Page 59 - World Aquaculture Magazine - March 2011

World Aquaculture 57
solved and particulate cadmium, suggesting the removal
and assimilation of dissolved metal by phytoplankton.
Laboratory experiments determined that the assimilation
efficiency of cadmium from diatoms was on average 44
percent during summer and increased to 58 percent dur-
ing fall (Ng et al. in press³). Increases in cadmium from
the ingestion of phytoplankton may be mitigated by the
effects of tissue dilution during summer, but not likely
during fall. In addition, dissolved cadmium uptake con-
stants and pumping rates were lower in oysters harvested
during summer than fall (Ng et al. in press) helping to
further explain the lower tissue concentrations observed
at most sites.
During fall, levels of dissolved cadmium were elevat-
ed as cadmium was likely distributed to the surface epi-
sodically by storm generated mixing, stormwater runoff
and/or riverine inputs. Concentrations of phytoplankton
and particulate cadmium also increased, suggesting that
oysters were subjected to sources of both dissolved and
particulate cadmium at that time of year. Despite the in-
crease in food supply, tissue weights declined because of
spawning and/or depletion of glycogen stores. Without
the benefit of tissue dilution, oyster cadmium concentra-
tions increased at eight out of 10 sites.
Oyster ploidy and culture method
No difference in cadmium concentration was detected
between diploid and triploid oysters in field experiments.
Similarly, no difference in cadmium concentration was ob-
served between oysters grown in off-bottom aqua-purses
versus bags on-bottom at half of our test locations (Figures
12 and 13). Oysters placed in HC, WA, however, had notably
elevated concentrations when grown in aqua-purses, reach-
ing 3.5 µg/g in only 117 days.
Oyster weights and lengths were significantly greater
in oysters cultured in aqua-purses than bags on-bottom.
Additionally, a negative relationship was observed be-
tween oyster weight and cadmium concentration, so any
increase in cadmium associated with the aqua-purses was
likely mitigated by tissue dilution. Growth rates were
similarly slow in oysters grown in SPS and HC despite
their vastly different tissue cadmium concentrations. This
disparity may, in part, be attributed to the physical topog-
raphy of HC.
HC is a Puget Sound fjord, separated by the main body
of Puget Sound by two sills (50 and 75 m) located at the
north entrance of the Canal. The physical topography
of HC supports sluggish circulation and a strong sum-
mer stratification gradient (Paulson et al. 1993), which
results in surface waters that are particularly warm and
fresh. ⁴ The mean July surface temperature for central HC
is 18.9°C (WDOE 2005) making the canal one of the few
places in Washington that is warm enough for Pacific oys-
ters to spawn consistently. Elevated temperature, particu-
larly 20°C, has been shown to increase cadmium uptake
in many marine organisms by raising metabolic activities
such as pumping and feeding rates (Jackim et al. 1977,
Denton and Burdon-Jones 1981).
Oysters caged in aqua-purses, and possibly other forms
of suspended culture, are held within this lens of warm,
fresh surface water for a longer duration of time. In this
manner, oysters may be exposed to conditions that in-
crease metal uptake rates and/or alter the chemical spe-
ciation of the metal to a more bioavailable form. Increas-
es in cadmium concentration may be significant in areas
with slow growth rates and undetectable in areas with fast
growth rates. However, even if growth rates are initially
slow, as observed in SPS, adequate circulation within a
basin may still support conditions that foster oysters with
lower cadmium concentrations.
Species selection
Cadmium concentrations from various shellfish types
(clams and mussels) and oyster species are shown in Figure
14. Results indicate that average cadmium concentrations
in manila clams, butter clams, geoduck clams, mussels and
Olympia oysters were consistently below 1 µg/g.
Recommendations
Our sampling indicates that cadmium enrichment in U.S.
West Coast Pacific oysters is limited to specific regions and
that the economic risk to HC oyster growers from import re-
strictions appears to be minimal at this time. The following
recommendations will help prevent future rejections, espe-
cially in light of the anticipated growth in international ex-
ports, and ensure that cadmium residues in oyster products
are minimized whenever possible.
• Test oysters for cadmium prior to harvesting and dis-
tributing to international locations.
• When possible, increase diversification by growing dif-
ferent shellfish types (clams, mussels, oysters) and es-
tablishing beds in various locations.
• Consider exporting to destinations that currently
do not enforce cadmium limits, such as Taipei and
Southeast Asia, or to the United States which is sub-
ject to the U.S. Food and Drug Administration’s 3.7
µg/g recommended guideline (USFDA 1993).
• Harvest oysters in the summer when tissue and dis-
solved cadmium concentrations are low and oyster
weights are high.
• When possible, select a site with fast shellfish growth
rates. Also select a location with adequate circulation
to discourage extreme stratification gradients that
support unusually warm and fresh surface waters and
phytoplankton blooms of greater magnitude and lon-
ger duration.
• Be cautious when growing oysters in aqua-purses or
other methods of suspended culture. While the tech-
nique may increase growth rates at some locations, it
may also expose oysters to conditions that increase ex-
posure to and uptake of cadmium.
Notes
¹Pacific Shellfish Institute, 120 State Avenue NE #142, Olympia,
WA 98501, USA. Corresponding author E-mail: aimee@pac-
shell.org.
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