Page 53 - World Aquaculture Magazine - March 2011
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W
orld
A
quaculture
51
Cadmium in Pacific oysters (
Crassostrea
gigas
): A survey of the United States west
coast and mitigation strategies
A
imee
E. C
hristy
,¹ D
aniel
P. C
heney
¹
and
I
an
S
tupakoff
²
U.S. Pacific Coast shellfish farm-
ers produce over 45,000 t of mollus-
can shellfish annually. It is no sur-
prise, therefore, that growers became
concerned when, in 2003, the Hong
Kong Food and Environmental Hy-
giene Department (HKFEHD) re-
jected shipments of Pacific oysters
from Hood Canal, Washington USA
for exceeding the 2 µg/g limit for cad-
mium in molluscan shellfish (HKFE-
HD 2007). The refusal came on the
heels of similar rejections by the HK-
FEHD of British Columbia oysters
in 1999 and 2000, which prompted
workshops and research efforts along
Vancouver Island and the Strait of
Georgia that continue today (Schallie
2001, Kruzynski
et al.
2002).
Research Objectives
With limited data, Pacific oys-
ter farmers were uncertain as to
the extent of the problem in both
geographic and economic terms.
Methods, if any, to minimize cad-
mium residues in oysters were also
unknown. In response, work was
completed to determine the spatial
distribution of cadmium in Pacific
oysters harvested from growing
areas located throughout Puget
Sound and several other west
coast estuaries. Based on these
results, specific locations were se-
lected for seasonal sampling of
oyster tissue, seawater, sediments
and shellfish growth rates to bet-
ter understand the mechanisms
behind the geographic distribu-
tion (Figure 1). Laboratory up-
Fig. 1. Map of seasonal sampling locations.
Fig. 2. Field-cleaning and processing of oysters
prior to transport to the analytical laboratory (Photo
by Ian Stupakoff).
take and depuration experiments, as
well as field experiments designed to
minimize cadmium residues in shell-
fish, were also performed. Finally, an
economic assessment was completed
to evaluate potential losses associ-
ated with this issue.
Study Methods
Geographic distribution of cadmium
in Pacific oysters
In September 2004, Pacific oysters
were sampled at 39 commercial, rec-
reational and tribal shellfish growing
areas throughout California, Oregon,
Washington and Alaska. Three com-
posites of 20 oysters each were har-
vested per location along transects
positioned along the zero tidal eleva-
tion (Figure 2). Intact oysters (10-15
cm) were delivered to an accredited
laboratory for composite weight (tis-
sue + nectar) and tissue cadmium
analysis (µg/g wet weight, includ-
ing nectar).
Seasonal variation in tissue, water
and sediment
Based on initial sampling re-
sults, seasonal collection of oys-
ter tissue, seawater and sediment
(Figure 3) was performed at 10
locations [Windy Bay, AK; south
Puget Sound, WA (SPS); Hood
Canal, WA (HC); north Hood
Canal, WA (NHC); north Puget
Sound, WA (NPS); coastal estuar-
ies, WA (CE); Tillamook Bay, OR
(TIL); Netarts Bay, OR (NET);
Humboldt Bay, CA (HUM); and
