Stable Isotope Analyses of Yellowfin and Blackfin Tuna Eye Lenses Reveal Life History Patterns

Spatial geography, ontogenetic movement, and trophic patterns of mobile species are key elements of species life history, and knowledge of them is critical for effective marine resource management. For yellowfin tuna ( Thunnus albacares ; YFT) and blackfin tuna ( Thunnus atlanticus ; BLFT), residency durations and ontogenetic movements in the Gulf of Mexico (Gulf) are uncertain. To evaluate changes in trophic position and movement of YFT ( n = 46) and BLFT ( n = 31) caught in the northern Gulf, we used nitrogen (δ 15 N) and carbon (δ 13 C) stable isotope analysis across eye lenses. Lifetime increases in isotopic values, significant correlations between each isotopic value and laminar midpoint (proxy for length; LM), and a significant correlation between δ 15 N and δ 13 C values would indicate fewer movements across isotopic gradients and the potential for long‐term residency. In this study, all individuals increased in δ 15 N from core to outermost lamina (YFT mean = 4.47‰, BLFT mean = 5.06‰), and 95% of individuals had significant, positive correlations between δ 15 N and LM, consistent with increasing trophic position and/or migration from lower δ 15 N areas to the northern Gulf. Intraspecific smoothed curves of δ 13 C by size exhibited early dips in δ 13 C values and subsequent increases around estimated age 1, revealing possible ontogenetic or seasonal movement by both species. This study was limited in giving a definitive interpretation of movement, but some individual profiles (YFT n = 15, BLFT n = 2) displayed significant, positive correlations between δ 13 C and δ 15 N, signaling a trophic position increase with potential long‐term residencies.