Powell et al. : Multiple stable reference points in oyster populations: Crassostrea virgimca in Delaware Bay 
113 
the highest fishing mortality rate 
observed after 1958 was about 
10% of the stock (Powell et al., 
2008). By circa 1960, the effect 
of an increase in natural mortal- 
ity, on the order of 5-10% of the 
stock, had been ameliorated by 
a decrease in fishing mortality 
at least that large. From 1957 
through 1966, natural mortality 
neared 15% of the stock in most 
years and exceeded 20% in two 
years (Fig. 4). Mortality substan- 
tively increased downbay and by 
1960, animals on the high-mor- 
tality beds were contributing a 
disproportionate share of the to- 
tal mortality of the population 
(Fig. 6). As a consequence, during 
the 1960s, individuals on the me- 
dium-mortality beds contributed 
more than their long-term me- 
dian proportion of the total stock 
in eight of ten years (Fig. 5). Al- 
though the fishery continued to 
target these beds (Powell et al., 
2008), the reduction in total re- 
movals minimized the influence 
of the fishery on the stock. 
The 1970 population expansion 
In 1970, the oyster population 
increased by more than a factor 
of two, and this high level of 
abundance was maintained for 
the succeeding 15 years. This 
was a period of high abundance 
in a number of other species of 
commercial importance (Gabriel, 
1992; Link et al., 2002), includ- 
ing many finfish species in the 
Gulf of Maine and Mid-Atlan- 
tic Bight, hard clams along the 
Long Island coast (Kraeuter et 
al., 2005; Hofmann et al., 2006), 
and Illex squid off Newfoundland 
(Dawe et al., 2000). In many of 
these cases, this abundance was 
rapidly impacted by overfishing 
(e.g., Kraeuter et al., 2008), which 
artificially limited its duration. 
A decline in population, however, 
did not occur for the Delaware 
Bay oyster stock. However, the 
general coincidence of abundance 
in bay and shelf species, both 
temperate and boreal, bespeaks 
of a large-scale climatic event that 
influenced much of the northeast 
Figure 4 
The fraction of eastern oyster ( Crassostrea virginica ) dying each year in the 
New Jersey waters of Delaware Bay, 1953-2006.. Horizontal line marks an 
arbitrary boundary between mortality in epizootic (above the line) and non- 
epizootic years. 
Year 
Figure 5 
The fraction of the total stock of eastern oyster ( Crassostrea virginica) in the 
New Jersey waters of Delaware Bay that was located on the medium-mortality 
beds, 1953-2006. The horizontal bar represents the 54-yr median of 0.417. 
