Scientific Journal Article Summary
Movements of pacific bluefin tuna (Thunnus orientalis) in the Eastern North Pacific revealed with archival tags
Andre M. Boustany, Robyn Matteson, Michael Castleton, Charles Farwell and Barbara A. Block
Progress in Oceanography (2010) 86:94-104
Two hundred fifty-three juvenile Pacific bluefin (Thunnus orientalis) were released in the California Current off the coast of California and Baja California with archival tags between 2002 and 2005. Data from 143 fish that were recaptured were analyzed to explore seasonal and inter-annual migratory patterns and the influence of the environment on movements.
Bluefin tuna showed repeatable seasonal movements along the west coast of North America. Only 7 fish migrated to the western Pacific. The vast majority (98.7%) of daily positions of the tuna were within 345 miles of the coast, documenting residency in the California Current throughout the year. Bluefin tuna were found farthest south in the spring when they were located off southern Baja California, Mexico and farthest north in the fall when fish were found predominately off central and northern California (Figure 1). North-south movement patterns were correlated with peaks in primary productivity. The productivity in the California Current results from upwelling, which brings nutrient-rich water to the surface, feeding phytoplankton and subsequently providing abundant food resources for animals at higher trophic levels. Two of the bluefin's main prey species, northern anchovy and sardines, are tightly affiliated with seasonally productive areas.
Figure 1. Density plots of Pacific bluefin tuna seasonal distributions in the California Current for: (a) winter, (b) spring, (c) summer, and (d) fall. Warmer colors indicate higher densities.
Figure 2. North-south movements of tagged bluefin in relation to coastal primary productivity.
When in areas of high productivity, fish were tightly clustered, whereas they were more dispersed in regions of low productivity. This is likely explained by increased prey density in areas of high productivity, which would in turn support higher densities of bluefin. In the spring through fall, bluefin tuna were located in areas with the highest levels of primary productivity available in the California Current ecosystem. However, in the winter months, tagged bluefin tuna were found in areas with lower productivity compared to other regions along the coast at that time of year suggesting that during the winter, bluefin tuna are feeding on aggregations of pelagic red crabs, sardines and anchovies that preferentially spawn in areas of reduced coastal upwelling.
This study revealed that although the total range of bluefin tuna in the Pacific Ocean is expansive, primary habitat utilization may be much more restricted. High recapture rates of Pacific bluefin tuna in this study (62%) indicate that the limited range of bluefin in the eastern Pacific can potentially expose these fish to greater fishing pressure.