Connectivity in juvenile yellowfin tuna Thunnus albacares between temperate and tropical regions of the western Pacific Ocean

Yellowfin tuna Thunnus albacares is a commercially important fish species widely distributed between 40 degrees S and 40 degrees N. Mixing rates between tropical and temperate areas of the western Pacific Ocean were investigated using stable carbon (delta 13Cotolith) and oxygen isotopes (delta 18Ootolith) in whole otoliths of small juveniles (mean: 5.8 cm standard length, SL) and in otolith cores of large juveniles (32.4 cm SL). The density distribution of delta 18Ootolith values for small juveniles showed a significant difference between the areas, whereas patterns for large juveniles were almost identical between areas. In addition, modal values for large juveniles spanned several small juvenile modes. The density distribution of delta 13Cotolith values for small juveniles had a single significantly different mode for each area, whereas the distribution of delta 13Cotolith values for large juveniles in these areas was bimodal, particularly in the temperate area; these modes corresponded to those for small juveniles in each area. These modal patterns indicate that the majority of large juveniles captured in the temperate area have tropical origins, and mixing from tropical to temperate areas occurs between the areas before recruitment to fisheries in each area. These connectivity patterns during early life history can contribute to refinement of stock assessments of this commercially valuable species.