δ 13 C and δ 15 N isotope biplot (a) showing Japan Sea (circles), Pacific Ocean (triangles) and Pacific freshwater populations (squares; note, individuals from Fushikobetsu not shown to aid interpretation) with putative dietary source values (mean ‰ ± SD); boxplots (b and c) representing differences in δ 13 C and δ 15 N values between allopatric and sympatric JA and PA populations.