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).