Variability in Trans-Pacific Migration Rates of Pacific Bluefin Tuna (Thunnus orientalis)

Abstract

Within the past several decades, Pacific bluefin tuna (Thunnus orientalis; PBT) stocks have dropped to historically low levels, currently estimated at just 3.3% of unfished levels. All PBT are spawned in the Western Pacific Ocean (WPO) and an unknown proportion of juveniles (Ages 1 and 2) make trans-Pacific migrations across the Northern Pacific Ocean (NPO). This study was conducted to investigate potential environmental drivers of PBT trans-Pacific migration and support management of the stock through a better understanding of when and where they interact with fisheries. Two primary questions were explored: (1) what are the environmental correlates that drive the residency of Age-1 and Age-2 PBT in the WPO; and (2) what environmental correlates increase the residency duration in the Eastern Pacific Ocean (EPO)? We used long term (1952-2014) oceanographic and catch data, which allowed us to examine the influence of decadal scale environmental regime shifts. Generalized Additive Models (GAMs) were used to quantify the strength in relationships between modeled PBT catch-at-age (CAA) data, and focal explanatory variables. The effects of sea surface temperature (SST), sea surface height anomalies (SSHa), the Pacific Decadal Oscillation (PDO), the Arctic Oscillation (AO), upwelling, iron input from Asian dust, and prey availability on the proportion of PBT making eastward and westward trans-Pacific migrations were examined. Residency of Age-1 PBT in the WPO was positively correlated with anchovy and sardine landings in the WPO, as well as SSHa, while Age-2 PBT had significant relationships with sardine landings, SST, aeolian dust, and the PDO. In the EPO residency of both Age-4 and Age-5 PBT were significantly correlated with increased sardine and lower mackerel landings. These findings are aligned with previous hypotheses that prey abundance is a key influencing factor on PBT residency in the WPO.