Integrating isoscapes and amino acid δ15N analyses to reveal migration patterns and habitat use of molids in the western Pacific Ocean

Understanding the movement ecology and resource use of pelagic fishes is essential for their conservation and management. Molids, as predators of gelatinous zooplankton, play important ecological roles in the gelatinous food webs and undertake long-distance migrations across the Pacific Ocean. This study used a novel combination of isotope-based approaches, including isoscapes, compound-specific isotope analysis of amino acids, and Bayesian mixing models, to investigate the migration patterns and habitat uses of molids (Mola mola and M. alexandrini) at a population level in the western Pacific Ocean. Results from isoscapes revealed that most molids in Japan, Taiwan, and New Caledonia might be local residents, with measured δ¹⁵N values aligning with predicted isoscape values. However, some larger individuals in Japan and Taiwan showed δ¹⁵N values higher than predicted, suggesting recent migrations from isotopically distinct regions. In New Zealand, the measured δ¹⁵N values of molids did not overlap with predicted values, indicating non-residency. Source amino acid δ¹⁵N values and Bayesian mixing models suggest that local residents in Japan and Taiwan primarily consumed local prey, whereas recent migrants with higher δ¹⁵N values may originate from the warm pool region, potentially linked to spawning behavior. Molids in New Caledonia and New Zealand likely migrate from regions with isotopic values distinct from the local food web base. The cross-regional movements of molids highlight the importance of understanding connectivity within the western Pacific Ocean. Identifying migratory corridors and key habitats, such as spawning and feeding grounds, is essential for developing regional and international conservation strategies that ensure the long-term sustainability of molid populations.