Otolith geochemistry of tropical and temperate tunas in the ocean

Geographic trends in otolith δ18O and δ13C provide insights into relevant spatial scale(s) of population discrimination, and the utility of these geochemical markers for elucidating the movements, stock structure and niche breadth in fishes. Otolith δ18O and δ13C of juvenile (age-0/1) tunas (genus Thunnus) were quantified to characterize regional and global variation across the Atlantic, Pacific and Indian Oceans. Tunas from the Atlantic Ocean commonly displayed higher otolith δ18O relative to individuals from comparable zones (tropical or temperate) in the Pacific Ocean, and to a lesser degree, the Indian Ocean. Otolith δ18O of tunas collected at higher latitudes was elevated and more variable, with temperate tunas (albacore, Atlantic and Pacific bluefin) enriched in otolith 18O relative to congeners in tropical waters (bigeye, blackfin and yellowfin). Inter- and intra-ocean trends in otolith δ13C were less pronounced, but tunas collected from several regions in the Atlantic Ocean were generally enriched in otolith 13C relative to individuals from the Pacific and Indian Oceans. Niche breadth derived from otolith δ18O and δ13C was often wider for tunas collected in temperate zones where individuals experience more variable environmental conditions compared with congeners in tropical waters.