Restoration of oyster reefs along a bio-physical gradient in Mobile Bay, Alabama.
KEY WORDS: oyster reef, Crassostrea virginica, survivorship, Gulf of Mexico, cost-benefit
Reefs formed by the eastern oyster, Crassostrea virginica (Gmelin, 1971), have long been recognized for the valuable fishery they support. This fact, combined with increasing recognition of the many ecological services oyster reefs provide to estuarine ecosystems, has resulted in increased efforts to restore degraded oyster reefs along the United States Atlantic and Gulf of Mexico coasts (Breitburg et al. 2000, Peterson et al. 2003, Luckenbach et al. 2005). Because dense aggregations of live oysters provide 3-dimensional habitat and filter large quantities of water, healthy oyster reefs provide a range of ecological benefits to estuarine ecosystems. For example, the complex structure provided by oyster reefs provides both refuge and foraging habitat for a diverse assemblage of invertebrates and small fishes (e.g., Breitburg 1999, Coen et al. 1999, Harding & Mann 1999, Lenihan et al. 2001, Peterson et al. 2003, Grabowski & Powers 2004). Oyster reefs can also stabilize substrates in areas where waves and currents may cause erosion (Meyer et al. 1997). In addition, the suspension feeding activity of oysters has the potential to remove large amounts of particulate material from the water column, which may in turn affect local phytoplankton assemblages, nutrient dynamics, and sediment biogeochemistry (e.g., Dame & Libes 1993, Newell et al. 2002, Cressman et al. 2003, Newell & Koch 2004).
Destructive harvesting practices, poor water quality, and disease have contributed to the dramatic decline of oyster reefs in many United States Atlantic estuaries (Rothschild et al. 1994, Ford 1996, Lenihan & Peterson 1998, Hargis & Haven 1999, Lenihan & Micheli 2000, Kirby 2004). The history of the oyster industry and oyster fishery in Alabama differs from Atlantic states. Harvest of oysters on public reefs occurs primarily by hand-tongs and dredging is permitted on a very limited basis (Wallace et al. 1999). Harvest records exist from 1880, but until 1948 most records were incomplete and actual harvest was unknown (May 1971). Commercial landings from 1950 to 2004 averaged 385,554 [+ or -] 240,404 kg of oyster meat and show high interannual variability with dramatic decreases associated with hurricane land fall (National Marine Fisheries Service 2006). In 1971, oyster reef habitat covered 2,040 hectares of natural bottom in Alabama with an additional 425 hectares of riparian and 374 hectares of leased bottoms for growing oysters (May 1971). Tatum et al. (1995) surveyed the Cedar Point reef area (SW part of Mobile Bay) and estimated 1,407 hectares of subtidal oyster reef coverage, which exceeds the amount of subtidal oyster habitat reported there in the 1970s. Consequently, there is some evidence of expansion of oyster reef habitat. Currently, the Alabama Department of Conservation and Natural Resources (ADCNR) deposits ~2,300 [m.sup.3] of shell cultch per year to enhance the oyster fishery, which may be responsible for the relatively constant mean annual landings (Alabama Department of Conservation and Natural Resources 2005, National Marine Fisheries Service 2006).
Oyster production in Mobile Bay is influenced by several physical, chemical, and biological factors including fecal coliform levels, the occurrence of hypoxic or anoxic bottom waters, abundance of oyster drills, dermo disease, tropical cyclone activity, and overfishing. High densities of oyster drills, Stramonita haemostoma (Conrad, 1837), in high salinity arcas have caused extensive mortality and have reduced populations of adult oysters to levels unsuitable for oyster fishery harvest in many areas. Perkinsus marinus (Mackin, 1950), a protozoan endoparasite that can cause high mortality in adult oysters is prevalent in Alabama; however, no record of large-scale mortality from this disease exists. In contrast, dramatic decreases in landings are well documented as a result of tropical cyclone landfall [e.g., Hurricanes Fredrick (1979), Elana (1985), Opal (1998) and Ivan (2004)]. Extended fisheries closures as a result of high rainfall amounts after high precipitation as well as burial or harvestable reefs by storm-induced sediment transport both contribute to the decline in annual landings and have been a primary motivation behind several oyster enhancement efforts.
Shellfish managers use three basic management and restoration options to repair damaged reefs: additions of shell or other substrate, fishery closures of existing reefs, and active transplanting of oysters (Coen et al. 1999). Central to most oyster reef enhancement/restoration programs is the addition of substrate, shell or other hard material. Several studies have demonstrated how the design and choice of material for reef creation can influence success, generally measured as density of live oysters (see Luckenbach et al. 2005). In most cases, bivalve shell produces higher oyster settlement than alternative material such as concrete or limestone (Soniat & Burton 2005). Recently, vertical relief of oyster reefs has been shown to be an important determinant of oyster recruitment and persistence, especially in areas that experience prolonged periods of hypoxia (Lenihan & Peterson 1998, Lenihan 1999, Breitburg et al. 2000).
We designed a restoration project to test the influence of reef design and reef location on the success of oyster recruitment within Mobile Bay. Specifically, the project addressed two questions: how does the design of an oyster reef influence the recruitment of sessile marine invertebrates and how does this relationship change over environmental gradients?
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MATERIALS AND METHODS
Mobile Bay is located in the north central Gulf of Mexico and is connected to Mississippi Sound on its western side and to the Gulf of Mexico at its southern end. Oyster reefs were created at three sites within Mobile Bay as part of the University of South Alabama's Oyster Reef Restoration Prograto. Two sites are on the west side of Mobile Bay, Cedar Point Reef Area H (30.31[degrees]N, 88.11[degrees]W) and Sand Reef Area A (30.27[degrees]N, 88.09[degrees]W). Cedar Point Reef Area H is located on the north side of the Intra Coastal Waterway (ICW) near the Dauphin Island Parkway Bridge and Sand Reef Area A is located on the south side of the ICW near little Dauphin Island. The third site, Shellbank Reef (30.26[degrees]N, 87.86[degrees]W), is on the east side of Mobile Bay in Bon Secor Bay (Fig. 1). Although all three sites have previously been documented as oyster reef habitat (May 1971), the three restoration sites differed in the amount of existing oyster habitat as well as water quality. Cedar Point, which is the current and historic center of the Alabama oyster fishery, has abundant oyster reef habitat and rarely experiences low dissolved oxygen. Sand Reef has little oyster reef habitat in close proximity and is located in an area with low occurrences of hypoxia or anoxia. In the Shellbank Reef area, live oysters are in very low abundance and the reef area does experience low oxygen conditions (May 1973). A study by Saoud et al. (2000) showed that Bon Secor Bay had periodic anoxic events that lasted anywhere from a few hours to six days. The sites in this study were chosen to compare restoration across biological (abundance of live oysters) and physical (water quality) gradients.
Reef Construction and Design
Four high relief and four low relief reefs were constructed at each of three restoration sites (Cedar Point, Sand Reef, and Shellbank). Reefs were 25 x 25 m and high relief reefs were 100 cm high whereas low relief reefs were 10 cm. All 12 low relief reefs had a limestone marl base (15.96 [m.sup.3]), which was covered by oyster shell (48 [m.sup.3]) and cost $2,068 per reef (material + cost of labor = $15 per [m.sup.3]). Six of the high relief reefs (2/site) had a concrete rubble base (434 [m.sup.3]) and a top layer of oyster shell (140 [m.sup.3]), whereas the other six high relief reefs had a limestone mart base (433.85 [m.sup.3]) and a top layer of oyster shell (140 [m.sup.3]). High relief concrete/shell reefs cost $18,600 and high relief limestone/shell reefs cost $20,292 to construct.
Beginning in March 2004 and continuing through June 2006 monthly depth profiles of physical/chemical measurements were collected at each of the three restoration sites. A Yellow Springs Instrument (YSI) multiparameter unit was used to collect profiles of temperature, salinity, and dissolved oxygen every 0.5 m from the bottom. One profile was usually performed in the early morning (07:00-09:00) and one in the late morning-early afternoon (11:00-13:00).
Oysters and Sessile Invertebrates
To quantify densities of sessile invertebrates on the restored reefs two 0.25 [m.sup.2] quadrats were taken by divers at all 24 reefs in May 2005, October 2005, and June 2006. One diver took a sample near the base (edge) of the reef, whereas another diver took a sample near the crest (center) of the reef. A sample was collected by placing the quadrat on the reef and removing the top 10 cm of oyster shell. The collected shell was placed in a mesh sack and samples were subsequently placed in a large aquarium tank at the Dauphin Island Sea Laboratory where they were held until processing (samples were processed as soon as possible to avoid additional mortality because of aquarium conditions).
In the laboratory, samples were processed by counting the numbers of live oysters, dead oysters, live spat (<30 mm), dead spat (<30 mm), and mussels. Twenty live and dead oysters were measured for shell height (umbo to distal shell edge). Live spat, dead spat, and mussels were each measured to establish a size range. Live oysters, dead oysters, and mussels were separated from the shells they were attached to, and an aggregate mass was recorded for each.
After the samples were processed, the numbers of organisms and their respective masses were converted from numbers or mass per 0.25 [m.sup.2] to numbers or mass per [m.sup.2], and an average size was calculated for each group. Data were analyzed using a three-way ANOVA (StatView Software) with site, reef height, and position as independent variables. Site (df = 2) had three levels (Cedar Point Reef, Sand Reef, and Shellbank Reef), reef height (df = 1) had two levels (high relief reefs and low relief reefs), and position (df = 1) had two levels (base and crest). All factors were fixed in the analysis and considered significant at P < 0.05 or marginally significant at 0.05 < P < 0.10. Cochran's test was used to check for homogeneity of variance in the data. If any calculations rejected the null hypothesis for Cochran's test, data were log transformed and the test repeated. Student-Neuman-Keuls (SNK)post hoc tests were run for all main effects, except those that were included in interactions. Only June 2006 data were analyzed because we felt that previous sampling in May 2005 and October 2005 occurred too early in the succession process of the reefs.
In addition, paired t-tests were run to compare the high relief reefs constructed with the two different materials for the June 2006 oyster quadrats. The numbers of oysters, live spat, dead spat, and mussels were compared on high relief reefs with a concrete rubble base/oyster shell top layer versus reefs with a limestone marl base/oyster shell top layer. Data values, for each species, were the average number of organisms from the base and crest quadrats for each high relief reef. All data were log transformed to meet the assumption of homogeneity of variance.
Bathymetric surveys of each restoration site were completed in January 2006. A Ceeducer DGPS/depth finder system was mounted to a 22 ft. skiff and recorded GPS coordinates and depth for the reef surveys. Each site was surveyed with multiple parallel transects. The data were downloaded into ArcView, converted into decimal degrees, depth was converted to rise off the bottom (subtracted deepest depth from each depth value), and bathymetric maps were made for each reef site. The surveys were done to determine reef elevation following major hurricanes: Ivan (September, 2004), Dennis (July, 2005), and Katrina (August, 2005).
To quantify oyster survival rates, oyster mats were deployed and quantified monthly during summer 2004 and summer and fall 2005. An oyster mat (1 [m.sup.2]) was constructed of 10 mm Vexar mesh. A total of 49 oysters (25-40 mm SH) supplied by the Auburn Shellfish Laboratory had a dab of Z-Spar epoxy resin applied to one hall of each oyster so that a cable tie could be embedded in the resin. After the resin had hardened, the oysters were cable tied to the mar. Mats were deployed at each of the three reef sites (8 mats per site, one mat on each reef) and each mat was cable tied to two rectangular concrete slabs. Mats were brought up monthly to measure oyster shell height and width and to replace any oysters that had died with live oysters.
Data were analyzed using a three-way ANOVA with site, reef height, and month as the independent variables. Site (df = 2) had three levels (Cedar Point Reef, Sand Reef, and Shellbank Reef), reef height (df = 1) had two levels (high relief reefs and low relief reefs), and month (df = 5) had six levels (June 04, July 04, August 04, June 05, July 05, and November 05). All factors were fixed in the analysis. Mortality was calculated as a proportion; the number of dead oysters divided by the total number of oysters on the mat (49) and that number was divided by the number of days the mat was on a reef to give a daily mortality rate. Data were transformed (2*arcsin [square root] x) because of heterogeneity of variances. Student-Neuman-Keuls (SNK) post hoc tests were run for all main effects.
Water quality parameters were typical for Mobile Bay and varied seasonally. Temperature averages ranged from 9.6[degrees]C to 32.8[degrees]C at the surface and 9.5[degrees]C to 32.7[degrees]C at the bottom during the study. High temperatures around 30[degrees]C were typical of all sites in July of 2004, temperatures dropped to around 10[degrees]C in January. Salinity averages ranged from 1-23 ppt at the surface and 1-26 ppt at the bottom. Lower salinity values (5-10 ppt) were associated with major precipitation events that occurred in the spring anal late summer. Higher salinities (18-25 ppt) were associated with periods of reduced rainfall in the fall of 2004 and 2005. Cedar Point Reef values were generally lower by 2-4 ppt than reefs at Sand Reef and Shellbank. Dissolved oxygen levels averaged 5-7 mg [L.sup.-1] during the summer and fall and were between 8-10 mg [L.sup.-1] during the winter and early spring. Average bottom dissolved oxygen values were 6.7, 6.9, and 6.6 mg [L.sup.-1] for Cedar Point, Sand Reef and Shellbank, respectively. Hypoxic conditions were detected only once during the study: 1.7 mg [L.sup.-1] at Cedar Point on July 7, 2004.
Bathymetric maps showed that the reef sites persisted through hurricanes Ivan, Dennis, and Katrina (Fig. 1). All high relief reefs still were approximately one meter of elevation off the bottom. Although height of each reef was not uniform, variation was mainly caused by initial reef construction. The low relief reefs still had areas of shell cover based on diver observations on the northwest sections of the reefs done by 8-10 m circular surveys.
Oysters and Sessile Invertebrates
Quadrat samples were dominated by eastern oyster, Crassostrea virginica, and hooked mussel, Ischadium recurvum (Rafinesque, 1820). Site was the only factor among all 3-way ANOVAs that was consistently significant for oyster abundances (Table 1). The number (P = 0.001, Fig. 2A) and total weight (P = 0.001) of adult oysters differed significantly by site with Cedar Point having higher abundances and weight than Sand Reef and Shellbank Reef. Average density of oysters >30 mm SH was 115 [+ or -] 21 [m.sup.-2] ([+ or -] 1 standard error) for Cedar Point reefs, 80 [+ or -] 27 for reefs at Sand Reef and 18 [+ or -] 5 for Bon Secour reefs. Average size of oysters was significantly (P < 0.001, Fig. 2B) larger at Cedar Point and Shellbank Reefs as compared with Sand Reef. Live spat numbers (P < 0.001, Fig. 2C) were significantly higher at Cedar Point and Sand Reefs versus Shellbank Reef, whereas dead spat numbers (P < 0.001, Fig. 2D) were highest at Sand Reef and significantly lower at the other two sites. Although mussels were abundant, particularly at Shellbank, there were no significant or marginally significant effects.
Comparisons between reef design elements, vertical relief and construction material, revealed trends of higher oyster abundances on high relief reefs, and only minor differences between construction materials. Density of live oyster spat (P = 0.002, Fig. 3) was significantly higher on high relief reefs across all sites, whereas density of dead spat (P = 0.056) were marginally significant. Comparison of oyster density between high relief reefs built with concrete rubble/shell versus limestone marl/shell were not significant for oysters, live spat, or mussels, but were marginally significant for dead oyster spat (P = 0.054 for paired t-test, n = 12) with higher numbers of dead spat on concrete rubble/shell reefs.
Mortalities were only calculated for June 2004, July 2004, August 2004, June 2005, July 2005, and November 2005 for marked oysters because of mat loss from hurricanes or other disturbances in the remaining months. The three-way ANOVA testing the effects of reef height, site, date, and their interactions on oyster mortality demonstrated significant two-way interactions between site and reef height (P = 0.05, Fig. 4) and site and month (P < 0.01, Fig. 5). Overall, mortality was highest at Sand Reef (0.015 oyster [m.sup.-2] [d.sup.-1]), followed by Cedar Point (0.010 oyster [m.sup.-2] [d.sup.-1]) and Shellbank (0.007 oysters [m.sup.-2] [d.sup.-1]). For the interaction between site and reef height, differences between low and high relief were significant only at Sand Reef where mortality was 31% lower on high relief reefs. Mortality rate during the six months included in our analyses were consistently high at Sand Reef showing minor variation among dates (Fig. 5). In contrast, the Shellbank and Cedar Point reefs showed significant differences between months. At both sites, mortality was generally low (<0.004 oyster [m.sup.-2] [d.sup.-1]) in the three months surveyed in 2004, but doubled in 2005 to levels approaching or exceeding those at Sand Reef.
[FIGURE 2 OMITTED]
Overall, the oyster reef restoration project in Mobile Bay was successful in that all reefs had been colonized by oysters during the first 2.5 y. Constructed oyster reefs at the three sites also survived three hurricanes: Ivan in September 2004, Dennis in July 2005, and Katrina in August 2005. Bathymetry plots confirmed high relief reefs maintained reef heights near 1 m off the bottom and visual inspection of the low relief reefs by SCUBA divers revealed that shell cover was still present. Although all reefs had been colonized by oysters, densities of oysters varied significantly among reefs. This variability was best explained as a function of reef location within Mobile Bay (i.e., site effect in the ANOVA) and to a lesser extent by differences in reef elevation.
[FIGURE 3 OMITTED]
The results of our oyster sampling support our original hypothesis that abundance of oysters would differ along a biophysical gradient in Mobile Bay. Oyster reefs at the Cedar Point Reef site had high numbers of spat, low numbers of dead spat, and high numbers of oysters. Reefs at the Cedar Point site were located 2-3 km northeast from the largest expanse of oyster reef in Alabama coastal waters, had high natural oyster recruitment, and the area is not prone to extended periods of low dissolved oxygen. The Sand Reef site had very high numbers of oyster spat, high numbers of dead spat, and had moderate numbers of oysters. The high mortality these spat suffered, based on the number of dead spat in quadrat samples and our monthly marked oyster assays, could be the result of poor water quality or predation by oyster drills. Visual inspection of the water quality data we collected on site as well as examination of water quality measurements collected continuously at the Dauphin Island Sea Laboratory (2 km South of Sand Reef), revealed no low oxygen events. Oyster drills were present in high abundance at Sand Reef (Gregalis and Powers, unpub, data) and were likely responsible for the large numbers of dead spat. The Shellbank Reef site had low numbers of spat, low numbers of dead spat, and therefore had low numbers of oysters. Low oyster production at the Shellbank Reef site was most likely a result of low larval supply, because predation by oyster drills was low (see assay data) as was the abundance of dead oysters in the quadrat, which would indicate no effect of low dissolved oxygen.
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[FIGURE 5 OMITTED]
Oyster mortality as assessed by our mark and recapture technique agreed with the patterns of dead oyster shell found in the quadrat samples. Mortality differed among the three reef areas (Sand Reef > Cedar Point > Shellbank). Based on scarring patterns and the overall condition of the still articulated shells, the vast majority of oyster mortality on the mats appeared to be caused by drills. At Sand Reef where oyster mortality was highest, oyster mortality was lower on high relief reefs. The mechanism creating this elevation pattern remains unclear.
Oysters were not the only sessile invertebrates on the reefs. Recurved mussels were also common at the three reef sites. Mussels are filter feeders like oysters and do provide structure and serve as prey. Despite Shellbank Reef's inability to support high oyster abundance, large numbers of mussels were established on the reefs. The mussels, along with their water filtering capabilities, created habitat for mobile invertebrate and resident fish species and provided feeding areas for demersal fishes.
Although it was encouraging that all reefs had oyster settlement, a cost/benefit comparison illustrates some important points to consider for future oyster reef restoration efforts. The high relief reefs cost on average $19,446.80 per reef and low relief reefs cost $2,068 per reef, so for the price of approximately 1 high relief reef, low relief reefs would cover 10 times the area. Our study showed that reef height did not result in significantly higher abundances of adult oysters; however, oyster spat were more abundant on high relief reefs. Lenihan and Peterson (1998) proposed that oyster mortality was related to vertical relief of reefs and that reef height loss caused by fishing disturbance combined with hypoxic/anoxic conditions increased mortality on low-relief reefs in deeper waters in the Neuse River estuary, NC. They found that experimental deep reefs had mass mortality in oysters and other invertebrates caused by anoxic conditions because of stratification of the water column. The reefs that were elevated into the water column or those in shallow water had oysters and other species that survived the low oxygen conditions. The reefs in Mobile Bay did not experience prolonged hypoxic/ anoxic conditions, but water quality sampling was limited in time and space.
Breitburg et al. (2000) described three general goals of oyster reef restoration that included fisheries, water quality, and habitat and discussed benefits of combining research and restoration activities. One important question they asked that directly relates to our study was whether the cost of building the high relief reefs outweighs the benefits of the restoration (oyster growth, recruitment, and survival). Upon completion of the project, we did find a benefit in reef design in that live spat had higher abundances on high relief reefs, which may be a driving factor for adult oyster abundance in future assessments. However, it is unlikely that the high relief reefs would produce a 10-fold difference in oyster abundance as compared with low relief reefs. In terms of oyster survival, abundance, and reef longevity, high relief reefs are more likely to show benefits with future sampling. It is entirely plausible that longer term monitoring of the reefs may find a more substantial benefit of vertical reef elevation as a result of greater resiliency to fishing disturbance or decreased frequency of anoxic and hypoxic events.
Funding for reef creation and monitoring was provided by a grant from the National Marine Fisheries Service via the University of South Alabama's Alabama Oyster Reef Restoration Program. The authors gratefully acknowledge the assistance of D. Byron, C. Davis, N. Geraldi, G. Miller, J. Herrmann, and B. Furman for assistance in sample collection and analyses. Elements of this project served as the basis for a M.Sc. thesis for KCG submitted to the University of South Alabama.
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KEVAN C. GREGALIS, (1,2) SEAN P. POWERS (1,3) * AND KENNETH L. HECK, JR. (1,3)
(1) Department of Marine Sciences, University of South Alabama, LSCB Room 25, Mobile, Alabama 36688; (2) Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, 350 Carroll St., Eastpoint, Florida 32320; (3) Dauphin Island Sea Lab, 101 Bienville Blvd., Dauphin Island, Alabama 36528
* Corresponding author. E-mail: firstname.lastname@example.org
TABLE 1. Summary of the P values for analysis of variance tests for the oyster quadrat data conducted to test the effects of reef site, reef height, reef position, and their interactions on the dependent variables listed. Analyses were performed on log (X + 1) transformed data. Oyster Quandrats Effects Reef Species Site Height (m) Position No. live oysters/[m.sup.2] 0.001 0.134 0.364 Live oyster avg. size (mm) <0.001 0.462 0.933 Live oyster weight g/[m.sup.2] 0.001 0.146 0.474 No. live spat/[m.sup.2] <0.001 0.002 0.656 No. dead spat/[m.sup.2] <0.001 0.056 0.942 No. mussels/ [m.sup.2] 0.403 0.113 0.202 Oyster Quandrats Effects [Site.sup.*] Reef [Site.sup.*] Species Height Position No. live oysters/[m.sup.2] 0.556 0.891 Live oyster avg. size (mm) 0.679 0.999 Live oyster weight g/[m.sup.2] 0.603 0.920 No. live spat/[m.sup.2] 0.519 0.404 No. dead spat/[m.sup.2] 0.263 0.284 No. mussels/ [m.sup.2] 0.109 0.980 Oyster Quandrats Effects [Site.sup.*] Reef Reef [Height.sup.*] [Height.sup.*] Species Position Position No. live oysters/[m.sup.2] 0.343 0.960 Live oyster avg. size (mm) 0.835 0.997 Live oyster weight g/[m.sup.2] 0.393 0.925 No. live spat/[m.sup.2] 0.512 0.663 No. dead spat/[m.sup.2] 0.382 0.932 No. mussels/ [m.sup.2] 0.147 0.420
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|Author:||Gregalis, Kevan C.; Powers, Sean P.; Heck, Kenneth L., Jr.|
|Publication:||Journal of Shellfish Research|
|Date:||Dec 1, 2008|
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