Page 54 - World Aquaculture Magazine - March 2011

52 March 2011
Tomales Bay, CA (TOM)]. Laboratory analyses were per-
formed for tissue cadmium concentration, total and dis-
solved cadmium in seawater, particulate cadmium, plankton
concentration and speciation, sediment cadmium concen-
tration and total organic carbon.
Growth rates and laboratory experiments
To better understand the effect of growth rate on tissue
concentration, oysters of similar age, size and family were
placed in on-bottom cages and distributed to five Washing-
ton locations from April to October 2005 (Figure 4). Addi-
tional cages were placed in Washington and Oregon in April
2005 and shipped to the Hong Kong University of Science
& Technology (HKUST) in July 2005 to test for cadmium
clearance rates, assimilation efficiencies for various food
types and dissolved metal uptake rates. A second group of
oysters was collected from California and Washington in
October 2005, and sent to HKUST for additional cadmium
uptake and depuration experiments (Figures 5 and 6).
Additional field experiments
To test the impact of culture method and oyster ploidy
on tissue cadmium concentration, diploid and triploid oys-
ters (3 to 6 mm) were placed at four Washington locations
in adjacent bags on-bottom and elevated aqua-purses from
April to August 2006 (Figure 7). After 117 days the oysters
were weighed, measured and sent to a laboratory for tissue
cadmium analysis. In April, different types of clams, mussels
and oysters were sampled at five Washington locations to
evaluate how species selection impacts cadmium concentra-
tion.
Economic evaluation
Northern Economics, Inc. conducted an economic as-
sessment for Hood Canal, WA to evaluate the financial im-
pacts associated with a 2 µg/g import restriction on oysters.
A small, yet representative sample of 10 shellfish growers
ranging in production from small (11 thousand oysters/yr)
to large (5 million oysters/yr) were interviewed to assess
Fig. 3. Collection of sediment sample at a typical shellfish farm
in Hood Canal, Washington (Photo by Ian Stupakoff).
Fig. 4. On-bottom cage containing oyster seed for the growth
rate experiment (Photo by Ian Stupakoff).
Fig. 5. Laboratory
experiments to test
dissolved cadmium
uptake, cadmium
clearance and assimi-
lation efficiencies for
different food types
and diatom concen-
trations (Photo by Ian
Stupakoff).
Fig. 6. Testing assimilation and depuration rates by feeding
radiolabeled diatoms to oysters and placing them in enclosed
recirculating tanks to depurate.