC residues in egg-laying Antarctic petrels. • Blood Hg was related to pre-laying trophic position, and not to breeding success. • Plasma PCBs were low, while Mirex was high when compared to similar polar seabirds. • Plasma OCs were not linked to pre-laying feeding ecology. • Plasma OCs were inversely related to body condition, but not to breeding success. [ABSTRACT FROM AUTHOR]