• Enhancement of subtidal eastern oyster, Crassostrea virginica, recruitment using mesh bag enclosures

      O'Beirn, F.X.; Walker, R.L.; Heffernan, P.B. (National Shellfisheries Association, 1996)
      Eastern oysters, Crassostrea virginica, in the southeastern United States are found predominantly in the intertidal zone. In this study, mesh bags (3 and 6 mm) were deployed over collecting frames, and the patterns of oyster settlement on these collectors were compared against unmeshed controls at three tidal heights (intertidal, low water, and subtidal) over three sampling regimes (biweekly, monthly, and seasonal) at two sites. Within the biweekly sampling regime, the meshed collectors and controls had similar patterns of settlement at the respective tidal heights. For monthly samplers, mesh treatments maintained higher settlement subtidally whereas controls had highest settlement on the collectors at mean low-water level. Controls had highest recruitment intertidally for seasonal collectors, whereas mesh treatments had higher recruitment lower in the intertidal zone. Conclusions from this experiment were that the use of mesh-covered collectors enhanced subtidal oyster recruitment. Causes of observed increases in subtidal settlement in mesh collectors over unmeshed controls over time could be the result of a combination of factors: predator exclusion, larval entrainment, or reduced desiccation, which seemed to overcome the detrimental effects of increased fouling, resulting in reduced flow and possible hypoxic conditions within the mesh bags. Given the degree of recruitment and the sizes of the recruits attained within the mesh bags, the use of these methods to attain juveniles for commercial purposes would appear to be both feasible and viable, particularly for long periods (up to 6 1110) of deployment.
    • Pea Crab, Pinna theres ostreum Say, 1817, in the eastern Oyster, Crassostrea virginica (Gmelin, 1791): prevalence and apparent adverse effects on oyster gonad development

      O'Beirn, F.X.; Walker, R.L. (California Malacozoological Society, 1999)
      Incidence of pea crab, Pinnotheres ostreum Say 1817, infestation in the eastern oyster, Crassostrea virginica (Gmelin, 1791), was recorded and related to oyster gametogenic activity over 18 months. Sampling occurred at twO tidal heights (high intertidal HI and low intertidal LI) at two sites (House Creek, HC and Skidaway River, SR) in Wassaw Sound, Georgia. Overall, incidence rates were 3% HC LI, 1 % HC HI, 8% SR LI, and 4% SR HI. At both tidal heights at HC, no differences were observed in gonad area between those oysters with and without pea crabs. At SR (where overall incidences were higher), oysters without pea crabs had significantly higher gonad area values than those oysters with pea crabs present. These results suggest that at higher incidences of pea crab infestation, oyster reproductive capabilities could be impacted, and support the claim that the pea crab/oyster relationship is a parasitic one.
    • Preliminary recruitment studies of the eastern oyster, Crassostrea virginica, and their potential applications, in coastal Georgia

      O'Beirn, F.X.; Heffernan, P.; Walker, R.L. (Elsevier, 1995)
      Oyster recruitment was monitored in Wassaw Sound, Georgia from April through October, 1991. The study was initiated to determine precise recruitment patterns of oysters over the 7 month spawning season. One of the goals was to determine the most suitable time for the collection of natural spat for maricultural, fisheries and recreational purposes. Three sites of varying hydrographic characteristics were chosen in Wassaw Sound. The sites varied in terms of temperature and salinity regimes and in their exposure to wind and wave action. Within each site, oyster recruitment was estimated at three tidal heights; subtidally, at mean low water and intertidally, approximately 2 h above the mean low water mark. Sampling took place so as to measure net recruitment over biweekly (BW), monthly (M) and seasonal ( S) periods. The results indicated that the recruitment of oysters in this region of coastal Georgia is protracted, lasting 6 months (May through October). The levels of recruitment were very high relative to other regions on the east coast of the USA, with peak recruitment for the entire study area (x = 2800 spat m- 2 for BW and x = 3020 spat m- 2 for M) occurring between July and September. The intensity of recruitment varied significantly among the three sites. The most sheltered site in terms of wave exposure experienced the highest recruitment (x =4380 spat m- 2 BW; x = 6260 spat m -2 M). This site also had higher overall water temperatures as well as greater daily temperature fluctuations. The least sheltered site had the lowest numbers of young oysters (x = 1000 spat m- 2 BW; X. = 686 spat m- 2 M), as well as having the most stable temperature regime. The biweekly samples generally experienced higher recruitment subtidally at the three sites. The monthly samples had higher numbers of oyster spat at the mean low water mark while the seasonal samples showed significantly higher recruitment intertidally (up to x =7353 spat m-2 ). The shift in recruitment patterns over time is partially attributed to increased subtidal predation pressure on the recruits. The collection of natural spat in relation to the natural fishery, potential maricultural activity and the recreational fishery is discussed.
    • Prevalence of Perkinsus marinus in the eastern oyster, Crassostrea virginica in relation to tidal placement in a Georgia tidal creek

      O'Beirn, F.X.; Dean, C.C.; Walker, R.L. (Marine Environmental Sciences Consortium of Alabama, 1994)
      This experiment was designed to evaluate the effects tidal zonation and bottom placement of the eastern oyster, Crassostrea virginica, have on the prevalence and intensity of the oyster parasite, Perkinsus marinus a suspected causative agent for subtidal oyster mortalities experienced in the coastal waters of Georgia. Twelve growout bags (1 m x 0.5 m; 12.7 mm mesh), each containing 200 oysters, were placed in Jointer Creek, Georgia In March 1992. Bags (3 replicates each) were placed lntertidally and subtidally on the creek bottom and offbottom. Ten oysters per bag were removed monthly for twelve months beginning March 1992, and were inspected for prevalence and intensity of Perkinsus marinus, using the thioglycollate method. Oyster mortality and shell length data were also evaluated. Neither prevalence (p = 0.3505) nor intensity levels (p = 0.2993) of Perkinsus marinus in oysters were significantly different among the treatments. Although there were no significant differences In prevalence or intensity of the pathogen among treatments, the intertidal offbottom treatment had the lowest values most frequently. Perkinsus marinus was present in all replicates every month. Prevalence and intensity of infection followed the typically observed pattern of maximum values in summer and fall and minimum levels in winter. Subtidal bottom oysters experienced higher mortalities (p = 0.0022), but the prevalence and intensity of Perklnsus marinus in oysters were not significantly different between treatments. It appears therefore, that the oyster parasite, Perkinsus marinus is not the discerning factor in the higher mortalities witnessed in oysters placed subtidally on the bottom in the southeastern U.S. coastal waters.
    • Reproductive biology and parasite (Perkinsus marinus) prevalence in the eastern oyster, Crassostrea virginica, within a Georgia tidal river

      O'Beirn, F.X.; Walker, R.L.; Jansen, M.L. (Elisha Mitchell Scientific Society, 1997)
      Recruitment, in 1992, of the eastern oyster, Crassostrea virginica, at one site (Flume Dock) within the Sapelo Island National Estuarine Research Reserve (SINERR) was lower than other sampling sites. Recruitment allied with gametogenesis and parasite (Perkinsus marinus) prevalence within the oysters was examined in 1993, in an attempt to explain the differences observed the previous year. Sampling occurred at three sites (Marsh Landing, Jack Hammock, and Flume Dock) along the Duplin River within the SINERR. Oyster recruitment in 1993 was reduced in all sites within the SINERR. No oyster recruitment was recorded at the Flume Dock site during any monthly collections or on collectors deployed for the duration of the sampling study. Oysters developed gametogenically and spawned at the Marsh Landing site two weeks prior to their occurrence at the Jack Hammock site. Gametogenic development in oysters at the Flume Dock site was retarded a further two weeks where the males appeared to spawn six weeks after the females, which could account for reduced recruitment levels at this site. Approximately, twice as many females as males occurred in all sites. Monthly sampling of prevalence and intensity of Perkinsus marinus (Dermo) revealed high levels of both parameters. Smaller oysters did display lower prevalences of Dermo than larger forms.
    • Toward design criteria in constructed oyster reefs: oyster recruitment as a function of substrate type and tidal height

      O'Beirn, F.X.; Luchenbach, M.W.; Nestlerode, J.A.; Coates, G.M. (National Shellfisheries Association, 2000)
      Restoration of degraded oyster reef habitat generally begins with the addition of substrate that serves as a reef base and site for oyster spat attachment. Remarkably, little is known about how substrate type and reef morphology affect the development of oyster populations on restored reefs. Three-dimensional, intertidal reefs were constructed near Fisherman's Island, Virginia: two reefs in 1995 using surfclam (Spisula solidissima) shell and six reefs in 1996 using surfclam shell, oyster shell, and stabilized coal ash. We have monitored oyster recruitment and growth quarterly at three tidal heights (intertidal, mean low water, and subtidal) on each reef type since their construction. Oyster recruitment in 1995 exceeded that observed in the two subsequent years. High initial densities on the 1995 reefs decreased and stabilized at a mean of 418 oyster/m2. Oyster settlement occurred on all reef types and tidal heights in 1996; however, postsettlement mortality on the surfclam shell and coal ash reefs exceeded that on the oyster shell reefs, which remained relatively constant throughout the year (mean = 935 oysters/m2). Field observations suggest that predation accounts for most of the observed mortality and that the clam shell and coal ash reefs, which have little interstitial space, suffer greater predation. Oyster abundance was consistently greatest higher in the intertidal zone on all reefs in each year studied. The patterns observed here lead to the preliminary conclusion that the provision of spatial refugia (both intertidal and interstitial) from predation is an essential feature of successful oyster reef restoration in this region. In addition, high levels of recruitment can provide a numerical refuge, whereby the oysters themselves will provide structure and increase the probability of an oyster population establishing successfully on the reef.