INSIGHTS | PERSPECTIVES
28 JULY 2017 • VOL 357 ISSUE 6349 357 SCIENCE sciencemag.org
dent and migrant tuna in the western Pacific.
Given that any harvested bluefin tuna can be
analyzed to obtain movement data at relatively low cost, the method can be applied
to large samples over time, helping to obtain
population-wide estimates of trans-Pacific
High bluefin residency in the eastern Pacific has important management implications. For an internationally fished species, it
is critical to understand the relative influence
of regional fishing fleets. The current stock
assessment is not a spatially explicit model
(1), and incorporating migration patterns
into stock assessments can be highly complex (7). However, the potentially deleterious
effects of ignoring spatial distribution of fish
populations in stock assessment models has
been demonstrated, with spatially explicit
models outperforming simpler ones (13).
For Pacific bluefin, basic implications of
extensive residency in the eastern Pacific can
also be inferred from catch trends. Bluefin
that are 1 year old or younger dominate catch
in number, and this high fishing mortality
among the youngest fish leads to high estimates (70 to 80%) of the impact of western
fisheries on future spawning stock biomass
(1). Because Pacific bluefin make the eastward migration late in their first or early in
their second year of life, only western fisheries have access to these youngest fish (1, 9). If
most fish then migrate to the east, conditions
there will predominantly dictate mortality
rates of juvenile bluefin older than 2 years.
The importance of the eastern Pacific
Ocean for Pacific bluefin tuna has manage-
ment implications across the Pacific. Popula-
tion declines have been attributed largely to
western fisheries and markets, and western
fisheries contribute most to current mortal-
ity (1). However, the assumption that only
a minority of Pacific bluefin travel to the
east implicitly minimized the importance of
eastern fisheries in Pacific bluefin tuna con-
servation. Western Pacific fisheries clearly
must do more to ensure that more bluefin
tuna survive the first 2 years of life (1), but
doing so without at least maintaining con-
servation efforts in the east will only serve to
delay mortality by another year or two. New
movement data may drive managers to reas-
sess the influence of eastern fisheries on the
number of maturing Pacific bluefin that can
survive several years, return west, and add to
the biomass of spawning fish.
Different mortality rates in the east and
west would influence chemical tracer–based
conclusions for the Pacific bluefin population. For example, if mortality rates for 2- to
7-year-old tuna are higher in the western
than the eastern Pacific, then the proportion
of returning eastern Pacific migrants would
swamp out the signal from the few western
Pacific residents that survived to these age
classes. In such a scenario, few fish that remained in the western Pacific would survive
into these older age classes; eastern migrants
would thus be overrepresented in chemical
tracer studies. However, fishing mortality estimates of these ages in the west are not sufficiently high to implicate unbalanced fishing
pressure, rather than migration dynamics, as
the driver of observed trends.
In addition, an assessment of previously
published catch data shows that extensive
eastward migration by young Pacific bluefin tuna may be historically common. From
1968 to 1984, eastern Pacific fisheries caught
no tuna younger than 1 year, and fewer age-1
tuna than in the west, but they consistently
exceeded the west in catch of age-2 tuna
(see fig. S1 in the supplementary materials)
(9). These findings suggest that most of the
tuna population arrives in the east sometime
in their second year of life. Thus, the eastern
fisheries only have substantial access to a few
juvenile age classes, whereas western ones
have access to all life stages from juveniles to
giants. Despite this distributional advantage
in the west, the east’s total catch in weight
has exceeded the west’s in some years (9).
Recent conservation measures for bluefin
instituted by the Inter-American Tropical
Tuna Commission (IATTC) in the eastern Pacific have been relatively well enforced, and
catches have been reduced over the past decade (14). However, these reductions have not
been matched by the Western and Central
Pacific Fishery Commission in the western
Pacific, resulting in western nations exceeding their agreed catch limits. This geographic
imbalance in conservation measures, coupled
with the belief that only a small proportion of
bluefin migrate to the east, has contributed
to calls by fishers in the east to forego conservation efforts and has led to the current
assertion from NOAA that consumers should
purchase U.S.-caught Pacific bluefin tuna as
a sustainable “good choice” (15). These regulatory incongruities and the movement patterns described above highlight the need for
both sides to enact more comprehensive, science-based management measures to rebuild
the stock of Pacific bluefin tuna. j
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Scientific Committee for Tuna and Tuna-like Species in the
North Pacific Ocean, La Jolla, CA, 2016).
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Scientific Committee for Tuna and Tuna-like Species in the
North Pacific Ocean, Shimizu, Shizuoka, Japan, 2012).
7. www.westcoast.fisheries.noaa.gov/fisheries/migra-tory_species/ pbt_esa_status_review.html
8. J. J. Magnuson, C. Safina, M. P. Sissenwine, Science 293,
9. W. H. Bayliff, FAOFish. Tech.Pap. 336, 244 (1994).
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14. IAT TC, Fishery Status Report No. 14 (Inter-American
Tropical Tuna Commission, La Jolla, CA, 2016).
15. NOAA Fisheries, “Pacific bluefin tuna: Limits on
fishery make U.S.-caught fish a good choice” (NOAA,
Published online 13 July 2017
Pacifc bluefn tuna
Return to the west Eastward migration
Between the ages of 3 and 7, many tuna return
to the west; by age 7 or older, most individuals are
recent migrants from the eastern Pacifc (3).
Many juveniles swim east in their frst
or second year and reside in the eastern
Pacifc for several years (4, 10).
A school of Pacific bluefin tuna in Baja California,
Mexico is shown. More young bluefin migrate east
than previously thought.
Bluefin tuna movements across the Pacific
By measuring the isotopic composition of muscle tissue, it is possible to distinguish recent migrants from
long-term residents in the western or eastern Pacific. The data suggest that most juvenile bluefin migrate to
the east and must eventually return to the west to spawn.