Oyster Crassostrea virginica Spat Settlement as it Relates to the Restoration of Fish River Reef in Mobile Bay, Alabama

Transcription

1 JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 31, No. 4 December, 2000 Oyster Crassostrea virginica Spat Settlement as it Relates to the Restoration of Fish River Reef in Mobile Bay, Alabama IMAD G. SAOUD, DAVID B. ROUSE,' RICHARD K. WALLACE, JEFFREY HOWE, AND BLAN PAGE Department of Fisheries and Allied Aquacultures, Swingle Hall 203, Auburn University, Alabama USA Ahsrrucr.4yster spat settlement at four oyster reefs in Mobile Bay, Alabama, USA, was studied from August through October 1998 and May through mid November Spat collectors at the reefs were replaced every 2 wk and spat-set estimated as number of oysters per meter square per day. Water quality data at Fish River Reef was monitored using remote sensors. Spatset data revealed significant variation between the four sites and between the 2 yr. Spat settlement was 5 to 10 times greater at the other three reefs than at Fish River Reef. Dates and intensity of oyster settlement at Fish River Reef were different from dates and intensity of oyster settlement at Shell Bank Reef, both on the eastern side of the bay. However, settlement was similar between Cedar Point Reef and White House Reef, both on the western side of the bay. Spat set appears to occur 3 wk after a rapid decline in water temperature, provided adequate oxygen concentrations are present at the time of settlement. Data collected suggest that intensity of settlement at Fish River Reef is considerably less than at other reefs in this study but could be adequate to reestablish the reef, if cultch and environmental conditions are suitable. The data also suggest that the source of larval oysters at Fish River Reef is different from the source of larval oysters at the other sites tested in the present study. Several hard-bottom, mud-free areas exist in Bon Secour Bay on the eastern side of Mobile Bay, Alabama, yet few oysters grow in these areas (Smith 1999). Historical data suggest that these areas were once viable oyster reefs (Ritter 1895; Eckmayer 1979). A variety of factors have been suggested as explanations for the lack of harvestable oysters on the relic reefs of Bon Secour Bay (Ritter 1895; Mackin 1951; May 1972, 1973; Eckmayer 1979; Cake and Eckmayer 1982; Eckmayer 1983). Most studies have reported die-offs during particular events but research-based data that explains why viable oyster populations I Corresponding author. are not present today is lacking. Reasons for lack of viable oyster populations discussed include a lack of cultch on the reefs, and possibly a lack of adequate oyster-spat recruitment. An integral part in the management of oyster reefs is the need for adequate substrate for larval settlement. A survey of the reefs of Bon Secour Bay in 1998 indicated a near-total absence of cultch material on Fish River Reef (Smith 1999). The Marine Resources Division (MRD) of the Alabama Department of Conservation and Natural Resources has indicated an interest in deploying oyster shell as cultch at Fish River Reef to help restore the reef. If cultch is deployed too early, siltation and fouling may decrease or eliminate potential settlement and recruitment of spat (Ingle 1951; Shaw 1967; Hoese et al. 1972; MacKenzie 1977; Osman et al. 1989). Timely cultch dispersal requires a proper understanding of oyster population dynamics, reproductive cycles and larval dispersal patterns. Kennedy (1996) provides a review of factors related to the timeliness of cultch dispersal, oyster reproduction, larval dispersal and spat settlement. Oyster spatfall in Mobile Bay normally has two peaks, the higher of which is the fall set (Hoese et al. 1972; Lee 1979; Hayes and Menzel 1981). The present study was performed to evaluate the incidence of oyster settlement at Fish River Reef in order to determine if settlement is adequate to restore the reef should new cultch be deployed. Settlement at Fish River Reef was compared to settlement at three other reefs in Mobile Bay during one spring and two fall spawning periods. 0 Copyright by the World Aqunculture Society 2000 L A n

2 OYSTER SPAT SETI'LEMENT IN MOBILE BAY 641 Guli of Mexico --- WALE 1.urn l0 KIIOHETEl)O FIGURE 1. Mobile Bay. Alabama. Point a = Fish River Reef; h = Shell Bank Reef; c' = Cedar Point Reej d = White House ReeJ Materials and Methods Study Sites Spat collectors were deployed at the Fish River Reef and at three other reef locations in Mobile Bay for purposes of comparison. Study sites included four reefs; two near the eastern shore and two along the western shore of Mobile Bay. All sites were in the southern half of the bay. Fish River Reef (FRR), the focus of this study, is a 12-ha reef located on the east side of Mobile Bay, approximately 3 km south of the mouth of Fish River and 0.5 km from the eastern shoreline of Mobile Bay (Fig. 1; point a). Spat collectors were attached to three pilings (one set per piling) set at the reef by MRD, and oyster shell was dispersed around the foot of the pilings. Temperature and oxygen concentrations 10 cm above bottom were measured hourly using remote measuring devices (Minisonde by Hydrolab) attached to one of the pilings. On 27 October 1998, 15 shells were gathered from the bottom at FRR and inspected for newly set spat. Shellbank Reef (SBR) is located about 10 km south of FRR, and about 0.5 km north of Fort Morgan Peninsula (Fig. 1; point b). This site has also been set apart with pilings installed by the MRD. Spat collectors were attached to three pilings and replaced following the same protocol as at FRR. Cedar Point Reef (CPR) is the primary reef of commercial oyster harvest in Mobile Bay, and is located on the western side of the bay between the mainland and Dauphin Island (Fig. 1; point c). Three sets of spat collectors were attached to pilings within the reef and monitored following the regimen described for FRR. White House Reef (WHR) is also located on the western side of Mobile Bay, south of the mouth of Fowl River. The reef is narrow, extending for more than 2 km in a north-south direction. MRD installed pilings similar to those of FRR and SBR about 5 km south of the mouth of Foul River and 1 km from the western shore of Mobile Bay (Fig. 1; point d). Spat collectors were attached to three of the pilings and monitored similarly to the three other sites. On 27 September humcane Georges destroyed two of the pilings at SBR, and only one set of spat collectors was attached to the remaining piling for the rest of No spat collectors were deployed at WHR in Spat Collectors Collectors used in the study (Fig. 2) were similar to the collectors described by Supan (1983). Square plates of cement-fiber board (10 cm X 10 cm X 0.5 cm) were cut and holes drilled in the middle and on one corner of each plate. Plates were held together

3 642 SAOUD ET AL. FIGURE 2. A-Top view of spat collecting plate with PVC ring around the hole in the middle. B-Side view of IWO plates separated by PVC spacer, and held together by cable-tie at corner. The polypropylene rope passes through the middle hole und is knotted to hold the plates. C-A piling with the spat collecting setup and the Minisonde attached to it.

4 OYSTER SPAT SETnEMENT IN MOBILE BAY 643 TABLE 1. Total number.?f' oyster spur on collectors (.spat/m' per d) at jbur locations in Mobile Bav between 10 August und 27 October Date FRR SBR CPR WHR 10 August August September ,728 6 October 1998 I October in pairs with cable-ties inserted through the comer holes (Fig. 2A). The plates were separated by a 0.6-cm piece of PVC pipe, 1.25 cm in diameter (Fig. 2B). Collectors were attached to polypropylene rope stretched between a brass ring on the piling 15 cm below mean low tide and a trailer anchor on the bay bottom. Spat collectors were attached 10 cm, 60 cm and 150 cm above the bottom of the bay (Fig. 2C). The top collector was approximately 90 cm below the surface of the water. Clean oyster shell was deployed on the bottom near the base of the pilings at FRR in Spat collectors were exchanged approximately every 2 wk. Visual observations of the shell deployed at the base of the pilings at FRR were made for Comparison with the spat collectors deployed at FRR. Spat Counting and Calculations After collection, spat collecting plates were rinsed and allowed to dry overnight. Spat numbers on the inside surface of each plate (surfaces facing paired plate) were recorded separately. Data from plates at each depth were pooled. Spat set (count/m2 per d) was calculated according to the formula: Spat set = (N X 100)/(A X D) Where: N = A = D = the sum of all spat counted on inside surface of each plate at one site. number of plate sides counted (100 cm2 per side). number of days the plates were deployed. TABLE 2. Total number of oyster spat on collectors (spat/m' per d) at three locations in Mobile Bay between 18 May and 19 November Date FXR SBR CPR 18 May June July August August September September October November November Colonization of fouling organisms such as barnacles, bryozoans, and mussels on the collectors was also estimated. Estimations throughout the study were performed by the same person in order to avoid bias in reporting the degree of plate coverage. Abundance was reported as degree of plate coverage on a scale of 0 to 5, with 0 indicating no occurrence and 5 denoting almost total plate coverage. Analysis of Variance (ANOVA) was performed to test the null hypothesis that spat set was equivalent at the four study sites (a = 0.05). Settling rates at the three experimental depths were compared using oneway ANOVA (a = 0.05). Student's paired two-sample t-test (a = 0.05) (Steel and Torrie 1980) was used to examine differences between settling rates on the various sides of the collector plates. Temperature was plotted against time in an attempt to examine the relationship between temperature changes and peak spawning events. Oxygen concentrations in the vicinity of the spat collectors were plotted to examine the relationship between time of set and dissolved oxygen concentrations. Results Spat data from the four reefs showed considerable variation between sites and between years (Tables 1, 2). As reported in earlier studies, the strongest peaks occurred in the fall at all sites. Cedar Point Reef had

5 644 SAOUD ET AL the most consistent spat set with at least some settlement from 10 August through 27 October 1998 and 18 May through 17 October Peak settlement at CPR occurred between 24 August and 8 September in 1998 (840 spat/m2 per d) and between 8 September and 27 September in 1999 (2,300 spat/m2 per d). White House Reef also had an extended settlement period throughout the fall of 1998 with a peak set occumng between 24 August and 8 September (1,730 spat/m2 per d). Spat settlement at the two sites on the east side of Mobile Bay was confined to fewer months and usually at reduced levels. Spat were collected at SBR from 24 August to 6 October 1998 with a peak occurring on 24 August (1,450 spat/m2 per d) and from 23 August through 17 October 1999, with a peak occurring on 27 September (660 spat/m2 per d). Fish River Reef had the shortest spat settlement period and the smallest peak set. Spat were collected on 6 October (170 spat/m2 per d) in 1998, and on 23 August (280 spat/m2 per d) in Over 95% of the spat settled on the interior surfaces of spat collectors. Significantly more oysters settled on the upper side of lower plates than on lower side of upper plates (66% vs. 34%; P < 0.05; N = 2,586). The rate of oyster settlement on plates at 10 cm, 60 cm and 150 cm above bottom was not significantly different when settling rates at each depth throughout the fall season are pooled together. However, the numbers of oysters that settled at the various depths during each sampling period were different from each other, with settlement occurring mostly in the upper water column during one period and near bottom during another. Barnacles Bulunus sp. settled on the plates in high numbers throughout the study. Most barnacles settled on the outer surfaces of the plates; few barnacles were observed on the inner surfaces. Bryozoans Membruniporu sp. seem to colonize inner and outer surfaces of the plates in equal abundance. Mussel Ischudium recurvum at- tachment to the collectors was rare to nonexistent throughout the duration of the study. Visual observations revealed a heavy infestation of adult mussels on the bottom at FRR. Mussels were less abundant on bottom at SBR and WHR and rare at CPR. Figs. 3 and 4 show peak sets of barnacles and bryozoans at the various reefs in 1998 and 1999, respectively. Out of the 15 shells gathered on 27 October 1998 from the bottom at FRR, only one shell had a single spat attached to it. The spat was 52 mm in height, suggesting that it had set early, probably during the first couple of weeks after the shells were deployed. Visual observations, when possible, revealed a fine layer of silt covering the shells. Disturbances to the shells and surrounding water raised a thick cloud of silt and exposed cleaner shell. Very small numbers of fouling organisms were found on the shells. During the fall of 1998 water temperature at FRR dropped by 6.3 C ( C) over the 3-d period from 31 August to 3 September (Fig. 5A). Oxygen concentrations near the bottom were above 1.0 mgl from 13 to 18 September, and periodically dipped below 1.0 mgl on 18 and 19 September (Fig. 6A). Water temperature dropped 3.3 C (32.1 C-28.9 C) between 7 August and 10 August 1999 (Fig. 5B). Oxygen concentrations between 16 August and 23 August 1999 never dipped below 1.0 mg/l (Fig. 6B). Discussion Oyster settlement occurred at all four study sites. At FRR, the total settlement observed during the fall was 4,844 spat/m2 in 1998 and 3,924 spat/m2 in In contrast, settlement observed at CPR, the only actively harvested reef of the four study sites, was 18,850 spat/m2 and 67,350 spat/ m2 during the fall of 1998 and 1999, respectively. Spat settlement at SBR (22,799 spat/m2) and at WHR (27,614 spat/m2) in 1998 was also considerably higher than at FRR.

6 OYSTER SPAT SETTLEMENT IN MOBILE BAY 645 1: Y 2 a: Dates cpr - +Barnacles Bryozoans FIGURE 3. Relative abundance of barnacles and bwozoans attached to collectors placed at four oyster reefs in Mobile Bay in FRR = Fish River Reej; SBR = Shell Bunk Reef; CPR = Cedar Point Reef; WHR = White House Reef Assuming a square meter of reef contains 200 oyster shells, a spat set of 20 to 24 oysters per shell was possible at FRR during the two study periods. Although total yearly spat set at the other reefs was much higher than at FRR, Lindsay et al. (1959) and MacKenzie (1981, 1996) suggest that between 2 and 35 spat per oyster shell constitute a commercial spat set. Therefore, it appears that the spat set on the collectors at FRR during the fall of 1998 and 1999 was within levels sufficient to constitute a commercial set. Hoese et al. (1972) drew a line from just northeast of Cedar Point to near Fort Morgan, dividing Alabama waters into an area of significant spat-fall to the southwest, and an area of negligible spat-fall to the northeast. Our study conducted 26 yr later indicates that these differences between the west and east side of the Bay are still very evident. Results of our study also point out other differences in spat settlement between various areas of the Bay. In 1998, settlement peaks in CPR and WHR on the west side of the bay were coincident, and settlement at SBR on the southeastern side occurred 2 wk prior to peak settlement at the two western sites. At FRR, peak settlement was 4-6 wk behind the other three sites in 1998, and 4 wk ahead of the other sites in This suggests a different source of larvae and/or stimuli to initiate spawning at FRR than the other three Iocations. Austin

7 646 SAOUD ET AL. 9: b 1 0 M. SBR at. FIGURE 4. Relative abundance of barnacles and bryozoans aiiached to colleciors placed at three oysier reefs in Mobile Bay in indicaies no occurrence and 5 indicaies full coverage. FRR = Fish River Reef; SBR = Shell Bank ReeJ CPR = Cedar Point Reef: (1954) described flood tides entering and flowing up the east side of Mobile Bay. However, during ebb tide, water from Fish River flows towards FRR and water from Bon Secour River and the intercoastal canal flows towards SBR. Water temperatures of the Bon Secour River and intercoastal canal may be different from water temperature at Fish River, leading to non-coincident oyster spawns in the two systems. Results of the present study corroborate the findings of Hoese et al. (1 972) and Lee (1979) concerning peak spat sets in the fall in Mobile Bay. Based on Hayes and Menzel s (1 98 1) suggestion that oyster spawning in the fall is cued by a rapid decline in water temperature, it is possible that spat that set on FRR in 1998, were spawned between 31 August and 3 September (Fig. 5A) and in 1999, between 7 August and 10 August (Fig. 5B). In 1998, the rapid temperature decline coincided with a rapid decline in salinity from 17.1 ppt to 12.9 ppt (Fig. 5A) suggesting a high input of cool freshwater into Bon Secour Bay. Similarly, salinity between 7 and 10 August 1999 fell from 24.1 ppt to 16.3 ppt. Prytherch (1928) reports a planktonic larval period for oysters of 13 to 16 d, when temperatures are between 21.3 C and 23.2 C. Baker and Mann (1992) found that oyster pediveligers avoid settling in hypoxic conditions. Therefore, in 1998, it is possible that pediveligers at FRR settled between 13 and 17 September, when oxygen concentrations were above 2 mg/l (Fig. 6A) and in 1999 they set on August 19 when oxygen concentrations were greater than 4 mg/l (Fig. 6B) for several hours. These findings suggest that by monitoring water temperature at FRR, one could predict appropriate times for cultch deployment. The fact that no oyster settlement took place on the oyster shell at the base of the pilings at FRR underscores the need for timely cultch deployment. Excessive fouling (Osman et al. 1989) and, as in the present case, silting can deter oyster settling (Ingle 1951; MacKenzie 1983). Shaw (1967, 1969) proposed dates of cultch planting in the Chesapeake Bay, but Kennedy (1980) and Hargis and Haven (1988) cautioned that dates were not identical every year and suggested the need for monitoring oyster spawning before spreading cultch material. If cultch deployment and spawning are well coordinated, silting can be minimized as a deterrent to setting (Ingle 1951). If a rapid temperature decline is the

8 OYSTER SPAT SETTLEMENT IN MOBILE BAY o^ e c g.f l5 I 10 35, E 1: c o-Temperature I r l5 I 10 0 ZO-AUg 25-Aug 3O-Aug 5-SOP 1O-Sep 16-Sep Pl-Sep FIGURE 5. A-Temperature and salinity at Fish River Reef from 20 August through 21 September B- Temperature and salinity at Fish River Reef from 23 July through 23 August main cue for spawning in the fall, monitoring water temperature over the seed producing beds would notify managers of an impending spat set, ergo optimum cultch dispersal times. Shaw (1967) and Kennedy (1980) discuss contradictory results observed by various workers in regard to the orientation of the surface on which pediveligers prefer to set, without coming to any definite conclusion. Baker (1997) reports that pediveligers prefer lower surfaces of oyster shell to upper surfaces as a way to avoid siltation. Ritchie and Menzel (1969) and Baker and Mann (1998) report negative phototaxis by settling pediveligers. Settling-site selection and subsequent survival of larvae was also shown to be affected by the presence of other resident species (Osman et al. 1989). In the present study, the majority of the spat settled on the two interior surfaces of the collector plates, with most (66%) setting on the top of the lower plate. The fact that most larvae chose the surfaces of the plates facing the interior of the pair could be due to the proximity of the two plates. The narrow space probably provides refuge from predation, and also appealed to the rugotropic characteristic of oyster spat. Another reason could be the shadowing effect of the plates, thus appealing to the negatively phototaxic larvae. Moreover, the narrow space between the plates may have reduced current speed, allowing the oysters more leisure in finding a suitable spot to attach. The period of peak barnacle settlement in

9 648 SAOUD ET AL $: r4 P3 O I FIGURE 6. A-Oxygen concrniralion (mg/l) near botiom at Fish River Reef from I3 September through 19 Sepiember B-Oxvgen concentration (mg/l) near boiiom ai Fish River Reef from 9 Augusi through 23 August Mobile Bay has been reported to occur in the spring with limited settlement in the summer and fall (Hoese et al. 1972; Lee 1979; Athanas 1996). We observed heavy barnacle sets throughout our study, especially on the western side of Mobile Bay. Our results do not agree with the findings of Hoese et al. (1972) that barnacle set in the bay is lowest on the eastern side of Mobile Bay. Bryozoans were found at all stations throughout the sampling period in In 1999, no bryozoans set on the collectors at any of the reefs in May, but were present in all subsequent samples. Lee (1979) reported bryozoan fouling on his collector plates throughout the year at Cedar Point Reef. In this study, collectors were suspended from 10 cm to 150 cm above the bottom. Spat set on these collectors was higher than spat set on the oyster shells deployed on bottom. Spat that settle on cultch dispersed on the bay bottom are susceptible to predation, siltation, and anoxia as well as abrasion from wave action. Therefore, it is possible that spat collectors might over estimate spat set on cultch dispersed on bottom (Pineda and Caswell 1997). The present work confirms the presence of oyster-spat settlement at FRR. However, numbers were lower than spat collected on other reefs in Mobile Bay known to be viable and productive. It is possible that deployment of cultch on Fish River Reef could help restore the reef if a lack of suitable settlement surfaces is the limiting factor. However, due to inter-annual variability in setting dates, cultch should not be spread

10 OYSTER SPAT SETTLEMENT IN MOBILE BAY 649 at similar times every year. Oyster spawning and larval presence should be monitored and cultch deployed when optimum set can be expected. Our work suggests that by monitoring water temperature, managers could predict oyster spawning events and consequently estimate the proper time to disperse new cultch. Literature Cited Athanas, S Incidence of fouling at two mariculture sites in Bon Secour Bay, Alabama. Master s thesis. Auburn University, Alabama, USA. Austin, G. B On the circulation and tidal flushing of Mobile Bay, Alabama, part 1. Texas A&M Research Foundation Project 24: Baker, P Settlement site selection by oyster larvae, Crrtssostrea virginica: evidence for geotaxis. Journal of Shellfish Research 16( I): Baker, P. and R. Mann Effects of hypoxia and anoxia on larval settlement, juvenile growth and juvenile survival of the oyster Crassostrea virginica. Biological Bulletin 182: Baker, P. and R. Mann Response of settling oyster larvae, Crassostrea virginica, to specific portions of the visible light spectrum. Journal of Shellfish Research 17(4): Cake, E. W., Jr. and W. J. Eckmayer The Alabama oyster industry: its history, recent advances, growth deterrents, and future research needs. Pages in K. K. Chew, editor. Proceedings of the North American oyster workshop, 198 I, Seattle, Washington. Louisiana State University, Baton Rouge, Louisiana, USA. Eckmayer, W. J The oyster fishery in Mobile Bay, Alabama. Alabama Marine Resources Bulletin: Eckmayer, W. J Growth and survival of hatchery-reared American oysters set on three types of cultch and in Bon Secour Bay, Alabama. North American Journal of Fisheries Management 3: Hargis, W. J., Jr. and D. S. Haven Rehabilitation of the troubled oyster industry of the lower Chesapeake Bay. Journal of Shellfish Research 7(2): Hayes, P. F. and R. W. Menzel The reproductive cycle of early setting Crassostrea virginica (Gmelin) in the Northern Gulf of Mexico, and its implications for population recruitment. The Biological Bulletin 160: Hoese, H. D., W. R. Nelson, and H. Beckert Seasonal and spatial setting of fouling organisms in Mobile Bay and Eastern Mississippi Sound, Alabama. Alabama Marine Resources Bulletin: Ingle, R. M Spawning and setting of oysters in relation to seasonal environmental changes. Bulletin of Marine Science of the Gulf and Caribbean 1: Kennedy, V. S Comparison of recent and past patterns of oyster settlement and seasonal fouling in Broad Creek and Tred Avon River, Maryland. Proceedings of the National Shellfisheries Association 70: Kennedy, V. S Biology of larvae and spat. Pages in V. S. Kennedy, R. I. E. Newell. and A. E Eble, editors. The Eastern Oyster Crassosrrea virginica. Maryland Sea Grant. College Park, Maryland, USA. Lee, C Seasonal and spatial study of oyster spat in Mobile Bay and East Mississippi Sound. Final Report Sea Grant Project # 40(4) MASGP Mississippi-Alabama Sea Grant Program, Ocean Springs, Mississippi, USA. Lindsay, C., R. E. Westley, and C. S. Sayce Prediction of oyster setting in the state of Washington. Proceedings of the National Shellfisheries Association 48: MacKenzie, C. L., Jr Development of an aquaculture program for rehabilitation of damaged oyster reefs in Mississippi. Marine Fisheries Review 39(8):1-13. MacKenzie, C. L., Jr Biotic potential and environmental resistance in the American Oyster (Crassostrea virginica) in Long Island Sound. Aquaculture 22: MacKenzie, C. L., Jr To increase oyster production in the Northeastern United States. Marine Fisheries Review 45(3): MacKenzie, C. L., Jr Management of natural populations. Pages in V. S. Kennedy, R. I. E. Newell and A. E Eble, editors. The Eastern Oyster Crassostrea virginica. Maryland Sea Grant. College Park, Maryland, USA. Mackin, J. G Histopathology of infection of Crassostrea virginica (Gmelin) by Dennocysridium marinurn. Bulletin of Marine Science of the Gulf and Caribbean l( 1): May, E. B The effect of floodwater on oysters in Mobile Bay. Proceedings of the National Shellfisheries Association 62: May, E. B Extensive oxygen depletion in Mobile Bay, Alabama. Limnology and Oceanography 18(3): Osman, R. W., R B. Whitlatch, and R N. Wac Effects of resident species on recruitment into a community: larval settlement versus post-settlement mortality in the oyster Crassostrea virginica. Marine Ecology-Progress Series 54: Pineda, J. and H. Caswell Dependence of settlement rate on suitable substrate area. Marine Biology Prytherch, H. F Investigation of the physical

11 650 SAOUD Er AL. conditions controlling spawning of oysters and the occurrence, distribution, and setting of oyster larvae in Milford Harbor, Connecticut. Bulletin of the United States Fisheries Commission 44: Ritchie, T. P. and R. W. Menzel Influence of light on larval settlement of American oysters. Proceedings of the National Shellfisheries Association 59: Ritter, H. P Report on a reconnaissance of the oyster beds of Mobile Bay and Mississippi Sound, Alabama. Bulletin of the U.S. Fisheries Commission 15: Shaw, W. N Seasonal fouling and oyster setting on asbestos plates in Broad Creek, Talbot County, Maryland, Chesapeake Science 8: Shaw, W. N Oyster setting in two adjacent tributaries of Chesapeake Bay. Transactions of the American Fisheries Society 98: Smith, L Comparative assessments of three unproductive oyster reefs and a productive reef in Mobile Bay, Alabama. Master s thesis. Auburn University, Alabama, USA. Steel, R. G. D. and J. H. Torrie Principles and procedures of statistics: a biometrical approach. McGraw-Hill Inc., New York. Supan, J Evaluation of a leased oyster bottom in Mississippi Sound. Gulf Research Reports 7(3):