Restoration of the Oyster Resource in Chesapeake Bay: The Role of Oyster Reefs in Population Enhancement, Water Quality Improvement and Support of Diverse Species-Rich Communities

Restoration of the oyster Crassostrea virginica resource to the Chesapeake Bay is a widely supported goal. The role of the oyster in restoration through benthic-pelagic coupling is examined in the context of current and projected watershed management problems, agricultural and urban development with associated nutrient and sediment erosion issues, in the entire Chesapeake Bay watershed. Efforts to date have focused on rebuilding three-dimensional reef structures, often with oyster broodstock enhancement, in predominantly small estuaries with retentive circulation to provide demonstration of increased resultant recruitment. Fishery enhancement activity is then based on local increases in recruitment. Such examples are used to increase public awareness of the success of restoration processes and increase long-term participation in such programs by schools, non profit and civic organizations, and commercial and recreational fishing groups

Distribution of bivalve larvae at a frontal system in the James River, Virginia

James River is the southernmost of the major subestuaries of the Chesapeake Bay, USA. A frontal system develops on the early flood tide in the Hampton Roads region of the lower James. This system, together with a cyclonic gyre in Hampton Roads, is in part responsible for partial retention of downstream-flowing water in the estuary and it\u27s injection into deeper, upstream-flowing water. The role of the frontal system in retention of bivalve larvae in the James was investigated In a 2-part study: a field exammation of larval distr~bution versus depth along a transect across the front in relation to salinity and temperature of the converging and diverging water masses, and a laboratory examination of the ability of oyster Crassostrea virginica larvae to swim in and through salinity gradients comparable to or greater than those encountered near the frontal system. Field studies indicate that larvae are passively transported through the frontal system and plunge to depth as the more saline water in which they are entrained encounters less saline water The deeper, more saline water flows upstream as it leaves the frontal system. Laboratory studies demonstrate that both straight hinge stage (mean length = 75~) and umbo stage (mean length = 157.5 to 159.7 pm) larvae actively swim through a salinity discontinuity of 3 %D when exposed in a column of 22 960 water overlayed by 19 9b. water (extreme values charactenstic of bottom and surface water at the frontal system). Further, their mean rates of vertical movement (0.37 to 1.02 mms-\u27) illustrate the ability of larvae to move through the depth of the water column in the James in less than one tidal cycle. Pediveliger stage oyster larvae (mean length 317.2 pm), by contrast, restricted swimming to small but frequent excursions above the bottom in the laboratory apparatus and did not swim through the salinity interface. Following passive transport through the frontal system in the lower James straight hinge and umbo stage larvae may employ active depth regulation to redistribute throughout the water column; however, pediveliger stage larvae probably remain near the sediment-water interface

Arctica islandica (Linne) Larvae: Active depth regulators or passive particles?

The seasonal change in depth distribution of Arctica islandica (Linne) larvae at a station on the Southern New England Shelf for the period April-December 1981 is compared with the output of a numerical model designed to predict distribution in a scenario where active depth regulation predominates. Larvae in excess of 200 J.Lm length were present in the field in May at 1-30 m depth and, at depths of 20-40 m from late July through November. The majority of larvae captured in November were shelled veligers of 110 J.Lm length. Good agreement of the model with field data exists with respect to absence of A. islandica larvae in the warm (\u3e 18°C) shallow (0-20 m) waters between July and early September, and the abundance of larvae throughout the depth range 20-40 m from July through October. The model predicts occurrence of larvae in June; however, they were not seen in the field . The discrepancy can be due to the combination of reduced spawning by adult A. islandica (which is not included in the model) and less than optimum conditions for larval development. The model predicts aggregation of the negatively geotactic larvae at the surface following decay of a seasonal thermocline. Such aggregations were not seen in the field indicating that vertical mixing of the water column in the fall months is sufficient to negate distribution patterns dominated by active depth regulation . Depending upon the nature, intensity and stability of stratification of the water column, it is evident that depth distribution of A. islandica larvae can be dominated by either active depth regulation or passive movement at the mercy of physical mixing. The conditions of transition from active to passively dominated dispersal and distribution are poorly defined

Environmental change in the coastal environment: challenges for the selection and propagation of filter feeding species in aquaculture, stock enhancement and environmental rehabilitation

Selection of species for aquaculture, fishery stock enhancement and environmental rehabilitation or restoration in the coastal zone requires consideration of the fact that species have evolved over geological time whereas changes in the coastal environment have occurred predominantly over recent historical time, often with the largest changes occurring within the past decades of human activity. The evolutionary issue is particularly noted with filter feeding molluscs, where extant species supporting both major natural fisheries and aquaculture have ancient lineages and evolved in environments that may have differed considerably from the locally turbid, nutrient enriched, disturbed (through watershed change and local activity) waters in which they now survive. We cannot presume that native species are strongly selected to survive in the environments in which they currently reside. Neither can we presume that they will be successful candidates for aquaculture, fishery stock enhancement, environmental rehabilitation (the restoration of ecological services in community structure), or environmental restoration (restoration of native community structure with associated ecological services). Watershed and coastal use impacts have, over recent human history, altered community structure in coastal waters, and diminished the ability of surviving community members to perform the ecological services that are one end product of their evolution. A challenge is therefore presented to students of intensive species culture, extensive fishery enhancement, and ecological rehabilitation or restoration: how to best use the tools of husbandry in concert with large and small scale environmental manipulation to promote progress in the designated area of interest? Ecological rehabilitation or restoration centered on cornerstone filter feeding species must employ local environmental rehabilitation, but this will only be successful if it is accompanied by a wider commitment to watershed management protocols that protect all life history stages, including the delicate early stages. A numerical argument for this approach, based on Paulik life history models, will be presented. Intensive aquaculture, by comparison, may be able to progress in marginal environments where delicate early life history stages are cultured in controlled situations, thus limiting mortality, before transfer to open systems. Fishery enhancement resides between these options, where a dual role of supplementing local reproduction is balanced against increased exploitation of commercial product

Field studies of bivalve larvae and their recruitment to the benthos: A commentary

A list of factors influencing the recruitment of bivalve larvae might include, but not be limited to, the following: egg quality, physical environment, food availability, loss to predation and disease during larval development, interplay of passive dispersal (horizontally) by water currents and depth regulation by active swimming, proximity of suitable and available substratum as metamorphic competency is achieved, and availability of sufficient metabolic reserves to complete metamorphosis to the benthic form. While tractable methods exist to quantify aspects of certain members of the above list, the focus of such work has usually been biased towards laboratory experiments or hatchery production. The purpose of this commentary is to suggest that a refocussing of efforts in bivalve larval biology on natural systems is both timely and needed

Restoring The Oyster Reef Communities In The Chesapeake Bay: A Commentary

Restoration of the oyster Crassostrea virginica resource to the Chesapeake Bay is a widely supported goal. This manuscript explores the questions of why, how, and in what time frame this should be attempted. Restoration goals based simply on support of a commercial fishery fail to address the role of the oyster as a cornerstone species within the Chesapeake Bay and should only be considered in the context of a long-term sustainable fishery exploitation. The argument is proffered that a restored resource sustaining a fishery at the historical harvest level is unrealistic, because: (1) harvest probably exceeded biological production for much of the recorded history of exploitation; and (2) maximum production, a desired end for fishery support, occurs at approximately half the maximum (virgin, unexploited) biomass, and, thus, can only be achieved with disruption of the virgin complex community structure. Thus, the direct harvest economic value of a fishery based on a restored resource will not reach historical levels if there is an accompanying goal of long-term community development that is self-sustaining in the absence of restoration effort. The role of the oyster as a cornerstone organism and the pivotal link in benthic-pelagic coupling is examined in the context of current and projected watershed management problems, including agricultural and urban development with associated nutrient and sediment erosion issues, in the entire Chesapeake Bay watershed. Restoration efforts to date have focused on rebuilding three-dimensional reef structures, often with subsequent oyster broodstock enhancement, in predominantly small estuaries with retentive circulation to provide demonstration of increased resultant recruitment. Such examples are used to increase public awareness of the success of restoration processes and increase long-term participation in such programs by schools, nonprofit and civic organizations, and commercial and recreational fishing groups

Seasonal changes in the depth distribution of bivalve larvae on the Southern New England Shelf

A limited survey was made of the seasonal change in occurrence, depth distribution, size distribution, and species composition of bivalve larvae at a single station on the southern New England shelf during the period April-December 1981. The data were related to temperature structure of the water column and chlorophyll a distribution. Bivalve larvae were most abundant during late August and September at depths greater than 10 m, in water temperatures of 14 to 18°C, and chlorophyll a concentrations of200 p.m length consisted predominantly of the species Modiolus modiolus (Linne), Arctica islandica (Linne) and Spisula solidissima (Dillwyn). Modiolus modiolus was present in the depth range 10-40 m from late July through December with highest concentrations in August through October. Arctica islandica was present at I to 30m depth in May and from 20 to 40 m from late July through November. Larvae of A. islandica that were captured in May possibly originated from spawning in late 1980; those that were captured in November were first shelled veligers of 110 p.m length. Those larvae may form the basis of an overwintering larval population. Larvae of S. solidissima were present from late July through October and extended into shallower, warmer waters than larvae of either M. modiolus or A. islandica

Sperm Swimming Speeds In The Eastern Oyster Crassostrea Virginica (Gmelin, 1791)

Oysters, like the vast majority of sessile marine invertebrates, shed sperm and eggs into the water column where fertilization subsequently occurs. The fate of the gametes depends on their passive movements at various scales in a high-viscosity environment, the longevity of the sperm\u27s ability to affect oriented movement, the rate of sperm movement toward the egg target, and the ability of sperm to effect fertilization. Oyster sperm swim in a helical pattern with a mean forward progression velocity of 0.057 +/- 0.010 mm/sec (SE; n = 25) with the 95 percentile range extending from 0.036-0.078 mm/sec, a value comparable with that reported for echinoderm sperm