Amino acid <scp> δ ¹⁵ N </scp> in eye lens laminae reveals life‐time ontogenetic trophic shifts of a highly migratory species

Investigating the feeding ecology through the ontogenesis of highly migratory species such as the Pacific Bluefin tuna (PBFT; Thunnus orientalis) is difficult due to its extensive home range and cross-oceanic migration. Here, we show the potential of conducting nitrogen stable isotope (δ¹⁵N) analyses in bulk tissue and amino acids (AAs) in consecutive eye lens laminae of PBFT to reconstruct the trophic life history for an individual tuna. The δ¹⁵N_bulk profiles between individuals caught in the wild and pen-raised were compared. For all individuals, δ¹⁵N_bulk values increased with increasing eye lens diameter or fork length, and exhibited low variation among individual profiles despite tuna being captured in different months. Large δ¹⁵N_bulk shifts (6.8‰-8.5‰) were quantified between the first and last deposited laminae for each individual, suggesting major ontogenetic changes in either foraging areas or trophic position. AA δ¹⁵N values indicate that this highly migratory schooling predator switches feeding areas from lower to higher δ¹⁵N baseline values, reflecting feeding on both sides of the north Pacific, and tends to feed on prey of higher trophic position as it grows. Together, stable isotope analysis in bulk tissue and individual AAs in eye lens laminae could be a powerful approach to investigate changes in the foraging habitat and trophic status of highly migratory species.