Combining otolith microstructure and trace elemental analyses to infer the arrival of juvenile Pacific bluefin tuna in the California current ecosystem

Juvenile Pacific bluefin tuna (PBT, Thunnus orientalis) are known to migrate from western Pacific spawning grounds to their eastern Pacific nursery and feeding grounds in the California Current Large Marine Ecosystem (CCLME), but the timing, durations, and fraction of the population that makes these migrations need to be better understood for improved management. To complement recent work focused on stable isotope and radiotracer approaches ("tracer toolbox"; Madigan et al., 2014) we explored the suitability of combining longitudinal analyses of otolith microstructure and trace elemental composition in age similar to 1-2 PBT (n = 24, 66-76 cm curved fork length) for inferring the arrival of individuals in the CCLME. Element:Ca ratios in transverse otolith sections (9-12 rows, triplicate ablations from primordium to edge, empty set50 mu m) were quantified for eight elements: Li, Mg, Mn, Co, Cu, Zn, Sr, and Ba, which was followed by microstructure analysis to provide age estimates corresponding to each ablation spot. Age estimates from otoliths ranged from 328 to 498 d post-hatch. The combined elemental signatures of four elements (Ba, Mg, Co, Cu) showed a significant increase at the otolith edge in approximately half of the individuals (30-60 d before catch). Given the different oceanographic properties of oligotrophic open Pacific vs. high nutrient, upwelling CCLME waters, this signal is consistent with the entry of the fish into the CCLME, which was estimated to occur primarily in July after a transoceanic migration of similar to 1.5-2.0 months. Our approach comprises a useful addition to the available tracer toolbox and can provide additional and complementary understanding of trans-Pacific migration patterns in PBT.