Scientific Journal Article Summary

Horizontal and vertical movements of juvenile bluefin tuna (Thunnus orientalis) in relation to seasons and oceanographic conditions in the eastern Pacific Ocean
Takashi Kitagawa, Andre M. Boustany, Charles J. Farwell, Thomas D. Williams, Michael R. Castleton and Barbara A. Block
Fisheries Oceanography (2007) 16(5):409–421

One hundred sixty 2-3 year old, juvenile Pacific bluefin (Thunnus orientalis) were released off Baja California with archival tags in the summer of 2002. Data from 18 fish that were recaptured were analyzed to assess migrations, diving behavior, and thermal regulation relative to the environment.

A cyclical migration pattern was observed for bluefin in the eastern Pacific (Figure 1).  In summer, fish were located principally in the Southern California Bight and off Baja California .  In autumn, bluefin migrated north to the waters off the Central California coast. In mid-winter, the bluefin returned to the Southern California Bight.  Movements appeared to be related to prey abundance – with bluefin presence correlated with that of Pacific sardines, and water temperature – with the northward migration coinciding with warming in the area.  Feeding events were estimated by a decline in the tag-recorded internal temperature of the tuna (due to ingestion of cool seawater and prey) and were most frequent during June through August.

Figure 1. Estimated track (colored circles with line) of an archival tagged Pacific bluefin tuna, showing the typical annual migration.

The tagged bluefin primarily occupied surface depths (< 165 ft) along the continental shelf, undertaking occasional, brief forays to depths below the thermocline (i.e., the depth where warmer surface waters rapidly transition to cooler, deep waters).  For all of the Pacific bluefin tuna, average diving depth was significantly greater during the day that at night.  Lunar cycle also influenced diving depth, as mean nighttime depth increased with increased fullness of the moon (Figure 2).  Because bluefin feeding behavior decreased at night, the lunar effect on diving was hypothesized to help the fish avoid predation.

Figure 2. Lunar illumination (dotted curve) and daily average nighttime depth of a Pacific bluefin tuna.

Like other bluefin tunas, Pacific bluefin have the ability to maintain their body temperature above that of the surrounding water.  Remarkably, the greatest difference of body and water temperature was seen in the cool waters off the Central California coast, with bluefin maintaining their body temperatures approximately 50ºF above that of the seawater. 

The results of this study will be valuable when exploring fisheries management options for Pacific bluefin tuna.  There are limited regulations in place for the species, and bluefin have a high commercial value.  As a result, the species may become overfished in the near future.

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