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over a range of dates after the Fukushima discharge (demon-
strating the utility of
134
Cs:
137
Cs ratios as a tool to assess the
timing of PBFT migrations).
The total load of radiocesium transported to the CCLME by
PBFT can be estimated from catches in the EPO. Catch data
varies yearly (10), but assuming PBFT commercial catches in
2011 were within the range of catch from 2000 to 2010, trans-
ported and harvested radiocesium in tuna muscle tissue in 2011
could range from ∼3–17 × 10
6
Bq (Table S4)or<<1% of total
radiocesium released into Japanese waters (4). Catch data rep-
resent a portion of the PBFT in the EPO, so total transport of
radiocesium by PBFT would likely be higher. Still, this is a small
quantity of radiocesium to be introduced to a large pelagic
ecosystem, but it is also a conservative estimate based on one
species. Other highly migratory species (HMS) (e.g., turtles,
sharks, and seabirds) that forage near Japan may assimilate
radiocesium and transport it to distant regions of the north and
south Pacific (Fig. 1B). Tissue concentrations of radiocesium in
these species would depend on time spent near Japan, foraging
strategies, and timing of migration. The potential for species in
Fig. 1B and other HMS (e.g., pinnipeds, whales, and billfish) that
forage in Japan to transport Fukushima-derived radiocesium is
speculative. However, the presence of Fukushima-derived radi-
ocesium in all 2011 PBFT individuals reported here suggests that
study of other HMS is warranted. Our results demonstrate that
Fukushima-derived radionuclides in animal tissues can serve as
tracers of both migration origin (presence or absence of
134
Cs)
and, potentially, timing (using
134
Cs:
137
Cs ratios) in mobile ma-
rine animals, providing valuable complementary movement data
to extensive tagging programs in the Pacific (8). Extensive data
regarding spatiotemporal variations in Cs concentrations in the
west Pacific, and consequent uptake by biota, are forthcoming,
which will sharpen the precision of these tracers. The Fukushima
disaster, thus, provides an opportunity to examine both the ex-
tent of transport of anthropogenic radionuclides by highly mi-
gratory species and an unexpected tool for examining migratory
origins of apex predators in the Pacific Ocean.
Methods
Tuna tissue samples were collected from recreational anglers in San Diego,
CA. Muscle samples were freeze-dried and ground with mortar and pestle
and analyzed using a low-energy germanium detector. We detected the γ
emissions of
137
Cs and
134
Cs,
40
K, and other naturally occurring radionuclides.
134
Cs and
137
Cs concentrations of post-Fukushima PBFT and YFT samples
were decay-corrected to the angler-estimated catch date for all fish. We
calculated
134
Cs and
137
Cs concentrations in 2011 PBFT for various times
before capture in California (0, 30, 60, 90, 120 d). We accounted for back-
ground
137
Cs in tuna muscle (1.0 Bq kg
−1
) by subtracting 1.0 from total
137
Cs
values. We then accounted for the radioactive decay of Cs isotopes, impor-
tant only for the shorter-lived
134
Cs (t
1/2
= 2.1 y) using an exponential decay
model. To account for the metabolic efflux of assimilat ed cesium out of fish,
we used an experimentally derived marine fish efflux rate constant k of
0.019 d
−1
(14). To address growth dilution of the Cs concentrations in muscle,
we calculated change in fish body mass (0–120 d before catch) and calcu-
lated dilution of Cs concentrations over this range of days due to growth.
We estimatedradiocesiumtransport to the CCLME by PBFTusingcatchdataas
a proxy for PBFT biomass in the CCLME. Catch data from 2000 to 2010 were
convertedtomusclebiomassandthenmultipliedbymeanmeasuredradiocesium
concentrations to generate a range of estimates of transported radiocesium.
Spearman’s ρ analyses (two-tailed, nonparametric; α = 0.05) were performed to
assess correlation between 2011 PBFT length and radionuclide concentrations.
Equations and further details of the methods are included in SI Methods.
ACKNOWLEDGMENTS. We thank S. Zegers for labo ratory assistance;
A. Carlisle for technical assistance; and K. Buesseler, M. Condrup, H. Dewar,
F. Micheli, and two anonymous reviewers for helpful comments. We also
thank the anonymous fishermen who donated fish samples toward this
study. This work was funded by the Gordon and Betty Moore Foundation.
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www.pnas.org/cgi/doi/10.1073/pnas.1204859109 Madigan et al.
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