Abstract

Disease and lack of suitable substrata, along with overharvesting and pollution, have been major factors in the decline of the American oyster, Crassostrea virginica, in Delaware Bay. Recent reports have indicated that some oyster stocks there have acquired a degree of resistance to the major disease causing organism, MSX. It is therefore appropriate that research be directed toward restoring cultch material to traditional oyster grounds. This paper reports on the feasibility of using stabilized coal waste as an artificial substratum for setting and growing oysters.

Introduction

Nearly three decades have passed since the outbreak of MSX, and populations of Crassostrea virginica in Delaware Bay still show no signs of nearing pre-disease levels. While MSX is still responsible for oyster deaths in both Delaware and Chesapeake Bays (1), recent evidence indicates that an acquired resistance to the parasite has greatly reduced mortality rates (2). In light of this renewed vigor, limited substratum availability must be considered when analyzing the continued slump of oyster populations in Delaware Bay. The timely implementation of a concerted and on-going effort to revitalize old Crassostrea virginica beds there could promote a resurgence of this valuable species.

Recently, the coal industry has become interested in providing a new form of artificial substratum for use in coastal waters. Their landfills are currently overburdened with byproducts of coal combustion and pollution abatement which, when stabilized, are chemically compatible with the marine environment (3). The feasibility of stabilized-coal-fired power plant wastes as cultch material for oysters was first suggested after studies on depleted beds in Chesapeake Bay (4). Combining coal waste with an additive such as lime makes it environmentally acceptable for oceanic disposal (3). By consolidating stabilized coal combustion by-products into blocks, a marketable product, suitable for artificial fishing reef formation, is obtained.

Materials and Methods

Larvae were produced in the laboratory at the College of Marine Studies, University of Delaware (5). Five different substrata were available for spat set. Mix 1 was composed of fly ash combined with a by-product of flue gas desulfurization (scrubber sludge) and lime kiln dust in a 58:38:4 ratio. Mix 2 was fly ash and lime kiln dust only in an 85:15 ratio. Mix 3 was bottom ash, fly ash, and lime kiln dust in a 54:28:18 ratio. Mixes 4 and 5 were controls of concrete and oyster shell respectively. Mixes 1-4 were obtained from Valley Forge Laboratories, Devon, PA, as 15.2 cm diameter by 5.1 cm thick discs. Mix 5 consisted of eight to ten whole oyster shells stacked on top of each, other so as to approximate, as closely as possible, the dimensions of Mixes 1-4.

To determine whether substratum age influences setting, mixes 1-4 were soaked in seawater for 39 days, eight days, or zero days prior to addition of larvae. All five substrata were encircled with a 1.75 inch thick section of PVC pipe and secured with nylon wire ties to plastic bread trays (Figure 1). Bread trays were modified so as to allow for exposure of both sides of substrata to larvae.

Figure 1. Modified bread tray containing three coal waste and two control substrata.

Starting one week after larvae had been added to the tank, the height of oysters, taken as the distance from hinge to bill, was measured to the nearest millimeter. Oysters growing on both sides of mixes 1-5 were measured at two week intervals for 11 weeks, and four week intervals thereafter for 16 additional weeks. Approximately three weeks after setting, when an average size of 4-6 mm in height was obtained, one-half of the oysters were transferred to the field. Trays were suspended in a modular form from a raft floating at the mouth of the Broadkill River (Figure 2). Laboratory oysters were maintained for six months after setting while the field portion of the experiment was run for seven weeks post-set. Extensive fouling made it necessary to terminate the field component of the study at that time as the tremendous tunicate and bryozoan population threatened to smother underlying oysters.

Figure 2. Module for field oyster culture. Each module contains three bread trays, each with five substrata per tray.

Oysters used in determination of condition index were dried in an oven at 60 C for 24 hours. Dried tissue and shells were then ashed in a muffle furnace at 500 C for three hours and percent organic content calculated. The two condition parameters analyzed were the ratio:

 (dry soft tissue (g) / dry shell weight (g)) x 1000 and percent organic content. The ash weight was converted to percent organics by the formula:

 (total dry weight (g) - total ash weight (g)/ total dry weight (g)) x 100

Tissue for use in heavy metal determinations was freeze dried and then digested (6). Total cadmium, copper, iron, nickel, and zinc were analyzed on a Perkin Elmer model 630 atomic absorption spectrophotometer. All metal concentrations were compared to uncontaminated oyster tissue standard 1566 from the National Bureau of Standards (NBS), Washington, DC.

Results and Discussion

Where noted, statistical analyses that are significantly different, are based on alpha = 0.05.

1.  Effects of substratum type

a.  Spat Set Density

Initial larval settlement varied significantly with substratum type. Of the 4688 spat that initially set on the five experimental substrata, a statistically higher proportion settled on mixes 3 and 5 (Figure 3). All together, 13.3% of the initial oyster set occurred on mix 1, 16.7% on mix 2, 30.4% on mix 3, 7.5% on mix 4, and 32.1% on mix 5.

The bottom ash, of which mix 3 is primarily composed, is a dark, coarse, granular material that produces a somewhat irregular surface. Most invertebrate larvae prefer to settle on rough as opposed to smooth surfaces (7). It is likely that many tiny bumps and crevices make mix 3 as acceptable a larval substratum as the oyster shells of mix 5 (Figure 3).

Figure 3. Density of initial oyster spat set by substratum (all substratum aging conditions, three weeks post-set, where mix 1= fly ash and scrubber sludge; mix 2= fly ash; mix 3= bottom ash and fly ash; mix 4= concrete; and mix 5= oyster shell. Bars= least significant differences at the 95 percent level of confidence.

b.  Survivorship:
The number of oyster spat that survived over the course of the experiment indicated mortality rates. In both laboratory and field experiments, mortality was essentially zero.

c.  Growth Over Time:
At harvest the mean size of oysters growing on mixes 1-4 was significantly greater than the mean size of oysters growing on mix 5. Rate of growth in field oysters was over seven times that of oysters grown in the laboratory. Perhaps the greatest single factor in the elevated growth rates in field oysters is the almost unlimited availability of food in the oyster's natural environment.
Growth rates of oysters on mixes 1 and 2 in laboratory culture are significantly greater than the growth rate of oysters on mix 5. It is likely that the large numbers of oysters on mix 5 bring about increased competition for food, thereby depressing their growth rate.

d.  Condition of Spat at Harvest

i.  organic content

The organic content of control oysters was lower than the organic content of coal waste oysters. The presence of higher proportions of lime kiln dust in mixes 2 and 3 may contribute to the significantly higher organic content in these oysters when compared to oysters growing on mixes 4 and 5. Mixes 2 and 3 contain approximately 15% lime kiln dust (64-72% CaMgO, 4% MgO) (8), a by-product of the firing of limestone to lime. Limestone, like oyster shell, consists of approximately 95% calcium carbonate (9). Mix 4 does not contain any lime at all.

The dissolution of calcium ions from coal waste containing lime (Ca(OH2)] in seawater has been conclusively documented (10). It has been suggested that encrusting cheilostome bryozoa could obtain some calcium for their calcareous skeletons from coal waste/ lime blocks upon which they were living in Chesapeake Bay (11). It is possible then that coal waste oysters are obtaining an extra source of calcium for their shells from their substratum.

Oysters growing in the lab had a significantly higher organic content than those growing in the field. The closed indoor system would serve to concentrate the calcium ions in the tank where they would be available to oysters growing on all five mixes. Higher calcium levels have been found in the foliated part of shells of both valves of oysters grown in a mariculture facility when compared to oysters grown in the Broadkill River (12).

ii.  dry tissue/shell weight ratio

Although the overall ratio of dry tissue to dry shell weight versus mix alone was not significantly different for lab and field oysters together at harvest, there appeared to be significant differences among individual mixes. The ratio for oysters growing on mix 4 was significantly greater than the ratio for oysters growing on mixes 3 and 5.

It is likely that the lower spat density of mix 4 (Figure 3) contributed to the significantly higher dry tissue to shell weight ratio in oysters growing on mix 4 when compared to mixes 3 and 5. Oysters were able to obtain more food, thereby increasing their meat weight. Indeed, the highest mean dry weight recorded was at harvest for field oysters growing on mix 4.

e.  Heavy Metal Accumulation

Oysters growing on mix 5 had significantly higher concentrations of both zinc and copper than oysters growing on mixes 1-4, although the levels of these two elements in mix 5 oysters were not significantly different from uncontaminated oyster tissue from NBS. Levels of iron, however, were significantly greater in oysters growing on coal waster mixtures 1-3 when compared to control substrata 4 and 5 and NBS uncontaminated oyster tissue. Concentrations of zinc, copper, and iron were significantly higher in oysters grown in the laboratory than in oysters grown in field culture. The concentrations of cadmium and nickel were below levels detectable by AAS without a graphite furnace.

The significantly lower concentrations of zinc and copper in laboratory and field animals growing on coal waste mixtures 1-3 over oyster shell control mix 5, is readily related to the behavior of these metals in coal waste mixtures in seawater. Zinc is actually removed from seawater over time; a similar pattern was also evident for copper (8).

The behavior of iron in coal waste in seawater also reflects the amount of this element measured in oysters growing on these substrata. Iron in mixtures 1, 2, and 3 showed an initial drop in levels in seawater, then a gradual increase in concentration over the duration of the experiment. Iron levels in mix 3 by day 30 actually exceeded the initial concentration in the seawater (8).

2.  Effects of Substratum Age

a.  spat set density and growth over time

Larval settlement varied significantly with substratum age for all mixes combined. A significantly higher number of larvae settled on test substrata aged 39 days when compared to substrata aged eight days or not at all. Nearly 44% of total initial set (as determined three weeks post-set) was found on those substrata that had been aged for 39 days, and is likely explained by the presence of surface films. Most marine larvae are attracted to set on surfaces covered with such films (7).

A two-way ANOVA of mean size versus mix and age revealed no significant differences among oyster mean sizes at harvest on substrata of different ages in either the lab or the field.

Conclusion

Percentage of spat that set on coal waste mix 3 was statistically indistinguishable from percentage of spat that settled on control mix 5, the natural cultch material. Soaking substrata in seawater prior to larval set statistically increased spat set density.

Comparison of growth, mortality, and condition in oysters grown on test and control substrata further demonstrated the acceptability of coal waste as a substratum for oyster spat set. While elevated iron levels were found in oysters on coal waste when compared to controls, concentrations of copper and zinc in these oysters were not significantly different from levels in uncontaminated tissue.

These data establish the suitability of coal waste as a substratum for oysters. Further studies should focus on the optimum composition and shape of coal waste for long term field tests of setting, growth, and harvesting of marketable size oysters.

References

1.  Ford, S.E. Chronic Infections of Haplosporidium nelsoni (MSX) in the Oyster Crassostrea virginica. Journal of Invertebrate Pathology 45: 94-107 (1985).

2.  Haskin, H.H., and S.E. Ford. Development of Resistance to Michinia nelsoni Mortality in Laboratory-Reared and Native Oyster Stocks in Delaware Bay. Marine Fisheries Review Jan/Feb: 54-63 (1979).

3.  Woodhead, P.M. J., J.H. Parker, and I.W. Duedall. Coal Combustion Products-New Substrates for Artificial Reef Construction., pp. 219-224. In D.Y. Aska (ed.), Artificial Reefs; Conference Proceedings. Florida Sea Grant College Report No. 41 (1981).

4.  Humphries, E.M. The Utilization of Stabilized Coal-Fired Power Plant Wastes in an Aquatic System. Chesapeake Science 4:269 (1981).

5.  Ewart, J.W. Support Activities- Hatchery., pp. 87-97. In E.T. Bolton (ed.), Intensive Marine Bivalve Cultivation in a Controlled Recirculating Seawater Prototype System. DEL-SG-07-82 (1982).

6.  Cross, T.E. Trace Metal Uptake by Mussels Exposed to Suspended Coal Waste Particulates. Master's Thesis, Department of marine Environmental Sciences, State University of New York at Stony Brook, 81 p. (1982).

7.  Crisp, D.J. Factors Influencing the Settlement of Marine Invertebrate LArvae., pp. 177-265. In P.T. Grant and A.M. Mackie (eds.), Chemoreception in marine Organisms. Academic Press (1974).

8.  Scudlark, J.R., T.M. Church, J.M. Tramontano, P.L. Salevan, and G.A. Cutter. Ashreef Project Trace Element Solubility Studies. CMS-03-86. College of Marine Studies, Univ. Newark and Lewes, DE (1986).

9.  Galtsoff, P.S. The American Oyster, Crassostrea virginica. Fishery Bulletin of the Fish and Wildlife Service. 64:480 p. (1964).

10. Duedall, I.W., J.S. Buyer, M.G. Heaton, S.A. Oakly, A. Okube, R. Dayal, M. Tatro, and others. Diffusion of Calcium and Sulfate Ions in Stabilized Coal Wastes., pp. 375-395. In I.W. Duedall, B.H. Ketchum, P.K. Park, and D.R. Kester (eds.), Wastes in the Ocean- Industrial and Sewage Wastes in the Ocean. John Wiley & Sons (1983).

11. Humphries, E.M., I.W. Duedall, and S.J. Jordan. Coal-Waste Fouling Blocks as a Fouling Substrate in Estuarine Water., pp. 613-649. In I.W. Duedall, D.R. Kester, P.K. Park, and B.H. Ketchum (eds.), Wastes in the Ocean- Energy Wastes in the Ocean. John Wiley & Sons (1985).

12. Carriker, M.R., R.E. Palmer, L.V. Sick, and C.C. Johnson. Interaction of Mineral Elements in Sea Water and Shell of Oysters (Crassostrea virginica Gmelin) Cultured in Controlled and Natural Systems. J. Exp. Mar. Biol. Ecol. 46: 279-296 (1980).