Stable nitrogen isotope analysis of amino acids
The δ 15NAAs measurements were performed only for some copepods and chum salmon samples to minimize experimental effort. We selected representative 3–6 samples from six copepod species for δ 15NAAs measurements so as to cover theδ 15NBulk range of each species. Collagen samples from the bone sections of two individuals of O. keta (IDs: OK2 and OK8) were used forδ 15NAAs measurements. Samples for δ 15NAAs were prepared based on amino acid derivatization procedures, and theδ 15NAAs values were determined by gas chromatography/combustion/isotope-ratio mass spectrometry (See Text S3).