Zooplankton and chum salmon sample collection
Zooplankton samples were obtained from the Odate Collection (Odate 1994) (n = 294) at the Tohoku National Fisheries Institute, and additional samples were collected by the Hokkaido University training ship (Oshoro Maru ; n = 49) in 2002 and 2012. They were stored in formalin until analysis. We also obtained frozen (n = 17) zooplankton samples from the Hokkaido National Fisheries Institute and a 2018 cruise conducted by the Oshoro Maru . From each sample bottle, we selected one of the following six copepod species, which are dominant in the NP: Neocalanus cristatus , N. plumchrus ,N. flemingeri , Oncaea venusta , Paracalanus parvus,and P. aculeatus (Table S1). Copepod samples were rinsed with pure water twice and freeze-dried. Then, we removed the gut of large copepods (N. cristatus , N. plumchrus , and N. flemingeri ) and placed the copepods in tin capsules for isotope analysis. Because it is technically difficult to excise the gut contents of small copepods (Oncaea venusta , Palacalanus parvus , andP. aculeatus ), we used whole body samples of these species for isotope analysis. These six species have relatively long lifespans ranging from 1 month to 2 years (Paffenhöfer 1993; Tsuda et al . 2001), which enables us to obtain time-averaged isotopic information. We also collected δ 15Nbulk data for these species from existing literature (Pomerleau et al . 2014).
Bone samples were obtained from three individuals of chum salmon (IDs: OK1–3) collected at the Nukkibetsu River, Hokkaido, Japan (42.59°N, 140.70°E), in December 2016. We could not identify the age, sex, or origin (wild or hatchery-reared) of these samples because we were not provided with any non-bone tissues. We also obtained three individuals of chum salmon from the Chitose River (42.80°N, 141.56°E, IDs: OK4–6), Hokkaido and from the Hokkaido National Fisheries Institute, as well as two individuals from the Otsuchi River (39.37°N, 141.90°E, IDs: OK7–8), Iwate, Japan. All salmon from the Chitose River were identified as hatchery salmon based on their otolith labels, whereas the origin of salmon from the Otsuchi River was unclear. Age and sex were identified for samples of both origins (Table S2). We extracted vertebral centra from each salmon, subdivided them into 10 sections and extracted bone collagen from the sections (see Text S1).