Powell et al.: Multiple stable reference points in oyster populations 
143 
tion goals and methods have received consider- 
able attention (e.g., Breitburg et al., 2000; Mann, 
2000 ). 
Restoration goals are dramatically impacted by 
the location of type-II reference points in relation 
to stock abundance. Type II is the goal under 
MSY management objectives, and by the presence 
of type IV and the differential between types II 
and III. The difference between type II and type 
III affects 1) the ease of transition from one sta- 
ble point to another and 2) the impact on fishery 
yield during the transition. As the differential 
increases, from the example in the surplus pro- 
duction trajectory of Figure 6 to that in Figure 7 
for instance, the limitation on fishery yield during 
the transition must increase. The obvious incon- 
gruity will be an observed increase in abundance 
of marketable stock during times of decreased al- 
location necessitated by the transitory limitation 
on surplus production coincident with the type-III 
reference point. This apparent inequity will likely 
exacerbate the natural adversarial relationship 
that exists between regulator and industry. The 
frequently complex relationship between economics 
and biology in fisheries management is well known 
(Lipton and Strand, 1992; Mackinson et al., 1997; 
Imeson et al., 2002). Thus 
several questions come to 
the fore. Can rebuilding to 
N 1 ^ be accomplished? This 
depends on the existence of 
type IV. Does one try to re- 
build to N^ sy ? This depends 
on the willingness of the 
fishery and management 
to forgo catch yields during 
times of increasingly high 
abundance, possibly for an 
extended period, so that 
the population shifts to the 
higher regime. 
Regime shifts of long- 
term stability almost cer- 
tainly come with a type- 
IV reference point. In this 
case, even the closure of 
the fishery will not gener- 
ate enough surplus pro- 
duction to rebuild past the 
type-III low. Recognizing 
the existence of such a bar- 
rier is critical. Presumably, 
a massive recruitment en- 
hancement program could 
be implemented to artifi- 
cially affect a regime shift. 
Patience may be the better 
alternative, using the N msy 
value of the present regime 
as the management goal 
o 
o 
=1 
"O 
o 
Q . 
w 
Q. 
3 
c n 
Population size 
Figure 8 
The relationship of the primary trends in population abun- 
dance and surplus production associated with the bimodal 
surplus production trajectory depicted in Figure 7 in which 
the minimum in surplus production is negative. When surplus 
production is positive, the population abundance increases. 
The opposite trend occurs when surplus production is negative. 
The type-I reference point, the carrying capacity, is a conver- 
gence. Trends in surplus production and population abundance 
converge at this point. The type-IV reference point, the point 
of no return, is a divergence. Trends in surplus production 
and population abundance diverge at this point. 
Figure 9 
Time series of oyster abundance, by bay region, with the abundance levels associated 
with types I-IV reference points identified. Regime shifts occurred in 1970 and 1985 
(Powell et al., 2009). The 1959 peak is a survey artifact. Total oyster abundance is 
the cumulative value. Bed groups are defined in Table 1. Bed locations are shown in 
Figure 1. Reference point legend and symbols are given in order as displayed on the 
graph, from top to bottom. 
