Page 57 - World Aquaculture Magazine - March 2011

World Aquaculture 55
often lowest at locations with adequate circulation and tidal
exchange. Well-mixed growing areas encourage fast growth
rates and also discourage high pressure stratification gra-
dients that subject oysters to fresh, warm surface water for
extended periods of time. Well circulated water also tends
to prevent large phytoplankton blooms that, in some cases,
persist into late fall and early winter, supplying oysters with
cadmium at a time when tissue weights are at their lowest.
The finger inlets in SPS, WA are typically well-mixed
with freshwater inputs contributing little to both the tem-
poral and vertical distribution in salinity (Brooks 2006). In
one prominent shellfish growing inlet, the mean difference
in salinity between surface and 10 meters ranged from 0.4
to 3.9 percent throughout the year with a mean January sur-
face salinity of 26.7 ppt (WDOE 2005, 10-year average). In
contrast, HC, WA is stratified for a large portion of the year
with a surface-to-depth salinity difference of 5.9 to 19.7 per-
cent, a mean January salinity of 22.7 ppt and extreme low
of 17.0 ppt.
Decreased salinity and elevated temperature and are both
known to increase cadmium uptake rates in many types of
marine bivalves, including oysters (Roesijadi 1996, Luoma
1983). A decrease in salinity from 30 to 20 ppt was shown to
increase cadmium uptake in four species of marine bivalves
by as much as 24 to 400 percent (Jackim et al. 1977) More
recently, Ke and Wang (2001) reported a 2.4-fold increase
in cadmium uptake with decreasing salinity from 30 to 15
ppt in the coastal oyster (Saccostrea glomerata). A change
in metal speciation because of decreasing salinity was deter-
mined to be a dominant factor in cadmium uptake for that
particular oyster.
Harvest time
Cadmium concentrations were significantly lower when
oysters were harvested during summer at the California, Or-
egon and Washington locations similar to findings reported
by and Boyden and Phillips (1981), Thomson (1982) and
Bendell and Feng (in press). At the California sites, spring
and summer concentrations were, on average, 148 percent
lower than fall and winter. At the Oregon and Washington
locations, summer concentrations were significantly lower
than spring and fall by 17 percent and 27.8 percent respec-
tively. With the exception of Alaska, no oyster sample ex-
ceeded 2 µg/g at any location during the July sampling pe-
riod.
Seasonal changes in tissue cadmium concentration were
partially a result of fluctuations in oyster mass. Oyster
weights were elevated during the summer resulting in dilu-
tion of the cadmium concentration. In the California sites,
tissue weights were 61 percent higher in the summer than
winter. In Oregon and Washington, summer weights were
14.5 percent higher than fall, but not significantly different
than winter.
Dissolved cadmium concentrations in seawater were also
low during summer. Our field sampling indicated that dis-
solved cadmium was undetectable (<0.00002 mg/L) in sur-
face water at almost all locations. Similar to Lekhi et al.
(2008), a negative relationship was observed between dis-
Fig. 12. Experimental plot at Hood Canal, Washington de-
signed to test the impact of culture method and oyster ploidy
on cadmium concentration (Photo by Aimee Christy)
Fig. 13. Cadmium concentrations in diploid and triploid oyster
seed cultured in bags on-bottom (BOB) and elevated aqua-
purses (AP) at four Washington locations.
Fig. 14. Cadmium concentrations in various types of shellfish
harvested at four Washington locations. (Continued on page 57)