Page 56 - World Aquaculture Magazine - March 2011

54 March 2011
HC, 60 percent of the interviewed growers expressed interest
in entering or expanding into the international market, par-
ticularly in Asia, within the next five years. Oyster exports
out of Seattle have grown considerably from 1995 to present
because of an expanding Asian market and rapid economic
growth in China (NOAA 2008, Figure 9). A majority of ex-
ports to Asia from Seattle are shipped to Taipei followed by
Hong Kong. While Hong Kong commands a premium price
in the Asian market, it routinely enforces a 2 µg/g cadmium
import restriction. China maintains a stricter limit of 1 µg/g,
but does not appear to enforce the regulation at this time
(MAPRC 2001).
If no preventive actions are taken to minimize rejec-
tions, a loss of $67,000 over the next 5-10 years can be
expected for HC growers from lost export revenues when
international ports reject oysters for excessive cadmium
concentration. Beyond that time, an ongoing annual loss
of $9,000 to $75,000 is estimated from the redirection of
oysters to the domestic market where the price for large
oysters is lower.
For growers interested in entering the international mar-
ket, awareness of cadmium enrichment and prevention will
greatly increase export opportunities. Testing oysters for
cadmium prior to shipping internationally is one way to
avoid losses associated with rejected product.
Fig. 10. Left, oyster weights and right, corresponding tissue cadmium concentrations after 169 days at five
Washington locations.
Fig. 11. Relationship between adult oyster weight and cadmium concentration at sites with faster growth rates
(SPS, NPS) at left and slower growth rates (NHC, HC) at right.
Methods to Minimize Cadmium in Shellfish
Site selection
Selecting sites with fast shellfish growth rates favors lower
oyster cadmium levels inasmuch as newly acquired tissue di-
lutes the final concentration. A significant negative relation-
ship was detected between oyster weight and tissue cadmium
concentration in composites pooled from two experiments
including the growth rate experiment depicted in Figure 10.
Similarly, oyster weight and tissue cadmium concentra-
tion was negatively correlated in adult oyster composites
sampled across our entire study especially in regions with
the fastest growth rates versus slower growth rates. Approxi-
mately 40 percent of the variation in cadmium concentration
was explained by oyster mass alone at locations exhibiting
faster growth rates. Conversely, oysters appeared to experi-
ence slight bioaccumulation, as opposed to biodilution, in
locations with slower growth rates although the relationship
was not significant (Figure 11).
While rapid growth rate is an important determinant of
cadmium concentration, it is not the only factor when con-
sidering site selection. Our research indicates that a compli-
cated interplay of physical, chemical and biological factors
influence both the geographic and seasonal variation in
cadmium. Our findings suggest that oyster concentration is