Graduate Thesis Or Dissertation
 

The effects of concentration and particle size of suspended materials on growth and condition of the Pacific oyster (Crassostrea gigas)

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  • 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 sediments. The results are discussed in relation to Northwest water quality problems and water resource management.
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