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Fishery Bulletin 107(2) 
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o 
0 . 5 - 
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I 
I III I I III III 1111 
I I I I I I I I I I I I I 
minmincocococDco 
cococooococDcncncncn 
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t— CM CM CM 
Year 
Figure 6 
The fraction of total deaths in the population of eastern oyster ( Crassostrea 
virginica ) in the Delaware Bay contributed by the individuals on the high-mor- 
tality beds. The horizontal bar represents the 54-yr median level of 0.482. 
U.S. coastline. Baines and Folland (2007) documented 
the climatic forcing that certainly provided the basis for 
the 1970 regime shift, although how climate change in 
the North Atlantic imposed the conditions for increased 
productivity on the local scale remains uncertain. 
Two noteworthy events preceded population expan- 
sion in Delaware Bay. First, 1968-70 were three suc- 
cessive years of relatively high recruitment (Fig. 3). 
Only one other trio of such years, 1997-99, exists in 
the time series. Relatively high recruitment in these 
three years occurred in three of four bay subgroups 
(medium-mortality, Shell Rock, and high-mortality). 
No equivalent coincidence of years and bay coverage 
exists in the time series. Second, beginning in 1967, 
natural mortality dropped below 10% after the largest 
MSX epizootic event of the 1960s and remained at or 
below this level through 1975 (Fig. 4). The coincidence 
of dramatically lower natural mortality and a triplex of 
high recruitment years was unique in the time series 
and certainly provided the proximate conditions for the 
population expansion of 1970. 
The 1970-85 high-abundance interval 
and its termination 
The 1970-85 time period was remarkable for its per- 
sistent high level of oyster abundance (Fig. 2). The 
period was characterized by a lower contribution of 
animals on the high-mortality beds to total population 
mortality (Fig. 6) and by natural mortalities that rarely 
exceeded 13% of the stock annually (Fig. 4). During this 
period, the fraction of deaths on the high-mortality beds 
exceeded the long-term median only six times (Fig. 6). 
In the first half of the period, the medium-mortality 
beds contributed proportionately more to the stock, as 
they had during most of the MSX-dominated decade that 
preceded this period (Fig. 5). High freshwater inflow 
contributed to sustainable high abundance by limiting 
mortality from MSX. A dramatic shift in stock disper- 
sion began in 1979, coincident with the cessation of 
consistently high freshwater inflows, and led, over a 
few years, to proportional increases in abundance in the 
more environmentally sensitive waters of the upbay and 
downbay margins. An increase in the susceptibility of 
the population to epizootic disease mortality consequent 
of the increased abundance downbay evolved from 1985 
to 1986 through a coincidence of climatic events into 
the largest epizootic event in the recorded history of 
Delaware Bay (Fig. 4). Interestingly, the 1985-86 stock 
collapse was not obviously associated with any unusual 
trends in recruitment immediately before or after the 
collapse (Fig. 3), nor did the distribution of deaths (Fig. 
6) or the dispersion of the stock (Fig. 5) change. Abun- 
dance declined in all bay regions. 
The post-MSX period 
The few years immediately following the 1985-86 MSX 
epizootic event and preceding the onset of Dermo circa 
1990 were not unusual in any way, and neither was 
the first half-decade after Dermo became an impor- 
tant contributor to population mortality. Total abun- 
