By Daniel J. Madigan,1 Andre Boustany,2,3
Bruce B. Collette4
High market values have incentiv- ized extensive fisheries for Pacific bluefin tuna (Thunnus orientalis, see the photo), a species whose lon- gevity, commercial value, and long eneration time make it particularly
susceptible to overfishing (1, 2). Today, the
population of Pacific bluefin tuna stands at
an estimated 2.6% of prefished levels (1). Yet,
knowledge of its basic life history is incomplete. Recent studies (3, 4) suggest that the
fundamental distribution of Pacific bluefin
across the North Pacific has been misunderstood. The results underscore the need for
basic movement ecology information to assist science-based fisheries management.
Decades ago, stock assessments identified overfishing of Atlantic bluefin tuna
(T. thynnus) and Southern bluefin tuna (T.
maccoyii), and stricter management measures were applied to these species. By contrast, Pacific bluefin tuna was still rated as
“least concern” by the International Union
for Conservation of Nature (IUCN) in 2011
(2), due in part to the absence of a stock as-
sessment and precipitous depletion that be-
gan before the three most recent generations
of Pacific bluefin, which is the threshold time
frame used in IUCN assessments. Optimistic
valuations of population stability predomi-
nated, despite qualitative inferences in 2010
from fisheries observations that a population
crash was imminent or occurring (5).
In 2012, the International Scientific Committee for Tuna and Tuna-like species in the
North Pacific Ocean (ISC) stock assessment
estimated Pacific bluefin spawning stock biomass to be ~4% of unfished levels (6). In its
most recent assessment, the ISC estimates
that Pacific bluefin are at 2.6% of prefished
levels, that the stock is heavily overfished,
and that overfishing is still occurring (1).
These estimates have driven calls for a fishing moratorium by conservation groups, a
downgrade in IUCN status to “vulnerable,”
and an ongoing status review at the National
Oceanic and Atmospheric Administration
(NOAA) to list Pacific bluefin tuna as threatened or endangered under the U.S. Endangered Species Act (ESA) (7).
Long-distance migrations make manage-
ment of all three species of bluefin logistically
and politically difficult (8). Adult Pacific blue-
fin spawn only in the western Pacific Ocean.
The proportion of juveniles that migrate to
the eastern Pacific Ocean and are fished by
the United States and Mexico is unknown (1),
but has traditionally been considered a mi-
nority (9). The current NOAA notice of ESA
status review describes the group migrating
to the eastern Pacific as “variable but small”
(7), and the petition for ESA listing refers to
this group as a minority (7).
However, recent results from the Sea of
Japan (3) suggest that in some years, the majority of young Pacific bluefin makes the journey to the eastern Pacific Ocean. In the study,
Tawa et al. used stable-isotope differences
between the eastern and western Pacific to
differentiate long-term residents from recent
migrants. As the authors show, a substantial
proportion of younger bluefin (3 to 5 years
old) were migrants from the east, and this
proportion increased by age 6; by age 7 or
older, most individuals were recent migrants
from the eastern Pacific (see the figure).
Other recent findings show that juveniles
swim east in their first or second year and
reside in the eastern Pacific for several years
(4, 10). Together, these studies suggest that
most juvenile tuna must survive several years
in the eastern Pacific before returning west to
spawn; these migrants from the east contribute substantially to the spawning stock in the
Sea of Japan (see the figure).
Tawa et al. present clear evidence of a majority of eastern Pacific migrants in western
Pacific bluefin tuna. However, trans-Pacific
migrations are known to vary from year to
year and possibly from decade to decade (11).
As Tawa et al. acknowledge, the sample sizes
in their study (7 to 58 tuna per age class) were
too small to make population-wide estimates
or to assess variability over time. Classification would be more robust with additional
chemical tracers (12). Nevertheless, the study
shows that isotope tracers can discern resi-
East not least for Pacific bluefin tuna
More Pacific bluefin migrate across the ocean than previously recognized
1 Harvard University Center for the Environment, Harvard
University, Cambridge, MA 02138, USA. 2Nereus Program,
Monterey Bay Aquarium, 886 Cannery Row, Monterey, CA
93940, USA. 3Marine Geospatial Ecology Lab, Nicholas School
of the Environment, Duke University, Durham, NC 27708, USA.
4IUCN Tuna & Billfish Specialist Group, National Museum of
Natural History, Washington, DC 20560, USA.