oysters that were shipped from the U.S. East Coast for growing 
in the Bay. Others arrived in ballast water or burrowed into 
the wood hulls of ships arriving from ports all over the world 
(Carlton, 1979). These are hardy, opportunistic species, much 
like weed plants, that are seemingly resilient to disturbance. 
They may be temporarily eliminated from a given location in the 
Bay as a result of some natural (e.g., a storm or a prolonged 
wet or dry period) or human-induced (dredging) disturbance. How¬ 
ever, these animals typically return soon after the disturbance 
has ceased. It is against this background of high variability 
and apparent resilience that we must assess human effects. 
Effects of Waste Discharge 
The effects of waste discharge into the Bay were noted as 
early as 1900, when oyster beds were observed to be contaminated 
with human and industrial sewage. Soon thereafter, the taste of 
the harvested oysters began to deteriorate, and growth was im¬ 
paired. By the 1930s, the oyster industry had failed. Through 
the 1950s, raw or poorly treated sewage killed bottom organisms 
through lack of oxygen, and shellfish were contaminated with hu¬ 
man enteric bacteria. Beginning in the 1960s, the construction 
of facilities to treat waste began to resolve the oxygen and 
coliform bacteria problems, and by the 197 0s, these problems had 
been largely resolved (Nichols et. al. 1986). 
Now, industrial chemicals (some of which are known to be 
toxic) have become the primary concern. The tissues of mussels 
and clams contain varying levels of industrial chemicals depend¬ 
ing on their proximity to sources of contaminants, time of year 
and, apparently, the relative rate of freshwater inflow. For 
example, concentrations of the trace metal silver (a contaminant 
whose sources are largely the photographic and electronics in¬ 
dustries) in South Bay clams vary seasonally and between years 
(Luoma et. al . 1985). Highest seasonal levels are found fol¬ 
lowing the initial storms and runoff of winter while highest 
annual levels are found during driest years, These results 
suggest that river flow is important in the assimilation and/or 
flushing of contaminants from the Bay. However, the mechanisms 
are not well understood. Experimental studies have shown that, 
through genetic flexibility, some individuals within a species 
can survive in environments with high contaminant levels while 
other cannot (Luoma et. al . 1983). These studies have demon¬ 
strated that clams are physiologically stressed during periods 
when contaminant levels in the environment are highest. 
Despite clear evidence that individuals of many species 
contain contaminants, we have difficulty demonstrating a re¬ 
lationship between contaminated individuals and a threatened 
population. That is, we have not clearly demonstrated that 
there have been significant declines in either the abundance of 
the species Bay-wide or in the importance of these species in 
the Bay's food webs because of contamination with toxic 
chemicals. This difficulty results from: (1) the extreme 
66 
