The optimal salinity zone for oyster yields in an estuary
is bounded by low and high salinities, with an intermediate
zone of high yields in between (Fig. 1). Freshening an estuary
will reduce oyster yields if the oyster reefs are already
on the freshwater side of the curve (line C). If the oyster
reefs are initially on the high salinity side of the curve,
freshening an estuary will (1) increase oyster yields if
the endpoint is also on the high salinity side (line A)
and (2) either increase or decrease oyster yields if the
endpoint is on the freshwater side of the curve, depending
on the location of the initial and final points
(line B). Complicating matters is the fact that the influence
of salinity on oyster establishment, growth, and commercial
yield are not necessarily constant throughout the life cycle.
Accurate prediction of the relationship between salinity
and oyster production for an entire estuary depends on the
shape of the curve in Fig. 1 and the location of the oyster
harvest on the curve.
A recent analysis (Turner, R. E. 2006. Will lowering estuarine
salinity increase Gulf of Mexico oyster landings? Estuaries
and Coasts 29(3): in press) addresses some of the contradictory
conclusions about the effects of varying salinity on oyster
yields in the Gulf of Mexico, an area whose 2003 production
was 69% and 54% of the u.s. oyster landings by weight and
dockside value, respectively. The approach used was to collect
harvest data for an entire state or estuary and to establish
the existence, if any, of positive or negative relationships
with river discharge, which was considered a surrogate measure
of estuarine salinity. Five representative estuarine systems
were chosen for analysis: (1) Apalachicola Bay, Florida,
(2) Mobile Bay, Alabama, (3) the western portion of Mississippi
Sound, Mississippi (4) Galveston Bay, Texas, and (5) several
Louisiana estuaries. Data on oyster landings and product
value were taken from the annual reports of the U.S. National
Marine Fisheries Service. Data on stream discharge were
taken from the U.S.G.S. and U.S. Army Corps of Engineers,
New Orleans District Office.
A general pattern that emerged from the analysis is that
annual oyster landings of Gulf of Mexico estuaries are negatively
correlated with freshwater inflow. In other words, the position
of most Gulf of Mexico estuaries is represented by line
C in Fig. 1. Mississippi data are shown in Fig. 2 as an
example. Twenty-one of twenty-three peaks in landings from
Florida, Alabama, Mississippi, and Texas were coincidental
with lows in river discharge. Seventeen of nineteen lows
in landings were coincidental with peaks in river discharge.
The landings in Mississippi were lower than in the other
GOM states after the Bonnet Carre was opened, in twenty-five
of twenty-eight examples. Furthermore, when the Bonnet Carre
Spillway was opened, which of course happens when Mississippi
River discharge is relatively high, landings decreased in
Louisiana for the next several years. The rationale for
diverting freshwater into Mississippi Sound cannot be supported
by an anticipated higher yield of oysters by either the
Mississippi or Louisiana industries
Figure 1. Possible effects of freshwater inflow on average
oyster yields for the whole estuary. For oyster reefs on
the high salinity side of the optimization curve, freshening
the estuary may result in higher yields (line A) or lower
yields (line B), depending on how low the salinity falls
when freshwater flow into the estuary increases. Potential
yields will be reduced with any amount of freshening for
oyster reefs located on the freshwater side of the optimum
curve (line C). From Turner, Estuaries and Coasts, in press.
Figure 2. An example result for Mississippi. Top panel:
The three year running average of oyster landings in Mississippi
and the discharge of the Pearl River at Monticello, Mississippi.
Both discharge and landings are normalized to the average
value for 1950 to 2003 (1 = mean). The shaded area is the
below average values for river discharge. The squares are
the years in which the Bonnet Carre was opened. Bottom panel:
the relationship between discharge and landings for 1990
to 2003 (from the data shown in the top panel). A linear
regression of the data is shown (y = -2.14 X+ 3.18; R2 =
0.77).
The "p" and "t" indicates peaks and
troughs, respectively, in landings. From Turner, Estuaries
and Coasts, in press.
Suggested additional reading about this article:
Will
Lowering Estuarine Salinity Increase Gulf of Mexico Oyster
Landings
