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Recovery of Oyster Reefs (Crassostrea Virginica) in a Gulf Estuary Following Disturbance by Two Hurricanes

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During the summer and fall of 1985, two hurricanes struck the Apalachicola Bay system, a center for oyster (Crassostrea virginica) production in the northeast Gulf of Mexico. The first storm, Hurricane Elena, physically destroyed the major oyster-producing reefs in the Apalachicola estuary in early September (9/1/85). This disturbance was followed a month later by considerable accumulations of spat on those reefs most affected by the storm. The second hurricane, Kate, struck the bay in late November (11/21/85) and probably contributed to the natural mortality of young-of-the-year oysters. However, overall oyster biomass did not seem to be affected by Kate. Subsequent oyster growth was substantial with full recovery of the oyster stock noted within a 12-mo period. A detailed evaluation was made of the response of this important estuarine population to these disturbances. The timing and nature of the disturbances relative to the natural history of the oyster were crucial to the overall recovery pattern of the population. Hurricane Elena occurred at the end of the oyster spawning activity in 1985. Effects of the storm probably increased habitat availability and reduced direct competition and predation such that the oyster population benefited from the successful recruitment. The subsequent storm, Kate, coming after the spawning period, was not as destructive to oyster populations as Elena and could have even enhanced growth of the survivors. Hurricanes are common along the Gulf coast during the spawning period of the oysters; it appears that C. virginica is well adapted for such natural disturbances. The observed response of the Apalachicola oyster population to successive disturbances has significant meaning in terms of the long-term ecological stability of estuarine populations and the evolutionary aspects of such biological response to temporally unstable habitats. In this case, such populations can be viewed as highly resilient under even the most extreme conditions of physical instability. However, the exact biological response to temporally irregular disturbances is highly dependent on the timing of such events relative to the natural history of population in question.
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BULLETIN OF MARINE SCIENCE. 64(3): 465-483. 1999
RECOVERY OF OYSTER REEFS
(CRASSOSTREA VIRGINICA)
IN A
GULF ESTUARY FOLLOWING DISTURBANCE BY
TWO HURRICANES
Robert
J.
Livingston, Robert L. Howell, IV, Xufeng Niu,
F. Graham Lewis, III and Glenn
C.
Woodsum
ABSTRACT
During the summer and fall of 1985, two hurricanes struck the Apalachicola Bay system,
a center for oyster
(Crassostrea virginica)
production in the northeast Gulf of Mexico. The
first storm, Hurricane Elena, physically destroyed the major oyster-producing reefs in the
Apalachicola estuary in early September (9/1/85). This disturbance was followed a month
later by considerable accumulations of spat on those reefs most affected by the storm. The
second hurricane, Kate, struck the bay in late November (11/21/85) and probably contributed
to the natural mortality of young-of-the-year oysters. However, overall oyster biomass did
not seem to be affected by Kate. Subsequent oyster growth was substantial with full recovery
of the oyster stock noted within a 12-mo period. A detailed evaluation was made of the
response of this important estuarine population to these disturbances. The timing and nature
of the disturbances relative to the natural history of the oyster were crucial to the overall
recovery pattern of the population. Hurricane Elena occurred at the end of the oyster spawn-
ing activity in 1985. Effects of the storm probably increased habitat availability and reduced
direct competition and predation such that the oyster population benefited from the successful
recruitment. The subsequent storm, Kate, coming after the spawning period, was not as
destructive to oyster populations as Elena and could have even enhanced growth of the
survivors. Hurricanes are common along the Gulf coast during the spawning period of the
oysters; it appears that C.
virginica
is well adapted for such natural disturbances. The ob-
served response of the Apalachicola oyster population to successive disturbances has signif-
icant meaning in terms of the long-term ecological stability of estuarine populations and the
evolutionary aspects of such biological response to temporally unstable habitats. In this case,
such populations can be viewed as highly resilient under even the most extreme conditions
of physical instability. However, the exact biological response to temporally irregular distur-
bances is highly dependent on the timing of such events relative to the natural history of
population in question.
Considerable life history data have been published concerning the American
or eastern oyster
(Crassostrea virginica
[Gmelin]). Massive oyster reefs occur
along the Gulf of Mexico coast. The location and condition of these reefs depend
on many interacting factors which include complex combinations of geological,
physical, chemical, and biological processes. Reef oysters, although tolerant to
broad ranges of important habitat variables such as temperature and salinity, are
susceptible to various forms of physical disturbances whereby reef structure can
be adversely affected or destroyed. The success of the American oyster along the
Atlantic and Gulf coasts of North America depends on various factors that influ-
ence spawning, larval development in the plankton, metamorphosis of the spat
stage, successful attachment to a suitable (solid) surface, and development of the
sexually mature adult. Harvesting, predation, disease, prolonged low salinities,
and physical processes such as wave damage and sedimentation (burial) are all
major causes of mortality in the developing oyster.
Research on the extensive Apalachicola oyster reefs dates back to the works
of Swift (1896) and Danglade (1917). The Apalachicola estuary accounts for
about 90% of Florida's commercial fishery (Whitfield and Beaumariage, 1977).
Historical surveys indicate that considerable destruction of the Apalachicola oys-
465
466
BULLETIN OF MARINE SCIENCE, VOL. 64, NO.3, 1999
ter resource occurred during the fall and winter of 1893-94 due to storm-related
burial and subsequent freezes. Reports of such losses have been relatively frequent
from this period to the present although scientific documentation of such impact
and subsequent recovery is lacking. Detailed reef descriptions (Swift, 1896) in-
dicate similar distributions of oysters existed over the 90-yr period from 1896 to
1985. Conditions in the Apalachicola Bay system are highly advantageous for
oyster propagation and growth (Menzel and Nichy, 1958; Menzel et aI., 1966;
Livingston, 1984) with reefs covering about 7% (4350 hectares) of bay bottom
(Livingston, 1984). Mass spawning takes place at temperatures between 26.5°and
28°C, usually from late March through October (Ingle, 1951). Growth rates of
oysters in this region are among the most rapid of those recorded (Ingle and
Dawson, 1952, 1953) with harvestable oysters taken in 18 mo. Overall, the oysters
in the Apalachicola region combine an early sexual development, an extended
growing period, and a high growth rate (Hayes and Menzel, 1981); effective
spawning is restricted to older oysters, although young-of-the-year are able to
spawn.
Tropical storms and hurricanes, relatively common along the northern Gulf
coast, are often accompanied by storm surges, waves, strong currents, erosion,
sedimentation, flooding, altered salinities, and changes in the physiographic struc-
ture of inshore waters (Hayes, 1978). Tabb and Jones (1962) indicated mortality
of fishes and invertebrates due to oxygen depletion, sedimentation, and other
habitat changes associated with Hurricane Donna. Andrews (1973) found that
reduced salinities in Chesapeake Bay caused by Hurricane Agnes accounted for
unprecedented changes in the distribution and abundance of various estuarine
species. Oyster populations suffered heavy mortalities and were severely stressed
but not eradicated. In various rivers, prolonged fresh-water conditions were ac-
companied by long-term oyster mortality. Thus, previous studies indicate that the
physical impact of hurricanes can lead to a broad range of biological responses
depending on the timing and nature of the storms.
On 1-2 September 1985, Hurricane Elena, with winds of approximately 200
Ian h-
I,
struck the Apalachicola system. A maximum storm surge exceeding 3 m
was noted near the Apalachicola system with heavy rainfall (>18 cm). The angle
of the storm and an extended fetch caused major disturbance in St. George Sound
(Fig. 1). The strong storm surge moved in a southwesterly direction along the
sound, and, together with heavy sedimentation, caused major physical damage to
the Apalachicola oyster reefs in eastern parts of the bay (Livingston, pers. observ.;
Berrigan, 1988, 1990). On 21 November 1985, Hurricane Kate struck the Florida
coast just west of Apalachicola Bay; at landfall, the storm carried 150 Ian h-
I
winds with a storm surge exceeding 3.5 m. This hurricane, because of the position
of landing and characteristics of the wind distributions as it came ashore, was not
associated with observed major physical effects on Apalachicola Bay as was the
case with Hurricane Elena (Livingston, pers. observ.). However, within a period
of less than three months, two hurricanes struck the Apalachicola Bay system,
and these storms occurred during a comprehensive study of the Apalachicola
oyster reefs.
The Apalachicola Bay system is the major commercial oyster-producing area
in Florida. Commercial oystering represents a potential disturbance to the natural
population changes of oysters on the chief producing reefs. The 71-80 mm range
marks the lower limits of legal oyster catches (>3 in). Oystering activities were
ongoing from March through May 1985 throughout the bay, and on selected reefs
from May through August 1985. Due to the effects of Hurricane Elena, com-
mercial oyster harvesting was suspended in the Apalachicola Bay system by the
012345
Km
10
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS
467
~
Study Area
\j
.'
Figure 1. Distribution of sampling sites showing general boundaries of each oyster reef. These dis-
tributions were based on historic oyster distributions, interviews with commercial oystermen and state
agency personnel, and recent field studies (Livingston, unpubl. data).
Florida Department of Natural Resources (FDNR) from September 1985 through
April 1986. In May 1986, several of the historically best-producing reefs in the
eastern section of the bay were opened to oystering. By September 1986, the bay
was fully opened to commercial harvesting.
The purpose of this paper is to present a series of field analyses of oysters in
the Apalachicola Bay system in an effort to define the response of such popula-
tions to two hurricanes and temporal patterns of commercial fishing in the estuary.
METHODS
Based on previous research in the Apalachicola system (Livingston, 1984), a series of sampling
stations was established on both major and minor oyster reefs throughout the Apalachicola estuary
(Fig. 1). Oyster reefs where sampling occurred comprised about 59% of the mapped bay reefs (Table
1) and represented the primary oyster-producing areas in the Apalachicola Bay system. Field locations
were maintained with LORAN. Monthly water quality data were taken at each station (surface and
bottom) from March 1985 through October 1986. Depth (m) was taken along with a standard Secchi
disc reading. Salinity was determined with a temperature-compensated refractometer calibrated peri-
odically with standard sea water. Dissolved oxygen and water temperature were measured with a Y.S.I.
dissolved-oxygen meter. Turbidity was taken using a Hach model 2100-A turbidimeter, and apparent
color was measured with an American Public Health Association platinum-cobalt standard test. Cli-
matological data concerning local meteorological conditions and hurricane developments were pro-
vided by the Environmental Data Service, National Oceanographic and Atmospheric Administration,
U.S. Department of Commerce.
Oyster samples (multiple) were taken with full head tongs (16-tooth head; 4.5 m handles) at each
station on a monthly basis from March 1985 through October 1986. The use of tongs was standardized
with respect to opening widths and sampling effort (area covered with one tong
=
0.33 m
2).
Toquantify
the sampling effort, a series of 30 standardized, random tong samples was taken at the Big Bayou
and Cat Point reefs in February 1985. The cumulative size frequency distribution (in lO-mm incre-
ments) was determined and plotted for each sampling site. The number of samples necessary for a
specified level of quantification was determined according to a method described by Livingston et al.
(1976). The method allowed determination of the number of subsamples necessary to achieve specific
levels of size class accumulation when compared to the results of 30 subsamples. Seven subsamples
468
BULLETIN OF MARINE SCIENCE, VOL. 64, NO.3, 1999
Table I. List of oyster reefs sampled with estimated areas of oyster distribution at each sampling
site. Reefs are grouped by position within portions of the Apalachicola Bay system. Abbreviations
(shown in parentheses) correspond to those sites shown in Figure I. Oyster data are given as total
numbers per reef, total biomass (g AFDW) per reef, numbers m-
2,
and biomass (g AFDW) m-
2
for
oysters averaged over the study period. Tableentries for total numbers per reef and total biomassper
reef should be multiplied by 106.(*)
=
oyster spatfall accumulation stations.
Total Total Density Biomass
Reef Area (ha) number biomass (Nm-
2)
(Bm-
2)
St. Vincent Sound
Scorpion* (SC) 190 63,7 46,9 33.5 24.7
Schoelles Lease (SL) 85 13.7 19.3 16.1 22.7
Paradise* (PA) 232 59.2 51.0 25.5 22.0
Big Bayou (BB) 17 5.0 6.6 29.2 38.7
Pickalene* (PK) 30 13.4 3.1 67.2 15.7
Cabbage Top* (CT) 44 17.4 14.9 39.6 33.8
Kirvin's Lease* (KL) 24 1.0 1.8 4.1 7.5
Dry Bar (DB) 72 49.5 11.7 68.7 16.2
Apalachicola Bay
St. Vincent (SV) 575 464.0 12.4 80.7 2.2
Pilot's Cove (PC) 20 13.4 3.1 67.2 15.7
Sike's Cut (SK) I 1.4 0.3 43.3 27.3
Nick's Hole (NH) 14 3.9 1.6 28.0 11.1
Hotel (HO) 23 6.2 1.4 26.8 6.3
Sweet Goodson* (SG) 98 89.1 25.2 91.0 25.7
East Bay
Gorrie Bridge* (GB) 67 34.8 16.3 52.0 24.3
St. George Sound
Cat Point Bar* (CP) 514 1,781 342 347 66.5
Platform (PL) 180 595 87.5 331 48.6
East Hole* (EH) 204 853 97.2 418 47.7
Porter's Bar* (PB) 137 28.9 2.9 21.1 2.2
Shell Point (SP) 18 2.7 0.1 14.8 0.7
accounted for 80.8% (Big Bayou) and 87.5% (Cat Point) of the sampling variability for the total
sample. Accordingly, this number of subsamples (located beyond the asymptote for size class accu-
mulation) was considered to constitute a representative sample taken in each of the test regions.
Numbers of oysters tong-
1
were recorded and converted to numbers m-
2
All oysters were measured to the nearest mm in the field according to the greatest distance from
beak to lip using linear calipers. A total of 140 oysters taken from four stations (n
=
35 at each site)
was used to determine the relationship of shell length and weight of oyster meat. Four separate length!
weight equations were developed to account for known differential growth characteristics in different
regions of the bay.
In(AFDW)
=
2.505·ln(LEN) - 10.980
In(AFDW)
=
2.303·In(LEN) - 10.306
In(AFDW)
=
2.202·ln(LEN) - 9.125
In(AFDW)
=
2.465·In(LEN) - 10.190
(Cat Point, r
2
=
0.83)
(East Hole, r
2
=
0.84)
(Paradise, r
2
=
0.90)
(Scorpion, r
2
=
0.86)
where AFDW is the ash-free dry weight of oyster meat and LEN is oyster shell length in mm. F-tests
(P< 0.05) indicated that equations from both bars in the eastern bay (Cat Point and East Hole) were
significantly different from equations developed for bars in the western bay (Paradise and Scorpion).
Although the differences were not significant within the respective regions, east and west, the site-
specific equations were used for the transformations of the length data into ash-free dry weights. This
allowed the most comprehensive and accurate use of the data for such transformations. A fifthequation
was developed for bars not in the immediate vicinity of the above four reefs and was generated from
the baywide pooled data.
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS 469
In(AFDW)
=
2.754·ln(LEN) - 11.742 (all reefs, r
2
=
0.86)
All tong data (in terms of numerical abundance and ash-free dry wt) were calculated on a unit m-
2
basis. These data were then transformed to estimates of total numbers and biomass on each bar (Table
1) based on the estimated size of the bars. Areas were established from computer estimates of oyster-
producing areas (Livingston, unpubl. data) based on historic records, interviews with oystermen and
state environmental agencies, and our past and ongoing field studies (Livingston, 1984).
To standardize our overall collection effort, tong data (oyster density and shell length) were quan-
titatively compared with those derived from a series of multiple (0.25 m-
2)
quadrats taken on the same
sampling reefs (Cat Point Bar, East Hole, Paradise) over the same period of study by other researchers
(Berrigan, 1990). Tong data were based on seven random samples mo-Istation-I while quadrat data
were derived from five random samples mo-
I
reef-I. Comparisons were made only for those months
when both types of collections were taken concurrently. Unfortunately, tong and quadrat samples were
taken neither on the same date nor from exactly the same location on each reef. These differences
undoubtedly introduced some error into the comparisons. Since the data sets deviated from normality,
Wilcoxon sign rank tests for paired differences were used to compare monthly densities and shell size
between the tong and quadrat collections.
Spatfall accumulation was analyzed from a subset of 10 stations (Table 1). Spat baskets, constructed
of plastic-coated wire (25 X 25 X 25 cm; 2.5 cm mesh), were filled with about 20 sun-bleached oyster
shells and placed at each site. Bricks were placed at the bottom of each basket so that the oysters
remained off the bottom to lessen problems with sedimentation. Samples were retrieved, and new sets
of oyster shells were set out at two- to three-week intervals. One spat basket was used at each station.
Seven shells were randomly chosen from each basket for analysis. Spat counts were made from the
inner surface of each valve; this standardization was based on test results that indicated less variance
of such adherence on inner surfaces than on outer surfaces.
Oyster data were grouped in two ways prior to analysis: (1) bay-wide totals and (2) eastern versus
western reefs. In the latter grouping strategy, selected reefs from the eastern bay (Sweet Goodson, Cat
Point, East Hole, Platform, Porter's Bar) were compared to selected reefs from the western bay (Big
Bayou, Paradise, Pickalene, Scorpion, Schoelles Lease). These reefs were chosen because of their
relative commercial importance to overall oyster production in the bay. Intervention models (Box and
Tiao, 1975; Pankratz, 1991) were used to analyze the effects of the two hurricanes on the monthly
total oyster numbers (in thousands) and the monthly average shell length (in mm) of oysters at the
selected reefs in the bay. Three major interventions occurred in the Apalachicola Bay during the study
period: Hurricane Elena (combined with the cessation of commercial oystering after September 1985);
Hurricane Kate; and the resumption of commercial oystering in May 1986. The models were used to
test the possibility that the interventions were significantly associated with level changes in the monthly
total oyster numbers
(N(t)}
and the monthly average shell lengths
(S(t)}.
Three indicator variables corresponding to the interventions are defined as
t<
May 1986
t
2:
December 1985
t
<
December 1985
t
2:
September 1985
t
<
September 1985
{O,
Xj(t)
=
I,
{O,
X2
(t)
=
1,
{O,
X3
(t)
=
I,
t
2:
May 1986
Using B as the backward shift operator such that
BX(t)
=
X(t -
I), the relationship between the
monthly total oyster number
(N(t)}
and the three interventions can be described by the following
general intervention model:
N(t)
=
[30
+
vi(B)XI(t)
+
v
2
(B)X
2
(t)
+
v
3
(B)X
3
(t)
+
~(t),
(1)
where vi(B),
viB),
and
viB)
are polynomials with the typical form;
v(B)
=
000-w,B - ... - whBh
where
{wo,w" ... ,wh}
are parameters to be estimated.
In model (I),
f30
is a constant and
~(t)
is a noise term which is often modeled as a stationary
autoregressive moving average (ARMA;
p, q)
process with
W) - <I>,W -
1) ... -
<l>pW -
p)
=
£(t) - 6
1
£(t -
1) - ... -
6.£(t -
q),
where
p
is the order of the autoregression (AR) term, and
q
is the order of the moving average (MA)
term. The
£(t)'s
are assumed to be independent and normally distributed with mean zero and variance
(]'2.
The intervention model (shown in Eq. 1) extends the traditional linear regression models in two
470
BULLETIN OF MARINE SCIENCE. VOL. 64, NO.3, 1999
35
fIll
III
1fH IffPffP
35
,
30 ~ 30
!
!
f:f :
!~!!
0
~25
L. 25
!
'1 '
!
Q.
e
CD
!! : ~
!
20
a
20
~
III
: '!
i
1
:~15 •...15
8-
o
'1
iii
o ,
en
10 E10
CD
5I- 5
0
00
140 Elena : : Kate 12 Elena : : Kate
,
Jii'120
fHI 'I
10
: :!!f!
§
100
~ 8
0
C)
ffthnf~ HUftt
0
80
.§.
8
.!.
~60
d
f
It :
f~
I
4
5
40
ftf
!
ci
,
0
20 2
,
0
,
,
0 0
MAMJJASONDJFMAMJJASO MAMJJASONDJFMAMJJASO
1985 1986 1985 1986
Figure 2. Temperature
CC),
dissolved oxygen (mg L-I), salinity
(%0),
and color (Pt-Co units) in
bottom waters of the Apalachicola estuary. Data were averaged
SO) over all stations by month
from March 1985 through October 1986.
directions. First, the biological response, in this case either monthly total oyster number series
(N(t)}
or monthly average shell length
(S(t)},
may react to an intervention with a time lag. For example,
the term
vl(B)XI(t)
=
WeXI(t) -
WIXI(t- I) - ... - WJCI(t-
h)
in model (I) indicates that the impact
of the first intervention is distributed across several time periods. Second, instead of assuming that
the errors
~(t)
are independently distributed,
ARMA(p, q)
models are used for
~(t)
which incorporate
possible serial correlations in the response series. All intervention models were fitted using the Linear
Transfer Function Identification Method proposed by Pankratz (1991). Only those coefficients where
the computed t-ratio exceeded the critical t-value (t
=
2.10; P
<
0.05) were incorporated in the final
models. Diagnostic checks included both residual autocorrelation analysis and cross correlation anal-
ysis of the model residuals with residuals of each input series. Significant correlations in either of
these checks may indicate an incorrect final model.
In addition to the overall oyster data, intervention models were fitted to eastern and western sections
of the bay using numbers m-
2
and average shell length (in mm) of oysters from stations in these areas.
The four series were denoted by EN(t) and
WN(t)
for eastern and western densities and by ES(t) and
WS(t)
for eastern and western shell lengths. Intervention models were fitted to the four series sepa-
rately.
RESULTSANDDISCUSSION
HABITATCONDITIONS:HURRICANEINFLUENCE.-The mean depth for all stations
was 1.2 m, ranging from 0.5 m to 2.1 m. The relatively shallow depths of the
oyster reefs constituted a factor in the effects of the hurricanes. Secchi depths
were relatively uniform throughout the estuary (range of means:
0.5-1.0
m). Dis-
solved oxygen values were lowest at some stations in September 1985 Gust after
Hurricane Elena) and from June through October 1986 following a major drought
in 1986 (Fig. 2). Dissolved oxygen concentrations (Fig. 2) remained above 4.0
mg
L
-I
throughout most of the study and were not considered an important stress
factor on the Apalachicola Bay reefs. High color values in the bay during Sep-
tember 1985 reflected the impact of Hurricane Elena. In addition, there were some
reductions of salinity after both hurricanes; however, such salinity changes were
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS
471
short-lived. Overall water quality effects due to the storms were considered mod-
erate compared to the natural seasonal variation in the Apalachicola system. The
ensuing drought during 1986 was associated with high summer temperatures and
salinities in the estuary and somewhat lower color and dissolved oxygen.
Although various water quality factors were somewhat affected by the hurri-
canes, the chief effects on the reefs were physical. The sustained winds of Hur-
ricane Elena, moving from northeast to southwest in the Apalachicola region,
caused considerable structural damage to the eastern reefs (Porter's Bar, Platform
Bar, Cat Point Bar, Sweet Goodson's, East Hole) due to abrasion, sedimentation,
and extreme turbulence from wave action and water movement (Livingston, pers.
observ.; Berrigan, 1990). By contrast, Hurricane Kate had winds from the south
that did not have the direct force of extreme water flow (and the associated phys-
ical impacts on oyster reef structure) observed during Hurricane Elena (Living-
ston, pers. observ.).
DENSITY,BIOMASSANDGROwTH.-Results of the comparison of the quadrat and
tong data are shown in Figure 3. Overall, the two data sets were comparable. The
primary differences in density and mean size were most evident in the Cat Point
and East Hole data during the heavy recruitment in October and November 1985.
Statistical analysis of the grouped (all stations, all dates) data indicated that the
monthly densities estimated from the tong data were not significantly different (P
>
0.05) from those estimated with the quadrat method. When the months of
October and November (the major recruitment period in 1985) were excluded
from the analysis, a significant difference (P
<
0.05) was noted. Analysis of the
average size data indicated just the opposite effect. Means from the tong collec-
tions were significantly different (P
<
0.05) from the quadrat samples when data
were grouped (all stations, all dates); however, no significant differences (P
>
0.05) were noted when the October and November data were excluded. These
results are consistent with the fact that small oysters (i.e., <25 mm) were not
counted in the quadrat analysis while individuals as small as 10 mm were included
in the tong samples. This inclusion increased the density and decreased the shell
size estimates based on tong samples during the recruitment period. Overall, col-
lections taken with both methods were reasonably similar; sampling results of this
study are thus directly applicable to quantitative estimates of the numbers and
mean size distributions of oysters present in the Apalachicola estuary over the
sampling period.
The overall distribution of oysters in the Apalachicola system is summarized
in Table 1. Cat Point, East Hole, and Platform bars were by far the most produc-
tive of the various oyster-producing areas of the bay in terms of total estimated
reef numbers and biomass (averaged over all sampling dates from March 1985
through October 1986). These three areas were also characterized by the highest
oyster densities and biomass m-
2
High oyster productivity in these areas was due
both to the extensive areas of the reefs as well as the high mean density and
biomass. Western sections of the bay produced relatively fewer oysters than the
eastern sections. This was true both in terms of numbers m-
2,
biomass m-
2,
and,
with the exception of Paradise reef, overall area of oyster distribution.
Temporal trends of total numbers, numbers m-
2,
biomass m-
2
and mean shell
length of the oysters in the Apalachicola system are given in Figure 4. In the bay
as a whole, there was a major decrease in total numbers, numbers m-
2
and biomass
m-
2
from August to September 1985 (coincident with Hurricane Elena); mean
shell size was little affected by the storm's passage. Numbers and biomass of
oysters reached low points in the study period during September 1985. Berrigan
(1990) also found substantial reductions of oyster numbers during this period
100
E
80
-o-tong
__ quadrat
S-
60
GI
~
N
7i
c:
40
I
:!
20
0
100
E
80
S-
60
GI
N
'ii
c:
40
til
GI
~20
0
140
120
E
100
S-
GI
80
N
7i
60
c:
til
40
GI
~20
0MAMJJASONDJFMAMJJAS
1985 1986
Otong
_quadrat
1986
BULLETIN OF MARINE SCIENCE, VOL. 64, NO.3, 1999
1985
472
Cat Point
1400
1200
co
E
1000
-.::
.a 800
.s
l:- 600
'ii
c:
400
GI
0
200
0
East Hole
2500
••• 2000
E
"i::
.a 1500
S
Paradise
140
120
~
co
E
100
1S
80
.s
l:-
60
'ii
i
40
o
20
o
MAMJJASONDJFMAMJJAS
o
~ 1000
II)
c:
GI
o
500
Figure 3, Mean oyster density (number m-2)and shell size (mm) compared from tonged (this study)
and quadrat samples (Berrigan, 1990) on three reefs in Apalachicola Bay. Comparisons were based
on seven tongs and five quadrat samples taken at the most productive oyster reefs in the bay during
selected months from March 1985 through September 1986.
which he attributed to the effects of Hurricane Elena. A primary feature of the
numerical trends was the sizable recruitment just after Hurricane Elena (Fig. 4);
numbers of oysters in the bay peaked during October-November 1985. During
the period from September to October, mean size was reduced to lows for the
period of record due to the combined loss of adult oysters during the hurricane
and subsequent recruitment of small oysters. Considerable losses of numbers (but
not biomass) occurred among the young-of-the-year throughout the fall of 1985.
These decreases in young-of-the-year numbers likely reflected natural mortality,
as commercial oystering was prohibited from September 1985 to May 1986. Dur-
ing the fall to early winter period, biomass m-
2
increased rapidly (Fig. 4). Num-
bers of oysters were relatively stable from December 1985 through April 1986
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS
473
12000
_Numbers
80
-
'"
-0- Average size
~
0
10000 70
•...
CD
~8000 60 iii
(Q
U)
CD
•...
CI)
6000 50
U)
.c
N"
E
CD
::J
4000 40
c:::
3"
(i'i 2000 30
2-
(5
I-
020
:
Elena: : Kate : Oystering
:resumed
300 70
:
_Density
N'
250
::
-o-Biomass
60
E
::
to
";;::
50 0"
.c
200
:
3
.s 40
ll)
:
en
~150
en
"iii
30
iO
c:::
100
--
Q)
20
3
Cl ~
50 10
0 0
MAMJJASONDJFMAMJJASO
1985 1986
Figure 4. Total numbers of oysters, numbers m-
2,
mean shell size (mm) and ash-free dry wt biomass
m-
2
for all stations (averaged) from March 1985 through October 1986. The timing of Hurricanes
Elena and Kate are shown along with the date when oystering was resumed (commercial harvesting
ceased immediately following Hurricane Elena).
with growth continuing from October 1985 to May 1986. Overall numbers and
biomass of oysters in the bay declined in May 1986 coincident with the resump-
tion of commercial harvesting.
The intervention model in (1) was fitted to the monthly total oyster numbers
in the bay. The final identified model for this series was
(2)
with no serial correlation indicated in the noise process
~(t).
The estimated pa-
rameters in the final identified model are presented in Table 2A. When Hurricane
Elena was followed by massive recruitment of young oysters on the damaged
reefs, which resulted in a substantial increase in oyster numbers after about one
month. The fitted model showed that the monthly total oyster numbers in the bay
increased significantly in October (with a I-mo lag after the intervention). When
hurricane Kate struck the bay on 21 November 1985, oyster densities were already
in decline before the hurricane, and it is likely that the lower numbers in Decem-
ber 1985 reflected both natural mortality and hurricane effects. Finally, the re-
sumption of oystering in the bay beginning in May 1986 was associated with a
significant reduction in the monthly total oyster numbers. The fitted model showed
that the levels of monthly total oyster numbers during the four different periods
(before Hurricane Elena, between Elena and Kate, between Kate and May 1986,
and after May 1986) were significantly different. The coefficient of determination
(r
2)
of the fitted model indicated that the three intervention variables explained
474
BULLETIN OF MARINE SCIENCE, VOL. 64, NO.3, 1999
Table 2. Results of the application of the intervention models to bay-wide oyster data in the Apa-
lachicola system taken monthly from March 1985 through September 1986.
Variable Estimated
coefficients Standard
error T-ratio
2.68
15.1
-6.87
-2.79
314
627
643
465
839
9,473
-4,415
-1,298
A. Estimated parameters in the intervention model for the monthly total oyster numbers (in ten thou-
sands).
Intercept
XI
(t -
I)
X
2
(t)
X,
(t)
R2
=
0.95
38.2
-11.0,
3.33
-2.00
1.76
3.05
3.29
2.78
67.2
-33.4
11.0
-5.55
B. Estimated parameters in the intervention model for the monthly average shell length of oysters (in
mm).
Intercept
XI (t -
I)
X
2
(t - I)
X,
(t)
R2
=
0.93
about 95% of the variation in the monthly total oyster number series during the
study period.
The intervention model for the monthly average shell length of oysters in the
bay was identified as
Set)
=
<Xo
+
<xIXI(t -
1)
+
<xzXz(t -
1)
+
<x
3X3(t) +~(t)
(3)
The estimated parameters in the model are given in Table 2B. Similar to the
monthly total oyster numbers
N(t),
the monthly average shell length series reacted
to the firstintervention event with a l-mo time lag and decreased because of the
substantial increases of numbers of young oysters after Hurricane Elena. The
passage of Hurricane Kate had little direct effect on shell size; however, during
the period between the storm and the resumption of oystering, shell size increased
significantly (again, with a l-mo lag after the intervention). The second interven-
tion was correlated with considerable losses of numbers of young oysters. The
third intervention, resumption of oystering in May 1986, coincided with a reduc-
tion in the average shell size. The three interventions explained about 93% of the
total variation in the monthly average shell size series. As with the previous
model, the noise term
~(t)
was not serially correlated.
The temporal trends of oyster populations in the Apalachicola system were not
uniform throughout the bay. Trends of numbers m-
z
on selected oyster reefs (Fig.
5) indicated losses of oysters at Cat Point, East Hole, Sweet Goodson's, Porter's
and Platform bars (eastern reefs) that coincided with the occurrence of Hurricane
Elena. Following the hurricane, mean numbers m-
z
increased and mean oyster
size decreased at these bars as a result of high recruitment. Oyster numbers also
decreased in December 1985 following Hurricane Kate. Although there were in-
dications of similar changes in oyster numbers and size in western sections of the
bay, oyster distributions in these areas showed far less pronounced trends. Re-
cruitment in eastern areas during the second year of study was relatively uniform
following the substantial reduction in numbers and biomass that accompanied the
resumption of oystering in the bay (May 1986). Recruitment in western sections
appeared to increase during the summer of 1986. Mean size in such areas was
reduced during such recruitment.
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS
475
Eastern Reefs Western Reefs
1600 400
~1400 350
N
~ 1200 300
...
,
:S
1000 250
,
800
0
200
,
~
0
,
'iii
600 I
iw
150
:....•!J!
c:
f
100
Gl
400
C
200
I!
50
00
100 Elena : :Kate :Oystering 140 Bena : ;Kate ;Oystering
,resumed ,resumed
e-
o
120
HdHlwuhI
80
s.
Iff·tfl 1.i'l.ilflf!
100
Gl
60 80
N
en
40 60
fa
40
Gl
20
" , fll!!
::i:
" 0
: t: : 20
'0 0
00
MAMJJASONDJFMAMJJASO MAMJJASONDJFMAMJJASO
1985 1986 1985 1986
Figure 5. Average number m-
2
and mean shell size (mm) of oysters taken at stations representing
the eastern (Cat Point, Sweet Goodson, Platform, East Hole, Porter's Bar) and western (Scorpion, Big
Bayou, Pickalene, Paradise, Schoelles Lease) sections of the Apalachicola Bay system. Data were
taken monthly from March 1985 through October 1986.
Monthly oyster density (numbers m-
2)
patterns differed between western
{WN(t)}
and eastern
{EN(t)}
sections of the bay. The intervention model for the
monthly oyster density series
{EN(t)}
had the same form given in (2) which is
the model for the monthly total number of oysters in the whole bay system. The
intervention model for western bars
{WN(t)}
was
WN(t)
=
133X3(t -
3)
+~(t), (4)
in which the noise series
[~(t)]
followed an AR(1)model with the form
~(t) =<l>1~(t-
1)
+e(t), (5)
where the
e(t)'s
were independent with identically distributed normal random var-
iables with zero-mean and variance
<1
2
The term
<1>1
is a constant.
The parameters in the intervention models for eastern
{EN(t)}
and western
{WN(t)}
reefs are given in Table 3A. The r
2
s for the two fitted models were 0.97
and 0.94, respectively. In contrast to the eastern reefs
{EN(t)},
the associations
between hurricanes Elena and Kate and western reef densities
{WN(t)}
were not
statistically significant. The third intervention, resumption of oystering, was re-
lated to an immediate decrease in the monthly oyster numbers m-
2
in the eastern
bay, and appeared to be associated with a density increase in the western bay
{WN(t)}
after a 3-mo delay. The model correlation of resumption of oystering
with spring 1986 oyster recruitment in western sections of the bay does not nec-
essarily indicate a cause-and-effect relationship of such factors. It is likely that
the correlation in the western bay was due more to natural recruitment processes
476
BULLETIN OF MARINE SCIENCE, VOL. 64, NO.3,
.999
Table 3. Estimated parameters in the intervention models in the eastern and western sections of the
bay for oyster data taken monthly from March 1985 through September 1986.
Variable Estimated
coefficients Standard
error T-ratio
A. Estimated parameters in the intervention models for the monthly density in eastern and western
sections of the bay. Eastern Reefs
Intercept 69.4 16.9 4.12
XI
(1 -
l) 668 33.7 19.8
X
2(1)
-406 34.5 -11.8
XJ
(1)
-128 25.0 -5.13
R2
=
0.97
Western Reefs
XJ
(1 -
3) 64,9 20.2 3,22
<1>1
0.97 0.12 8.08
R2
=
0.94
B. Estimated parameters in the intervention models for the monthly average shell length of oysters
in the eastern and western sections of the bay.
Eastern Reefs
Intercept 55.0 2.15 25.6
XI
(1 -
I)
-37.1 3.72 -9.97
X
2(1 -
I)
19.8 3.46 5.73
R2
=
0.87
Western Reefs
Intercept 88.7 4.24 20.9
XI
(1 -
l) -20.8 5.78 -3.59
XJ
(1)
-25.4 5.78 -4,40
R2
=
0.80
than to some lagged influence of the resumption of oystering at eastern bars.
Unlike other intervention models identified in this study, the noise series
{E(t)}
in the model for
{WN(t)}
showed strong serial correlations.
Intervention models were identified for the monthly average shell size for east-
ern
{ES(t)}
and western
{WS(t)}
reefs. The model for
{ES(t)}
was identical in
form to that specified in (3) for the monthly average shell size of oysters in the
whole bay, except that the third intervention was not statistically significant. The
identified intervention model for
{WS(t)}
had the form
WS(t)
=
ao
+
ajX.(t - 1)
+a3X3(t) +E(t). (6)
The estimated parameters in the two models for
{ES(t)}
and
{WS(t)}
are listed
in Table 3B. The relationships of the two hurricanes with the series
{ES(t)}
were
similar to those on the monthly average shell size of oysters in the whole bay.
The second hurricane, Kate, was not significantlyrelated to the monthly shell size
of oysters in the western bay. This was probably influenced by the limited damage
to adult oyster populations here along with a relatively poor recruitment in western
areas during 1985. Berrigan (1990) discussed the generally poor recruitment in
the western bay as a consequence of the patchy, unconsolidated nature of the
reefs. Contrary to these findings, the resumption of oystering was correlated sig-
nificantly with shell size in western reefs but not in eastern bars. As with
{WN(t)},
this was likely due to good recruitment following peak: spatfall in June 1986 and
the loss of the 31-40 mm size class (Figs. 6,7).
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS
Eastern Reefs Western Reefs
50
~
~
40
I
"iii
.r;
30
rn
~
as
20
a.
CJ)
.0
10
!
z
0
MJJASONDJFMAMJJASO MJJASONDJFMAMJJASO
477
1985 1986 1985 1986
Figure 6. Oyster spatfall (mean number of spat shell-I wk-') in eastern (Cat Point, Sweet Goodson,
Gorrie Bridge, Platform, East Hole) and western (Scorpion, Cabbage Top, Pickalene, Paradise, Kirvins
Lease) reefs within the Apalachicola estuary collected every
2-3
wks from March 1985 through
October 1986.
The data indicated that, following a stable period with relatively low densities
of oysters during the summer of 1985, Hurricane Elena destroyed the primary
oyster-producing areas in eastern portions of the Apalachicola system. Major oys-
ter recruitment was noted in eastern sections that were most affected by the hur-
ricane; oyster numbers may have been thinned out in November 1985 by Hurri-
cane Kate thus exacerbating existing, natural levels of oyster mortality. The com-
bination of specific attributes of the hurricanes and the timing of these storms
relative to the natural history of the oysters defined to a considerable degree the
nature and extent of the impact and response of this population. Although the
.=
10
9
oysters
141-150
~Ie,:,al. I~a~ · Il?ys~ri~g .
resumed
131-140
·
\.
121-130
1
I
1
,....,
E
111-120
I. I. · I.
E
101-110
'-'
I'
I'
·I'
91-100
l/)
Q)
81-90
··
·
I' I' I •
··
l/)
l/)
71-80
·
.·.
.
CIS
I.
I.
• I.
U
61-70
·.
Q)
51-60
·
·
·.
.1.
•••
N
.~
......
Vi
41-50 ·
·
· I·
· I·
31-40
• •
·.
·
•••••••
21-30 ·
·
I
•••••••
11-20
I.
•• 1 ••••••
1-10
•••
N
A
N
J
A
S
0
N
0
J
F
N
A
N
J J
A
S
0
1985 1986
Figure 7. Oyster numbers by size class by month from March 1985 through October 1986. The area
of the circles is proportional to the square root of the total numbers of oysters by size class in the
Apalachicola estuary.
478
BULLETIN OF MARINE SCIENCE. VOL. 64. NO.3. 1999
storms (especially Hurricane Elena) had a major economic impact in virtually
destroying the Apalachicola oyster industry (Berrigan, 1988, 1990), the resilience
of the oyster and the fortuitous timing of the storm (before spawning was com-
pleted) allowed rapid recovery. Hurricane Kate, in possibly thinning out the newly
settled oysters, may have provided improved growing conditions for the survivors
by reducing competition. Subsequent increases in biomass were substantial during
the winter-spring of 1985-86 prior to the resumption of oystering in May. Modest
recruitment during the summer-early fall of 1986 enhanced full recovery of system
numbers and biomass in less than 1 yr from the occurrence of Hurricane Elena.
SPATFALLANALYSIs.-Theseasonal pattern of spatfall (Fig. 6) differed between
eastern and western sections of the bay over the period of study. During 1985,
oyster spat were first detected in eastern sections of the bay in June with incre-
mental increases in July and August. By early October, there was a major spatfall
event that was indicative of increased spawning around the time of the early
September hurricane. In 1986, the spatfall in the eastern parts of the bay was
highest during June; subsequent levels were relatively low, with slight increases
in August and September. The lowest spatfalls were noted in May and late Sep-
tember-October 1986. Western sections of the bay were characterized by rela-
tively low oyster spatfall during 1985. In 1986, there was a major spatfall in early
June with a subsequent smaller peak during September. Thus, there were consid-
erable differences in oyster spatfall between eastern and western areas of the
Apalachicola estuary. Ingle (1951) attributed this disparity to greater fluctuations
and lower averages of temperature in western parts of the bay. Berrigan (1990)
noted limited reproductive success in the western areas relative to the eastern
sections of the bay. He suggested this difference was a function of the high
density, consolidated nature of the eastern reefs compared to the low density,
patchy nature of the western reefs. Fertilization and subsequent settling of larvae
on suitable substrata (particularly other oyster shells) are presumably enhanced
over dense, highly aggregated reefs. Olguin-Espinoza (1987), in a study of repro-
ductive processes of Apalachicola Bay oysters (March 1985-March 1986),found
that peak levels of gametogenesis occurred during spring months with spawning
at temperatures above 25°C. Spawning ended in October 1985 and was timed
with reductions of water temperature. Hurricane Elena did not have an effect on
the gonad condition of the surviving oysters. There was no indication that Hur-
ricane Elena had an effect on the spawning activities of the Apalachicola oysters.
The relatively high standard deviations of spatfall indicated substantial station-
to-station variation. In terms of spatfall (number of spat shell~1wk-
I),
the primary
regions of the bay contributing to the October 1985 spatfall event included Gorrie
Bridge (60.6), Sweet Goodson's (45.2), East Hole (16.3), and Cat Point (14.6).
These eastern reefs were also the most seriously damaged areas during Hurricane
Elena although the connection of the storm with spatfall occurrence remains un-
documented. The highest observed densities of spat and young oysters over the
entire study were found in these areas during October 1985. Most of the oyster
reefs in Apalachicola Bay and St. Vincent Sound had moderately low to low
numbers of oyster spat at this time. During the June 1986 peak of spatfall, the
primary recruitment areas included Pickalene (38.3), Cabbage Top (35.8), and
Scorpion (16.2) in the western bay with lesser concentrations at eastern reefs such
as Porter's Bar (14.6), Cat Point (12.7), and East Hole (9.6). These data indicate
that at least part of the high productivity of the eastern bars is due to continued
high levels of spatfall. It is possible that the combination of available habitat and
the lack of competition and predation from existing oysters and oyster predators
contributed to the success of the spatfall in fall 1985. Thus, the primary factors
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS
479
that contribute to the population structure of the oysters in the Apalachicola es-
tuary include habitat features (temperature, salinity), the extent of suitable bottom
type, successful spatfall on a year-to-year basis, and the response of the population
to natural disturbances such as storms and biological factors such as competition
and predation.
TEMPORALCHANGESINPOPULATIONCOMPOSITION.-Monthlysize frequency data
of oysters in the Apalachicola Bay system (Fig. 7) reflect the effects of various
disturbances over the period of study. Between March 1985 and July 1985, re-
cruitment of newly spawned oysters was low. Spatfall was low during this period.
By June, there were reductions in most size classes; such low levels persisted
during the early summer months, particularly among the larger oysters (>71-80
mm). By August 1985, increased spatfall was accompanied by recruitment of the
smaller size classes (10-30 mm) which was associated with increased local rain-
fall and periodic reductions in salinity. Hurricane Elena, occurring at the begin-
ning of September, had a devastating effect on the oyster population of the Ap-
alachicola estuary with a decrease of oysters by almost an order of magnitude to
the lowest levels recorded in this study. By October, there was a major recruitment
of young oysters; this recruitment was consistent with the spatfall record (Fig. 6)
and continued into November 1985. Over this period, there was no appreciable
increase in oysters above the 71-80 mm size class despite the fact that commercial
oystering had been suspended. The effects of Hurricane Kate could have contrib-
uted to ongoing reductions of the youngest size classes; there was, between No-
vember and December 1985, a loss that approached 50% of the new crop of
oysters. By the end of December 1985, there was evidence of increased growth
of the remaining oysters. It is possible that this growth was aided by reductions
of oyster numbers due to Hurricane Kate. Increases of the remaining young-of-
the-year oysters were noted in the size classes between 21 and 100 mm with
substantial increases in the 21-30 mm increment. Thus, the impact of the two
storms depended to a large degree on the timing of the individual disturbances
relative to the stage of the life cycle of oysters in the Apalachicola Bay system.
Hurricane Elena occurred prior to the end of the spawning season and, although
the hurricane was extremely destructive to existing adult oysters, it did not pre-
clude major recruitment success during the following fall period. This event could
have created a set of environmental conditions that actually contributed to the
success of the succeeding cohort of young oysters. Hurricane Kate, with less
adverse impacts on the existing oysters in the bay, did not substantially affect
biomass increases of the surviving oysters and could have actually enhanced
growth of these oysters by reducing competition.
From January 1986 through April 1986, there was a generally high growth rate
of the younger cohorts with no discernible changes in overall numbers of oysters.
The numbers of legally obtainable oysters did not change substantially. The return
of commercial oystering in the bay during the late spring of 1986 coincided with
reductions in the bay oyster numbers. From June through August, there was re-
cruitment of young oysters which brought total numbers up to post-Kate levels.
Commercial-sized oysters more than doubled to levels comparable to or exceeding
pre-hurricane numbers as the growth of the post-Elena population continued. The
commencement of full-scale commercial oystering in the fall of 1986 was accom-
panied by reductions of selected size classes. By October 1986, oyster numbers
stabilized; there were increased numbers of commercial-sized oysters due to
growth at most levels of the overall population. In this way, the oyster population
in the Apalachicola estuary reflected growth of the fall 1985 young-of-the-year
cohort together with summer 1986 spatfall recruitment. The resumption of com-
480
BULLETIN OF MARINE SCIENCE. VOL. 64. NO.3. 1999
mercial oystering during the spring and fall of 1986 may have affected the pop-
ulation age-class distribution although such changes were also undoubtedly af-
fected by the high June spatfall in that year. It appears that the Apalachicola Bay
oyster population had fully recovered from the effects of the fall 1985 storms.
Such resilience was due in large part to the successful recruitment that followed
Hurricane Elena in eastern sections of the bay that were most affected by the
hurricane.
EXTREMEEVENTSANDBIOLOGICALRESILIENCE.-The observed changes in the
Apalachicola oyster population should be placed within the context of long-term
changes of major habitat controlling features such as Apalachicola River flow.
Meeter et al. (1979) found that oyster landings from 1959 to 1977 were correlated
negatively with river flow. The highest oyster landings coincided with drought
conditions. Wilber (1992), using oyster landings from 1960 to 1984, found that
river flows were correlated negatively with oyster catch per unit effort within the
same year and positively with catches 2 and 3 yrs later. Highest oyster harvests
occurred in 1980-1981, coinciding with a major drought. Predation on newly
settled spat during periods of high salinity was given as a possible explanation
of the 2-yr time lags between low flow events and subsequent poor production.
Livingston et al. (1997), however, found that increases of the Apalachicola Bay
non-oyster bivalve mollusk populations during droughts were related to changes
in the trophic organization of the estuary. They suggested that relatively high,
non-oyster bivalve production during low flow years was probably due to in-
creased primary productivity as a function of altered physical conditions (i.e.,
increased light penetration) in the receiving estuary. Increased productivity con-
tributed to increased growth rates and ultimately increased bivalve production.
Such changes in river flow and productivity were regular and occurred within
prescribed progressions of river flow fluctuations. This trophic explanation was
suggested as an alternative to the predation hypothesis put forth to explain the
observed relationship between flow and oyster productivity (Wilber, 1992). Hur-
ricane effects, on the other hand, represent asymmetrical events, with population
response dependent on both the timing of the incident and the state of the pop-
ulationjust prior to the storm.
The concepts of biological stability and resilience have been defined in various
ways (see Harrison, 1979, and Santos and Bloom, 1980, for a brief review of the
semantic problems encountered). Stability is generally defined as the ability of a
given system, once perturbed, to return to its previous state. Resilience refers to
the degree, manner, and pace of restoration of the initial system function and
structure following a disturbance (Westman, 1978). Cairns and Dickson (1977)
referred to various parameters of a recovery index: proximity of recolonization
sources, mobility of propagules, physical and chemical suitability of habitat for
recolonization, toxicity of the disturbed habitat, and effectiveness of human man-
agement initiatives to facilitate rehabilitation. By these criteria, the high degree
of resilience of the Apalachicola oysters to massive disturbance can be examined.
Following a devastating physical event, the remaining oysters in the bay, along
with a relatively prolific mode of reproduction, provided the means of the noted
recovery. At the time, there was ample habitat available for settlement. Within
one or two weeks, habitat conditions were highly favorable for spat accumulation.
Salinity conditions showed a rapid recovery to pre-storm conditions. The loss of
existing oysters and various natural predators apparently enhanced spat coloni-
zation, and could have been an important factor in the oyster recovery process.
Subsequent shell-cultching activities of the Florida Department of Natural Re-
sources also aided in the habitat rehabilitation. Thus, the habitat conditions and
LIVINGSTON ET AL.: HURRICANES AND GULF OYSTERS
481
natural history of the oysters were favorable to the high level of resilience of the
Apalachicola population in response to the destructive effects of the hurricanes.
The response to repeated disturbances may take various forms (Westman,
1978), but, in most instances, such response is more extreme with each repetition.
The timing of successive interventions is crucial in the effects on subject popu-
lations. Hurricane Kate occurred after the spawning season had ended so that
recruitment was no longer a factor. The subsequent declining numbers due to
natural mortality of the new cohort could have been further affected by Hurricane
Kate, although oyster biomass continued to increase indicating that the survivors
could have benefited from the selective mortality engendered by the storm. Once
again, the nature of the disturbance relative to local habitat conditions and the
life history stage of the organism contributed to the population response. Bohn-
sack (1983) has pointed out that past experimental studies of disturbances have
suffered from scaling problems, and relatively few field studies have sufficient
data taken prior to the disturbance for an adequate evaluation. In a review of the
influence of the record cold spells on fishes in the Florida Keys, Bohnsack (1983)
found a high rate of recruitment of juvenile fishes, presumably due to reduced
competition and/or predation, which added to the generally high resilience of reef
fishes to regional disturbances. Such response parallels that of the Apalachicola
oyster population. Although the competition/predation explanation is favored by
parsimony, other explanations of the observed resilience of the oyster population
remain possible. These include the increased success of succeeding spatfall as a
result of possible increases of primary productivity, altered current patterns, and
increased levels of habitat availability due to as yet unknown mechanisms. Thus,
the processes of recovery depend on the timing of the disturbance relative to the
life history stage of the subject population; this amounts to a stochastic response
within a relatively structured life history progression.
CONCLUSIONS
The occurrence of two hurricanes in the Apalachicola estuary during a field
study of oysters provided a natural experiment that allowed the close surveillance
of the response of a population to a series of natural and anthropogenous distur-
bances. The nature of the impact of the storms depended on various factors. The
effective aspects of the disturbance included the dimensions and timing of the
storms, local existing habitat conditions, and the life history stage of the subject
population at the times of the disturbances. Storms, including hurricanes, occur
frequently along the northern gulf coast. Hurricanes occur with the highest fre-
quency during summer-fall months, overlapping the usual spawning season for
oysters. The key to the adaptive response to the destruction of major portions of
the oyster population by the first storm was a highly successful spatfal!. Had
Hurricane Elena struck during the winter months, it is doubtful that there would
have been a rapid recovery of this population. Conditions were such, in terms of
adequate habitat, that the spat were able to survive in considerable numbers after
the hurricane. The second hurricane, Kate, occurred after the spawning season,
was not associated with major reductions of the new cohort and could have aided
in the rapid increase in biomass of the survivors through some possible combi-
nation of factors such as reduced competition for food, space, and other factors
associated with crowding. Reproduction and subsequent successful recruitment
and growth in eastern parts of the bay were likely due to the generally higher
reproductive capacity of the oysters here relative to the less productive western
oyster reefs. There were thus differential effects on the overall population that
482
BULLETIN OF MARINE SCIENCE, VOL. 64, NO.3, 1999
depended on specific biological differences among different oyster reefs. Obvi-
ously, oysters are relatively well adapted to disturbances such as storms, even
when the immediate effects include substantial damage to the existing population.
Under such circumstances, the resilience of the oysters was enhanced by specific
aspects of its life history that included rapid and considerable spawning capabil-
ities, a relatively high rate of growth due to the usually optimal habitat conditions
in the Apalachicola estuary, and the return of habitat availability and natural
productivity of this system within days to weeks of the disturbance. The return
of the Apalachicola oyster population was aided by other by-products of the storm
that included the absence of natural predators (including human beings). Com-
mercial oystering has a significant impact on the density and population structure
of oyster reefs. The overall outcome of oyster population changes thus indicated
an adaptive response and high resilience of this species that allows survival under
even extreme conditions of natural stress. Stochastic natural disturbance could
even be viewed as an important stimulus to the long-term productivity of species
such as oysters that are already well adapted to the rigors of a river-dominated
estuary.
ACKNOWLEDGMENTS
Funding for this project came in part from the Center for Aquatic Research and Resource Manage-
ment (Florida State University), the Franklin County (Florida) Board of County Commissioners, the
Florida Department of Environmental Regulation, the U,S, Man and the Biosphere Program, and the
Florida State University COFRS program. The data analysis was aided by a grant from the Northwest
Florida Water Management District. Analytical work was carried out by L. E. Wolfe and J, Jimeian.
This manuscript benefited greatly from the comments and suggestions of M, Berrigan (Florida De-
partment of Environmental Protection) and two anonymous reviewers.
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DATESUBMITTED:November 26, 1996. DATEACCEPTED:June 25, 1997.
ADDRESSES:(R.J.L., R.L.H.,G.C.W.)
Center for Aquatic Research and Resource Management, Flor-
ida State University, Tallahassee, Florida
32306-2043,
Email: livingst@bio.fsu.edu;
(X.N.)
Depart-
ment of Statistics, Florida State University, Tallahassee, Florida
32306-3033;
(F.G.L.) Northwest Flor-
ida Water Management District, Route 1, Box
3100,
Havana, Florida
32333-9700.
... Oyster landings from Franklin County (dominated by Apalachicola Bay) fluctuated but overall increased from 1950 through the early 1980s, peaking at 3,000 metric tons (6.6 million pounds) in 1981 ( Fig. 3.5). In September 1985, Hurricane Elena caused extensive damage to the bay's reefs, particularly on the east end (Livingston et al. 1999; also see discussion above). Many of the reefs that had historically been the most productive suffered high mortality of live oysters, loss of cultch, and extensive sedimentation (Berrigan 1990). ...
... Landings were nearly an order of magnitude lower than the pre-hurricane harvest in 1985. Oyster populations recovered relatively quickly as a result of successful recruitment, shelling, and restricted harvests (Berrigan 1990, Livingston et al. 1999, but commercial harvests never returned to the levels recorded before Hurricane Elena (Fig. 3.5). ...
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River flow and rainfall patterns in the Apalachicola drainage system were analyzed by time-series methods, and a preliminary comparison was made with commercial harvests and trawl-tow collections of organisms taken during a long-term sampling program. Spectral analysis revealed long-term (5–7 year) cycles in river flow ranging over 25% of the mean flow figures. Cross- spectral analysis indicated that these cycles were somewhat correlated with local (Florida) rainfall but were strongly related to upstream (Georgia) rainfall. A two-parameter model gave a satisfactory fit to the monthly series of annual river flow changes. Dam construction and filling did not appear to be related to long-term cyclic patterns, but weekly cycles in river flow, evident during periods of low water, appeared after dam installations were made. A preliminary comparison of annual river flows with commercial harvests showed strong correlations with oyster and crab catches in associated coastal areas. River flow was weakly correlated with shrimp and crab numbers obtained from a long-term sampling program. There were indications that while long-term river fluctuations were closely associated with commercial landings, such correlations should be carefully scrutinized for possible influence of economic and sociological conditions. Cyclic biological changes driven by key climatological factors may be complicated by highly individualistic species strategies which tend to mask direct phase relationships. However, long-term periodic changes in climatological features of drainage systems need to be considered if the biological variability of such areas is to be explained. There are indications that such cycles may differentially influence population changes at various levels of biological organization.
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A long-term (9.5 yr) study addressed the relationship of the trophic organization of a river-dominated Gulf of Mexico estuary with interannual trends of freshwater input and biological controlling features. Alluvial river flow characteristics were evaluated with respect to Seasonal and interannual changes in physical, chemical, and biological trends in the receiving estuary. Infaunal and epifaunal macroinvertebrates and fishes taken over the period of sampling in the Apalachicola Bay system were transformed into their trophic equivalents. The long-term trophic organization of the bay was then related to observed changes in the physical and chemical conditions in the receiving estuary with particular attention to long-term response to a 2-yr drought. Within limited natural bounds of freshwater flow from the Apalachicola River, there was little change in the trophic organization of the receiving estuary over prolonged periods. The physical instability of the estuary was actually a major component in the continuation of a biologically stable estuarine system. However, when a specific threshold of freshwater reduction was reached during a prolonged natural drought, we suggest that the clarification of the normally turbid and highly colored river-estuarine system led to rapid changes in the pattern of primary production, which, in turn, were associated with major changes in the trophic structure of the system. Increased light penetration due to the cessation of river how was an important factor in the temporal response of bay productivity and herbivore/omnivore abundance. There was a dichotomous response of the estuarine trophic organization, with herbivores and omnivores responsive to river-dominated physicochemical factors whereas the carnivores responded to biological factors. Trophic response time could be measured in months to years from the point of the initiation of low-flow conditions. The reduction of nutrient loading during the drought period was postulated as a major cause of the loss of productivity of the river-dominated estuary during and after the drought period. Recovery of such productivity with resumption of increased river hows was likewise a long-term event. Based on the observed trends in the bay, postulated permanent reductions of freshwater flows due to anthropogenous activities could lead to major reductions of biological productivity in the Apalachicola Bay system. The long-term data indicated that, with reduction of freshwater flow below a level specific for the receiving system, the physically controlled, highly productive river-estuarine system would become a species-rich, biologically controlled bay with substantially reduced productivity.
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Oysters which set early in the spawning season reach sexual maturity and spawn before the end of their first year. The early sexual development in the southern latitudes is the result of both an extended growing season and an increased instantaneous rate of growth. Widespread spawning of oysters older than one year occurs twice during the reproductive season, with a period of renewed development in between. Spawning by young-of-the-year oysters is of minimal importance to population recruitment during the year they set.
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A hurricane caused heavy mortality among aquatic animals in north Florida Bay in September 1960. Fish and invertebrates were stranded by retreating salt water which had been driven inland, or were killed by mud suffocation or turbulence. Oxygen depletion due to decomposition of organic material caused subsequent mortality. Salinities returned to normal within 6 weeks, but dissolved oxygen concentrations remained abnormally low for a longer period. Fish and invertebrates were scarce for several months in the areas of greatest oxygen depletion. When environmental conditions again became suitable, the stricken areas were recolonized from surrounding regions. Sport-fish catches in the area declined immediately after the storm, but recovered within one to several months, depending on the locality. Catch statistics indicate that after the storm juvenile pink shrimp moved from their estuarine nursery grounds into deeper water about 60 miles offshore, where they were caught by the fishery. There is no evidence that the aquatic fauna of the area suffered any permanent damage.
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Four main contributions are made toward understanding stability and clarifying the ambiguity that has surrounded the term stability in the past. (1) Four different aspects of the response of populations to environmental stress-resistance, resilience, persistence, and variability-are given mathematical formulations. (2) Methods are given to analyze a differential equation model of population growth for resistance and persistence, while resilience corresponds to the traditional Lyapunov asymptotic stability. (3) The relation of resistance and resilience to each other depends on the underlying characteristics of negative feedback to population changes and sensitivity of the growth rate to the environment. Strong feedback mechanisms which are independent of the environmental factor causing the stress increase both resilience and resistance, but if the feedback mechanism makes the population growth rate sensitive to an environmental factor, the amount of resilience and resistance under changes in that factor w...
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The resilience of a natural ecosystem here refers to the ecosystem's ability to repair itself following disturbance and inertia to its ability to resist change when stressed. This paper defines four components of resilience, each of which is amenable to quantification using existing ecological methods. Some standardization in the way in which we conceptualize and measure ecosystem inertia and resilience could aid in the development of improved methods of environmental impact assessment.