Migration and Spawning Affect the Stable Isotope Values of Multiple Tissues in Pacific Salmon

AbstractMigration can be energetically demanding for animals, especially when individuals have only one chance to reproduce and rely on stored energy to complete both tasks. We investigated whether protein and fat catabolism, measured by stable isotope values, predicted successful migration and reproduction in semelparous sockeye salmon (Oncorhynchus nerka) in the Fraser River, British Columbia. We used stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) from adipose fins, blood, and scales sampled upon initial capture to assess an individual's oceanic habitat use; used passive integrated transponders to measure migration timing and success; and then collected isotope samples from the same individuals upon death to assess the level of protein and fat catabolism. We also assessed catabolism in pink salmon (Oncorhynchus gorbuscha) using stable isotope values from scales and adipose fins collected at death. We found consistent increases in δ¹³C over time across sockeye salmon tissues, showing that δ¹³C values collected from dead fish no longer represent ocean conditions. In contrast, δ¹⁵N increased only in adipose tissue of sockeye males and was particularly high in large male pink salmon, likely because of their extreme morphological changes for spawning. Migration time through lakes was related to δ¹³C, suggesting that males with lower energy reserves spent less time in lakes before spawning, and successful female sockeye spawners had higher δ¹³C values, suggesting that they catabolized more fat than unsuccessful females. Even though we were unable to link ocean habitat use to migration or reproductive success, we found several patterns of isotopic increases due to protein and lipid catabolism. These findings have implications for reinterpreting past and future studies using stable isotope values collected from migrating or dead salmon and, by extension, other animals.