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Fishery Bulletin 107(2) 
Figure 2 
Time series of abundance of the eastern oyster ( Crassostrea virginica) in 
Delaware Bay, showing four subgroups defined by location and natural mor- 
tality rate. Total oyster abundance for any year is the sum of abundance in 
the subgroups. Beds in the subgroups are listed in Table 1. 
Figure 3 
Time series of spat recruitment per >20-mm eastern oyster ( Crassostrea vir- 
ginica) in Delaware Bay. Solid and dashed lines mark the 1.0 and 0.5 spat- 
to-oyster levels, respectively. 
Shell Rock, leading to its designa- 
tion as a medium-mortality bed; 
the remainder are high-mortality 
beds (Table 1). 
Powell et al. (2008) have de- 
scribed the Delaware Bay time 
series in detail. The pertinent 
findings are summarized in the 
following sections. 
Pre-1970 period of low abundance 
In the few years before 1957 when 
survey data were available, the 
Delaware Bay oyster population 
was characterized by relatively 
low abundance (Fig. 2), an unre- 
markable rate of recruitment (Fig. 
3), relatively low natural mortality 
(Fig. 4), and a spatial distribution 
in which the fraction of the stock on 
the medium-mortality beds was rel- 
atively low in comparison with the 
54-yr median of 0.417 (Fig. 5). The 
dispersion of the stock was likely 
maintained by overfishing because 
the fishery predominantly targeted 
the medium-mortality beds during 
this time (Powell et al., 2008). 
Given that natural mortality rates 
averaged below 10% during this 
period, and fishing rates routinely 
exceeded 10%, we speculate that, 
had fishing rates been the same as 
those in later years (typically <7% 
of the stock), the medium-mortality 
beds likely would have contributed 
a larger proportion of the stock, and 
stock abundance likely would have 
been higher than that observed. 
MSX entered the picture circa 
1957. Abundance was unchanged, 
in part because of implementation 
of reference point-based manage- 
ment that curtailed overfishing 
(Fegley et al., 2003; Powell et al., 
2008). The early reference point 
referred to as “the 40% rule” lim- 
ited removals from individual beds 
when the volume of live oysters de- 
clined below 40% of a bushel haul 
(Powell et al., 2008). The 40% rule 
successfully limited harvest from 
the late 1950s until the 1985 re- 
gime shift, after which changes in 
the fishery imposed by low abun- 
dance and Dermo required develop- 
ment of management alternatives 
and new reference points (Powell 
et al., 2008). Under the 40% rule, 
