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Feeding ecology of Atlantic bluefin tuna (ABFT, Thunnus thynnus), from the Gulf of Saint Lawrence, Canada, was assessed by stomach content and stable isotope analyses. Stomachs were collected from 161 individuals averaging 249 ± 20 cm in curved fork length and 248 ± 64 in round weight. The fish were caught by commercial fishing boats using rod and reel gear during fall 2015-2017. Liver and muscle samples were taken from 133 fish for stable isotope analysis. Atlantic mackerel (Scomber scombrus) was the most abundant prey item in 2015 (%Alimentary index, %AI = 88.94), whereas in 2016 and 2017 the ABFT diet was mainly composed of Atlantic herring (Clupea harengus) (%AI = 56.73 and %AI = 77.23, respectively). PERMANOVA analysis revealed significant differences in the dietary composition between years. δ 15 N values varied between 12.3 and 15.4‰ for muscle and between 11.1 and 14.6‰ for liver. δ 13 C values ranged from -19.9 to -16.5‰ for muscle and from -20.7 to -17.3‰ for liver. PERMANOVA analysis also detected inter-annual isotopic differences in both muscle and liver samples (p < 0.05). Daily rations (DR) were similar in 2015 and 2016 (DR = 1.02 ± 0.32 and DR = 1.10 ± 0.21, respectively), but higher in 2017 (DR = 1.73 ± 0.54)., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The abundance of ungulate populations may fluctuate in response to several limiting factors, including climate, diseases, and predation. In the northern Richardson Mountains, Canada, Dall sheep (Ovis dalli dalli) have undergone a major decline in the past decades and predation by grizzly bears (Ursus arctos) and wolves (Canis lupus) was suspected as a leading cause. To better understand the relationship between these three species located in this rugged and remote ecosystem, we relied on a combination of indirect methods. We investigated the apparent role of predation on the Dall sheep population using spatial ecology and stable isotopes. We examined seasonal variation in predation risk, focusing on how it may affect Dall sheep habitat use and sexual segregation, and we evaluated the proportion of Dall sheep in the diet of both predators using stable isotopes. The movements of the three species were monitored by satellite telemetry. Dall sheep habitat use patterns were analyzed using topographical features, greenness index, land cover, and apparent predation risk. The diets of grizzly bears and wolves were examined using a Bayesian mixing model for carbon and nitrogen stable isotopes. We found that Dall sheep habitat use varied seasonally, with different patterns for ewes and rams. Exposure to grizzly bear risk was higher for rams during summer, while ewes were further exposed to wolf apparent predation risk during winter. The importance of safe habitats for ewes was reflected in space use patterns. Stable isotopes analyses suggested that the diet of grizzly bears was largely from animal sources, with mountain mammals comprising about one quarter. Wolves mostly fed on both aquatic browsers and mountain mammals. Diet variation between individual predators suggested that some individuals specialized on mountain mammals, likely including Dall sheep. We conclude that grizzly bear and wolf apparent predation risk are important in driving Dall sheep habitat use and play a role in sexual segregation. Overall, this study presents an innovative combination of indirect methods that could be applied elsewhere to better understand predator-prey dynamics in remote ecosystems. [ABSTRACT FROM AUTHOR]

We compared the carbon isotope composition of ecosystem-respired CO2 (δ13CR) from 11 forest ecosystems in Canada and the USA and examined differences among forest δ13CR responses to seasonal variations in environmental conditions from May to October 2004. Our experimental approach was based on the assumption that variation in δ13CR is a good proxy for short-term changes in photosynthetic discrimination and associated shifts in the integrated ecosystem-level intercellular to ambient CO2 ratio (ci/ca). We compared δ13CR responses for three functional groups: deciduous, boreal and coastal forests. The δ13CR values were well predicted for each group and the highest R² values determined for the coastal, deciduous and boreal groups were 0.81, 0.80 and 0.56, respectively. Consistent with previous studies, the highest correlations between δ13CR and changes in environmental conditions were achieved when the environmental variables were averaged for 2, 3 or 4 days before δ13CR sample collection. The relationships between δ13CR and environmental conditions were consistent with leaf-level responses, and were most apparent within functional groups, providing support for our approach. However, there were differences among groups in the strength or significance, or both, of the relationships between δ13CR and some environmental factors. For example, vapor pressure deficit (VPD) and soil temperature were significant determinants of variation in δ13CR in the boreal group, whereas photosynthetic photon flux (PPF) was not; however, in the coastal group, variation in δ13CR was strongly correlated with changes in PPF, and there was no significant relationship with VPD. At a single site, comparisons between our δ13CR measurements in 2004 and published values suggested the potential application of δ13CR measurements to assess year-to-year variation in ecosystem physiological responses to changing environmental conditions, but showed that, in such an analysis, all environmental factors influencing carbon isotope discrimination during photosynthetic gas exchange must be considered. [ABSTRACT FROM AUTHOR]

Mercury biomagnifies in aquatic foodwebs in freshwater lakes, and common loons ( Gavia immer) breeding in eastern Canada can be exposed to reproductively toxic concentrations of mercury in their fish prey. We assessed the bioaccumulation and biomagnification of mercury in juvenile and adult common loons, and their preferred prey: yellow perch ( Perca flavescens) in Kejimkujik National Park (KNP), Nova Scotia by measuring mercury levels and stable isotope ratios in tissues. Total mercury levels and stable-carbon (δ13C) and nitrogen isotope ratios (δ15N) were determined in composite whole-fish samples from lakes in KNP and blood samples from juvenile and adult loons captured on lakes in KNP and southern New Brunswick. Geometric mean mercury concentrations were 0.15 and 0.38 μg/g (wet wt.) in small (9-cm fork length) and large (17-cm fork length) yellow perch, and were 0.43 and 2.7 μg/g (wet wt.) in blood of juvenile and adult common loons, respectively. Mercury concentrations in perch and loons were positively associated with body mass and δ15N values. Juvenile loons and large yellow perch had similar mercury levels and δ15N values, indicating similar trophic status despite their 22-fold difference in body mass. Mercury concentrations were higher in yellow perch and common loons in acidic lakes. Our findings highlight the importance of both chemical and ecological factors in understanding mercury biomagnification in lakes and associated risks to fish-eating wildlife. [ABSTRACT FROM AUTHOR]