Seasonal variation in the lipid content of Fraser River Chinook Salmon (Oncorhynchus tshawytscha) and its implications for Southern Resident Killer Whale (Orcinus orca) prey quality.

In Southern British Columbia (BC), Canada, declines in southern resident killer whale (SRKW-Orcinus orca) populations have been linked to declines in numbers and average size of their preferred prey, Chinook salmon (Oncorhynchus tshawytscha). However, the life history diversity of Chinook suggests that there is a need to assess stock-specific differences in energy density to evaluate prey quality as a factor in SRKW declines. In this study, we calibrated a Distell fat meter to estimate Chinook whole-body lipid content, a proxy for energy density. The fat meter was deployed at the Fraser River, BC, Chinook test fishery during 2020, collecting lipid, weight, and length measurements from 1566 genetically stock identified individuals encompassing all major Fraser River Chinook population units (management units, MUs) at river entry. We found that MU-specific lipid content increased with distance and elevation to spawning grounds and was highest in the Spring-52 (12.8%) and Summer-52 (12.7%) MUs, intermediate in the Summer-41 MU (10.8%), and lowest in the Fall-41 MU (7.3%). Lipid content also decreased by up to 6 percentage points within MUs from the beginning to end of their migration period. Our data revealed SRKWs' most endangered prey sources, the Spring-52 and Summer-52 MUs, are also its most energy rich. It also indicated SRKWs have access to progressively lower energy density Chinook through the year, requiring up to ~ 30% more fish to meet energy demands in the fall than in the spring.

Experimentally derived trophic enrichment and discrimination factors for Chinook salmon, Oncorhynchus tshawytscha.

RATIONALE: Stable isotope analysis (SIA) can provide important insights into food web structure and is a widely used tool in ecological conservation and management. It has recently been augmented by compound-specific stable isotope analysis of amino acids (CSIA-AA), an innovation that can provide greater precision when analyzing trophic level and food web connectivity. The utility of SIA rests on confidence in its constituent parameters such as the trophic enrichment factor (TEF). There is increasing emphasis on the need to experimentally derive species and tissue specific TEFs for studies utilizing SIA. Chinook salmon, Oncorhynchus tshawytscha, is a species with high potential for study using SIA due to the difficulty in observing its ecology during its marine phase and the significance of the conservation consequences of recent population declines. METHODS: Bulk and amino acid-specific TEFs were determined for juvenile and adult Chinook salmon fed specific diets. Three controlled feeding studies were performed: adult salmon were fed a biofeed, juvenile salmon were fed a biofeed, and juvenile salmon were fed krill. Bulk and compound-specific stable isotope data were collected from diet samples and from salmon muscle tissue after a minimum of 8 weeks of controlled feeding. Bulk isotope signatures were measured using EA-IRMS and CSIA-AA signatures using GC/C-IRMS, allowing the TEFs to be calculated. RESULTS: The bulk isotope TEFs were higher than those predicted for similar marine organisms and averaged 3.5‰ for ∆15 N and 1.3‰ for ∆13 C. The TEFs derived for nitrogen isotopes of amino acids were in line with expectations for this approach: the mean value for ∆15 NGlu - ∆15 NPhe was 7.06‰ and, using a multi-AA approach, the value for ∆15 NTrophic - ∆15 NSource was 6.67‰. For carbon isotopes of amino acids, the derived TEFs of Iso, Leu and Phe were near 0‰, as was that of Met, supporting their use of as source amino acids in future CSIA studies. CONCLUSIONS: This study presents Chinook salmon-specific TEFs for bulk and amino acid SIA. It supports the application of future research applying SIA to the study of Chinook salmon and validates previous research on species-specific TEFs.

Evaluating the use of stable isotope analysis to infer the feeding ecology of a growing US gray seal (Halichoerus grypus) population.

Gray seals (Halichoerus grypus) have been rapidly recolonizing the Northeast US coast, eliciting concern from the fishing industry. However, the ecological effect of this recovery is still unknown and as such, research is needed to better understand how the diet composition of gray seals in US waters will contribute to the ecological impact. While previous research on seal diets has focused on the analysis of hard prey remains, stable isotope analysis presents an alternative method that can be used to describe marine mammal diets when direct observation is impossible. To address this issue, we used stable isotope analysis of gray seal pup vibrissae and lanugo from Monomoy Island, Cape Cod, MA during the 2015/2016 winter breeding season to estimate adult female diet composition during pregnancy. Stable isotope mixing models (SIMM) suggested adult female gray seals were consuming greater amounts of cephalopod prey and less sand lance than previously indicated from analysis of hard prey remains. However, using SIMMs to estimate the diet composition of gray seals remains difficult due to the large number of isotopically similar prey species and uncertainty in tissue-specific, stable isotope trophic enrichment factors. Even so, by combining prey sources into ecologically informative groups and integrating prior information into SIMMs it is possible to obtain additional insights into the diet of this generalist predator.