Trophic position increases with thermocline depth in yellowfin and bigeye tuna across the Western and Central Pacific Ocean

•A unique data-driven baseline δ15N isoscape was provided.•Large spatial variations in baseline and tuna bulk δ15N values were observed.•Higher trophic positions for tuna, particularly bigeye, in the southern latitudes.•Higher trophic positions were observed as the depth of the 20°C isotherm increased.•Vertical habitat use was shown as key to our understanding of foraging ecology.