Bacterial contamination . Since shellfish (clams) filter huge volumes 
of water, tests of these animals can be used to imply water quality 
conditions. Intertidal shellfish beds are monitored for bacterial 
contamination. As a result of this monitoring, many parts of the Sound 
are closed to collection and marketing of clams for commercial sale. 
However, these areas are not posted as off-limits for recreational clam 
diggers. This topic is explained more thoroughly in a paper by John 
Armstrong in this volume. 
Microlayer toxicity . Many flatfish such as English sole disperse their 
eggs into the water to maximize their distribution. The eggs are very 
buoyant and float to the water's surface where they remain for up to two 
weeks. Many other species of fish and invertebrates also occur in the 
upper water column and/or at the surface during part of their development 
in the Sound. The air-sea boundary at the surface is a film called the 
microlayer. It is usually about one-half millimeter thick. Toxicants may 
concentrate there at concentrations several hundreds to two thousand-fold 
over those of the underlying water. 
Laboratory bioassays of microlayer samples from parts of Elliott and 
Commencement Bays (near Seattle and Tacoma, respectively) showed that 
some samples were very toxic (lethal) to flatfish eggs. Whereas 74 to 96 
percent of the eggs exposed to microlayer samples from a reference area 
(Sequim Bay) and the Central Basin hatched to live larvae, as few as 
none hatched following exposure to samples from parts of Elliott and 
Commencement Bays. Toxicity appears to be highest in samples taken from 
visible surface slicks contaminated with aromatic hydrocarbons and certain 
trace metals. 
Sediment bioassays. 
The five most frequently used sediment bioassays are listed in Table 2. 
Others involving use of fish larvae, sea urchin larvae, clam larvae, 
polychaetes, copepods, and shrimp have also been used. 
Oyster larvae. Recently the oyster larvae bioassay has been adapted for 
use in bioassays of sediments. As with the bioassays of water, the tests 
are performed in a laboratory with samples from the Sound. However, in 
this case the samples are of the surface sediment. In the oyster larvae 
bioassays the tests are run with the elutriates of sediment samples. 
Among the areas tested thus far, samples from parts of Bellingham Bay, 
inner Everett Harbor, the lower Duwamish Waterway, inner Sinclair Inlet, 
and the waterways of Commencement Bay have shown significant toxicity. 
Those from Samish Bay, Port Gardner, outer Elliott Bay, Port Madison and 
off the west shore of Whidbey Island were much less toxic or were not 
toxic. 
Amphipods. Small shrimp-like animals, called amphipods, have been used in 
tests of the toxicity of over 600 sediment samples from the Sound. These 
tests are performed by exposing 20 animals to bulk sediments for 10 days. 
Figure 3 illustrates the average number of survivors (out of 20) for many 
33 
