|Abstract or Summary
- The effects of exposure to suspended silt, kaolin and fuller's
earth on growth and condition of Pacific oysters, 16 months of age,
were studied. The exposure apparatus maintained a continuous flow
of turbid water through chambers containing test oysters. Additional
studies measured the filtration rates of oysters exposed to suspended
sediments, and microscopic examinations of sediment transport on
excised gill tissue were made.
Growth was assessed as total wet weight increase over two-week
exposures to sediments. Normal wet weight increase under
field conditions averaged 2.30 percent and was similar to that found
under laboratory conditions. Laboratory growth, under conditions
of flowing, unfiltered seawater, was comparable to growth when test
animals were fed laboratory cultured algae in filtered seawater. Silt particles, 104 to 149 μ and 74 to 104 μ in size, did not
inhibit growth. Silt particles 38 to 74 μ in diameter appeared to
reduce growth at 0.65 and 1.06 g/liter, with the intensity of effect
increasing with an increase in silt concentration. No statistically
significant growth reduction occurred after exposure to silt particles
<38 μ in size, although visual inspection suggested that some reduction
may have occurred at 1.61 g/liter.
Kaolin particles 0.2 μ , 1.5 μ and 9.5 μ in size significantly
reduced growth of oysters in the range of 0.36 to 1. 81 g/liter, with
the level of growth inhibition increasing with kaolin concentration.
Many animals appeared to lose weight as a result of exposure to
kaolin sediments. Similarly, growth was reduced as the concentration
of 5 μ fuller's earth increased from 0. 20 to 1.37 g/liter.
None of the sediments tested had a marked effect on condition
index, except in tests employing silt particles 74 to 104 μ in size.
Although a significant reduction in condition was observed in this
test, it was concluded that this may have been an anomalous result.
Filtration rates of oysters exposed to all particle sizes of kaolin
and fuller's earth appeared to decrease in response to increase in
sediment concentration. Normal filtration rates averaged approximately
1.60 1/hour, and decreased to below 0.40 1/hour at sediment
levels of about 0.40 g/liter. A 50% reduction in filtration rate
occurred at about 0.02 g/liter of 0. 2 μ kaolin and at 0.2 g/liter of 9.5 μ kaolin. The concentration of 50% effect with fuller's earth
was intermediate to that of 0. 2 μ and 9.5 μ kaolin, and the same as
that for 1.5 μ kaolin.
Since apparent clogging of the gills in in vitro tests occurred
at concentrations of sediments similar to those affecting growth and
filtration, overloading of the gills was considered as a possible
sensory mechanism regulating the response of oysters to suspended
The results are discussed in relation to Northwest water quality
problems and water resource management.