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In the late 1940s and early 1950s Thurlow C. Nelson and Harold H. Haskin conducted a series of mark-recapture experiments to assess the growth of hard clams (Mercenaria mercenaria) on Delaware Bay mud flats adjacent to the Rutgers University Oyster Research Laboratory in Cape May County, NJ. Two specimens from these experiments were recovered alive on September 7, 1980. The growing shell margin of one of these clams (Specimen #1) had been notched with a hack saw between March and December 1947. The growing shell margin of the second clam (Specimen #2) had been notched with a small triangular file between March 1948 and December 1951. Examination of polished shell sections (cut along the axis of maximum growth) of specimens #1 and #2 revealed, respectively, 33 and 30 dark (translucent) bands within the postnotch regions of the outer and middle shell layer of each specimen. The presence of light (opaque) shell material in the outer and middle shell layers at the growing margin of each of these specimens is consistent with the observations of other workers that indicate the formation of dark (translucent) bands in this species during the winter months in certain coastal environments from the MidAtlantic region northward along the east coast of North America. The present study provides convincing evidence that alternating light (opaque) and dark (translucent) bands within the outer and middle shell layers of Mercenaria mercenaria from Delaware Bay, NJ mud flats reflect annual cycles of growth and can be used for estimating the age of hard clams from this region. It is believed that the present study represents the longest mark-recapture study of growth patterns within the shells on any bivalve mollusc. [ABSTRACT FROM AUTHOR]

Owing to the high cost of microalgal production for bivalve culture, a need for alternatives to live microalgal diets exists, particularly in facilities where space and personnel required for live microalgal culture are limited. This study examined the feasibility of utilizing commercially available microalgal concentrates in place of or in addition to live microalgae as a feed for post set northern quahogs (= hard clams), Mercenaria mercenaria and sunray Venus clams, Macrocallista nimbosa. Clams were fed one of six dietary treatments for 6 wk, and growth, survival, and fatty acid (FA) profiles were evaluated. Treatments included two live microalgal diets Tisochrysis lutea and T. lutea + Chaetoceros neogracile, which served as controls for complete or partial replacements with two commercially available concentrate products, ISO 1800 and Shellfish Diet 1800. The production of both clam species was negatively impacted compared with the live microalgae controls when only concentrates were fed. Survival decreased by 29–33% for hard clams and 42–62% for sunrays; a similar decrease in the growth rate (25–34%, 42–62%) was seen for hard clams and sunrays, respectively. Survival was not affected by feeding the partial replacement diets; however, the growth of sunrays was negatively affected (32–38% decrease) compared with live microalgae controls. Clam tissue FA profiles generally reflected the FA profiles of the dietary treatment fed, yet production was decreased with complete replacement diets, suggesting nonnutritional factors as the cause for the poorer performance seen. These results suggest that a live microalgal diet may be able to be sufficiently replaced with up to 50% of the microalgae concentrates tested without affecting the production of hard clams. For both post set hard clams and sunrays, production will be decreased if the two concentrates used in this study are utilized exclusively. [ABSTRACT FROM AUTHOR]

Overharvest of the once highly abundant northern quahog, or hard clam [Mercenaria mercenaria (Linnaeus, 1758)], has decimated local populations on Long Island, NY, and the ecosystem services they provide. Among the potential ecological consequences of the loss of hard clams, impacts on benthic community structure and function have not been well documented. This study took advantage of a large-scale hard clam restoration effort by The Nature Conservancy in Great South Bay, NY, to investigate these parameters. The benthic communities between areas planted with high densities of adult clams (i.e., spawner sanctuaries) and adjacent sites with no transplants were compared. In clam-stocked areas, there were greater trophic and taxonomic group densities, especially for some environmentally sensitive groups such as crustaceans, and increased species richness, which in turn altered community structure of the resident infauna. Differences between paired sites for macrofaunal diversity and community structure were also observed, likely reflecting larger scale differences in invertebrate communities. Minimal differences were observed in sediment porosity, carbon and nitrogen content, and total organic matter between adjacent areas with and without clams, although all of these parameters were significantly associated with community structure. Although shell density was significantly higher in clam-stocked plots, it explained little of the variation in macrofaunal composition on its own or in combination with other environmental parameters, but in combination with the living clams may have added enough environmental complexity to enhance densities of different trophic groups. Within the short period of 3 y, differences in the benthic community with the presence versus the absence of clams were already detectable. More long-term data incorporating the effect of processes at multiple scales are needed to gain insight into the complexity of rehabilitating coastal benthic environments following the removal of important species such as clams. The results of this work support the notion that M. mercenaria acts as an ecosystem engineer, modifying the environment by creating habitat and enhancing the abundance of other species. [ABSTRACT FROM AUTHOR]

Three selected aquaculture Strains of the commonly used hard clam, Mercenaria mercenaria (Linnacus, 1758)—Massachusetts (MA), New Jersey (NJ), and South Carolina (SC)—were reared in a single hatchery. They were planted in replicate plots in Barnstable Harbor, MA, and Dry Bay, NJ, in spring 2008 and grown to market size. Growth, survival, and QPX (quahog parasite unknown) prevalence and severity were measured in fall 2008, spring 2009, and fall 2009. Growth was similar at both sites for the first summer, but during the second year growth was better in Massachusetts. Overall survival was better for all strains in New Jersey when compared with Massachusetts. Survival of strains in New Jersey was 53.4% and 34.8% for NJ and MA, respectively, closely followed by those in Massachusetts (41.8% and 26.2% for NJ and MA, respectively). Strains from SC seed had the highest prevalence and severity of QPX and the lowest survival (36% and 6.6% in New Jersey and Massachusetts, respectively), Infections with QPX were low at both sites, but this study confirms earlier work indicating that QPX infection rates appear to be strain specific, with strains of southern origin being more susceptible than those of northern origin. Our study also supported anecdotal reports that QPX disease is more severe in Massachusetts than in more southern sites where it has been found. As in most previous field studies, we found that although mortality was correlated with QPX levels, it was considerably higher than infection prevalence would indicate, which suggests that strain interactions with stressful environmental conditions or unidentified factors may be also involved in mortality. [ABSTRACT FROM AUTHOR]

The northern quahog Mercenaria mercenaria, also known as the hard clam, is an important aquaculture species in the United States. Genetic and genomic studies in this species require a large set of genetic markers. In this study, microsatellite markers were developed from an enriched DNA library. Five hundred sixty-seven clones were sequenced, producing 337 microsatellite-containing sequences with di- (82.0%), tri- (7.8%), tetra- (7.8%), hexa- (1.1%), hepata- (1.1%), and octanucleotide (0.2%) repeats. Sixty primer pairs were designed and screened in 24 clams from a wild population, of which 30 primer pairs showed good amplification and produced no more than 2 alleles per individual. The other 30 primer pairs either failed to amplify consistently or amplified multiple fragments. Of the 30 primer pairs with good amplification, 29 were polymorphic with allele numbers ranging from 3-21 per locus. The expected and observed heterozygosity ranged from 0.538 I-0.9628 and from 0.0417-0.9167, respectively. Fourteen of the 29 loci showed significant (P < 0.05 after Bonferroni correction) deviation from Hardy-Weinberg equilibrium, probably because of the presence of null alleles. Most of the microsatellite markers developed here should be useful for genome mapping and some for population genetics studies in this species. [ABSTRACT FROM AUTHOR]

In many molluscs, it has been found that Ca2+ signaling pathway is involved in the resumption of meiotic maturation in oocytes. To better understand the possible role of Ca2+ signaling pathway in regulating meiotic maturation in oocytes of the northern quahog Mercenaria ,nercenaria, free extracellular Ca2+, A23187 (calcium ionophore), verapamil (calcium channel blocker), and trifluoperazin (calmodulin antagonist) were used to incubate oocytes or serotonin-induced oocytes by pharmacological methods. Results show that extracellular Ca2+ (50~200 mM) and A23187 (1~10 μM) can stimulate the meiotie maturation. In addition, verapamil (1~100 μM) and trifluoperazin (10~1,000 μM) could inhibit serotonin-induced oocyte maturation. Therefore, Ca2+ is essential for the reinitiation of meiotic maturation in oocytes of the northern quahog. Moreover, an increase in [Ca2+] can promote meiotic maturation. [ABSTRACT FROM AUTHOR]

Larval size and developmental rate can vary tremendously within and among cohorts because of genetics. environment, and maternal investment. This natural variation in larvae may have effects that span multiple life stages. Here we investigate the effects of larval size and developmental stage on the subsequent life stages of the commercially and ecologically important clam Mercenaria mercenaria. Fifteen days after fertilization, we divided larvae into two groups based on their developmental stage (umbonal or pediveliger) and recorded survival, size, and developmental stage of individuals over the next 4 months. Results revealed that after four months larvae that had only reached the unibonal stage by Day 15 were significantly smaller than those that had reached the pediveliger stage. These smaller and less developed larvae were less successful than the larger and more developed larvae across late larval and juvenile stages. In particular, smaller and less developed larvae were less likely to metamorphose, required more time to metamorphosis, and had lower survival and growth rates. These results suggest that natural variation in larval size and developmental rate can affect recruitment in a variety of ways: (1) Increased time to metamorphosis may increase the cost to larvae via predation or exposure to environmental stress, (2) Decreased proportion of larvae able to metamorphose may directly reduce the number of settlers, and (3) Decreased growth and survival rates for juveniles may reduce the number of new recruits. We also discuss the persistence of natural variation in larval size and developmental rate in light of the observed negative effects associated with smaller and less-developed larvae. [ABSTRACT FROM AUTHOR]

In 2000, the northern quahog (= hard clams) Mercenaria mercenaria population was surveyed in Raritan Bay with the purpose of determining sustainable harvest levels. To complement this population survey, we determined the size-at-age structure and experimentally determined mortality rate and size specific growth of adult clams. Clams of a range of sizes, obtained from the sampling program, were measured, cleaned, and aged by counting growth rings in sectioned shells. Experimental plots were established in the low intertidal zone at two sites in the Raritan/Sandy Hook Bay system. Marked clams of five sizes were planted in three seasons and harvested quarterly. Experimental estimates of mortality and survival were based on collected live and dead individuals and are thus conservative because they do not address the numbers missing. Some of the clams from both sites were removed from the area by predators. Estimated mortality for individuals >25 mm by instantaneous rate yielded a mean of 0.0176. Integrating the size specific information with the size-frequency distribution from field survey yielded an average instantaneous mortality rate of 0.0187. Growth, based on the difference between the mean size planted and the mean size of the same size class retrieved was analyzed with a general ANOVA, and exhibited typical seasonal growth. The smallest size individuals grew faster than larger individuals. Survey data indicated an increasing clam population and increasing harvests. The survey mortality estimates, based on box counts, seem to overestimate losses. Our experimental work suggests adult mortality rates of nearly 2%, but loss of individuals from the plots made computation of exact mortality rates difficult, and 2% probably underestimates adult natural mortality rates. The results indicate that current levels of fishing mortality are sustainable with 3% natural adult mortality, but a natural adult mortality rate just above 5% would reduce the population growth to near zero. This information is important, because there has been interest in establishing additional depuration facilities to take advantage of the clam population and put more people to work. To sustain current levels of harvest, it will be essential to increase population level monitoring activities over time to assure the population is not being over harvested because of slight changes in recruitment or mortality rates. [ABSTRACT FROM AUTHOR]

Spawner sanctuaries, harvest-free areas planted with high densities of adult clams, are currently being used to restore self-sustaining populations of Mercenaria mercenaria (L.) to Great South Bay, New York. To evaluate and guide this restoration, we monitored the condition and spawning of clams transplanted from two source locations in Long Island Sound since April 2004. Transplanted clams were in relatively high condition and gonad ripeness at time of transplant, spawned the first spring and/or summer after transplant, reconditioned and spawned in subsequent years, but rarely reconditioned to as high of levels as when they were first transplanted. All populations exhibited similar annual patterns of condition and gonad ripeness: both peaked in mid to late spring, dropped steeply through summer with spawning, and they were lowest in fall. In some years condition increased during fall, and the higher the condition attained by the end of fall (mid-December) the greater the peak in condition the following spring. Across years and populations, condition at the end of fall explained ~89% of the variance in spring peak condition. Consistent differences in condition through time among some populations suggested that location within the bay as well as clam size impact condition. Because of interannual and locational variability, long-term monitoring of this long-lived species is essential for determining factors affecting condition and reproduction, and the ultimate restoration of sustainable hard clam populations. [ABSTRACT FROM AUTHOR]

A numerical bioenergetics simulation mode! based on the physiological processes affecting individual clams across a range of phenotypes describing a cohort has been developed and applied to the conditions in Great South Bay, New York. The clam population is relatively sensitive to food and to a lesser extent to temperature within this system. The timing of temperature and food in the spring, and more importantly in the fall, can increase population sensitivity beyond the effects of one factor operating alone. The effects of fishing on the stocks in proportion to the size structure present, and as directed fisheries on various size classes (littleneck, cherrystone, chowder) was simulated. Recruitment overfishing was responsible for the stock decline in the 1970s and 1980s, but the continued decline into the late 1990s and 2000s cannot be attributed to fishing alone. Recruit-per-adult declined after the mid 1990s. Modeled stock recovery times under constant environmental conditions are on order of 10-15 or more years depending on the exploitation rate. Under base conditions a proportional fishery that removes approximately 25% of the stock, or a littleneck fishery that removes approximately 37.5% of that size class annually would provide the best economic returns under constant average environmental conditions. Slightly less harvest would be desirable to avoid overfishing in years of less than optimal environmental conditions. [ABSTRACT FROM AUTHOR]

To determine if population density in areas closed to fishing in Narragansett Bay is causing differences in the reproductive potential of the organisms, this study used two approaches to determine the reproductive condition of the animals. The first approach consisted in employing a gravimetric condition index (Cl) to evaluate the general condition of quahogs from nine different sites, 3 sites open conditionally for fishing (conditional areas) and 6 sites closed to fishing. The second approach was a determination of gonadal index (GI) of a subset of the sample sites, by histological observation of gonadal tissue sections. Initial sampling included determination of Cl only, and lasted from March 25 to Sep. 22, 2005. The subset of six of the Sites was sampled the next year (2006), every three weeks from April 15 until Sep. 28 to determine Cl and the gonadal index (GI). Results show that there is a significant difference between the Cl of northern quahogs from conditional areas and quahogs from closed areas (P < 0.001). There is a significant difference in GI between sites at (P < 0.10). The Cl in conditional areas was consistently higher than in closed sites indicating that maturation and reproductive stages are more prevalent in conditional areas than in closed sites. When sites were separated into three categories: conditional fishing sites; coves; and Providence River sites, the GI and Cl of the conditional areas were always higher than the Cl and GI of all the other sites. The coves had intermediate indices, and the Providence River sites always had the lowest indices. The results indicate that the populations of quahogs in the closed sites sampled are not completing the gonadal cycle as expected and that the reproductive capability of the quahogs in conditional areas is higher than those in closed areas. Possible explanations to this condition include lower water quality in the closed areas, low dissolved oxygen concentration especially during the summer, poor bottom conditions and high population density. Although, not one single characteristic of the environment is solely responsible for the reproductive condition of the quahog population, density appears to have significant effect. [ABSTRACT FROM AUTHOR]

Northern quahogs (=hard clam; Mercenaria mercenaria) is a commercially important bivalve naturally distributed along the American Atlantic coast. Because the hard clam was first introduced into China as a candidate for aquaculture in 1997, research has been conducted on its ecologic habits, ecological physiology, metabolism, growth, reproduction, artificial propagation, and seed culture. Little information about their chromosomes and karyotype is available. The present study is designed to study the karyotype of the diploids and triploids to provide a reference to genetic research on introduced M. mercenaria. Triploids were produced by inhibiting the release of second polar body (PBII) with a treatment of 450 µmol/mL 6-deimethylaminopurine (6-DMAP), with untreated fertilized eggs developing as normal diploid controls. Chromosome preparations were made from trochophore larvae with drop-splash method. Chromosomes were measured and classified according to Leven criteria. Statistical analysis of relative length and arm ratio revealed that, this species had a normal diploid chromosome number of 2n = 38, and the karyotype consists of 15 pairs of metacentric (M) and 4 pairs of submetacentric (SM) chromosomes, NF = 76. The triploid karyotype formula was 45 M + 12 SM, NF = 114. The heterochromosomes and satellite chromosomes were not observed in all chromosomes in the complement. [ABSTRACT FROM AUTHOR]

The power of sclerochronology as a tool for interpreting the life history of aquatic organisms and the marine environment they inhabited lies in the occurrence of periodic and repeating structures within the skeletal parts of these organisms. Previous work examining the internal growth patterns of northern quahog (=hard clams), Mercenaria mercenaria (L.), along the Atlantic Coast has established their periodicity; however, the two existing descriptions of the annual growth pattern for hard clams from Narragansett Bay, RI are not consistent with one another. One study identified a translucent slow growth band in the summer and the winter, whereas the other study identified a translucent slow growth band only in the winter. To facilitate future sclerochronological analyses using Narragansett Bay hard clams, variations in the existing descriptions of the annual growth pattern were clarified. We found high variability in the annual growth pattern of M. mercenaria collected during the summers of 2005 and 2006. In approximately half the mature individuals observed, the winter growth breaks were separated by three bands, an opaque band of rapid growth in the spring, a translucent slow growth band in the summer, and a second opaque band of rapid growth in the fall. As these individuals approached senility, the pattern became compressed into two seasonal bands, a single opaque band of rapid growth in the spring and a single translucent band of slow growth encompassing both summer and fall. Growth in the remainder of the samples exhibited only two seasonal bands throughout the mature and senile period, an opaque band of rapid spring growth and a translucent band of slow growth throughout the summer and fall. To confirm the patterns observed, we analyzed δ18O from a subsample of shells. After placing the earlier descriptions and the current description of growth in Narragansett Bay hard clams in chronological order, it appears as though the changing environment of Narragansett Bay was reflected in the growth pattern of these clams. This study emphasizes the importance of validating annual growth patterns even in a region where the periodicity of the species has been confirmed. Changes in the marine environment may alter the patterns in the skeletal structures of aquatic organisms as well as their growth rates. [ABSTRACT FROM AUTHOR]

Along the mid-Atlantic coast of the United States in numerous areas where the water is shallow enough for light to reach the bottom, heavy spring-summer growth of drift seaweed (macroalgae) covers sessile benthic fauna for periods of days to weeks. Coverage for such prolonged periods can exert sublethal effects and eventually lead to death. This study examines the effects of seaweed coverage on body weight, relative abundance, and mortality in an economically important bivalve Mercenaria mercenaria Linnaeus (hard clam) within the Delaware Inland Bays, a shallow estuary representative of systems in which the species occurs and where mortality events have been observed in recent years. Mean body weights (dry weight) were compared between five groups of clams (80-90 mm shell length), taken from two paired sites (each of which had adjacent areas of heavy and light seaweed), and one seaweed-free site during September of 1999. Heavy seaweed density persisted into mid-to-late summer and the groups of clams from these sites had the lowest mean weights. Both heavy seaweed areas also had low abundance of live clams and numerous recently dead clams, identified as such by their shells being still hinged together. Clams at heavy seaweed density sites were consistently out of the bottom, indicating stress. No hinged-dead were found at either of the light seaweed areas and clams were always buried as normal. These results confirm that prolonged coverage by heavy density mats of drift seaweed affects M. mercenaria detrimentally, and document a negative result of human-accelerated eutrophication in coastal waters. The findings emphasize the importance of good management of nutrient loads entering these and other similar shallow estuaries such that the ecological benefits of moderate seaweed growth are retained, whereas extremely dense growth that harms clams and other fauna, creating a public nuisance, is minimized. [ABSTRACT FROM AUTHOR]

Increased numbers of benthic predators, especially crabs, have been proposed as a factor contributing to the decline of hard clams (Mercenaria mercenaria Linnaeus, 1758) in Great South Bay, NY. The long-term trend in benthic predators in this system was examined using observations on the distribution and abundance of predators that have been collected by the Town of Islip, NY as part of an annual survey of hard clam populations. The survey began in 1978 and extends to the present and provides concurrent observations of habitat (sediment type, and presence/absence of eelgrass), and hard clam size-frequency distribution and abundance. Predator type and abundance were reported from 1978 to 1981 and 1991 to 2003, which represents one of the most comprehensive benthic predator data sets currently available for any estuarine system. The annual averages of predator abundance in the survey area primarily show interannual fluctuations in abundance. Xanthid crabs (mud crabs, primarily Dispanopeus sayi Smith, 1869) were the numerically dominant predator in the system; blue crabs (Callinectes sapidus Rathbun, 1895) appeared in the late 1990s. Hard clam abundance has declined by 44% since the early 1990s. An Empirical Orthogonal Function (EOF) Analysis of the predator and hard clam data sets showed that fluctuations in predator abundance are: 1) mostly in phase over the survey region and 2) dominated by year-to-year fluctuations in abundance. The EOF results for the hard clams show that hard clam abundance fluctuations are: 1) in phase over the survey region and 2) dominated by a decreasing trend in abundance over the time series. The primary EOF modes essentially were uncoupled, which implies no strong predator-prey interactions between the predators and hard clams. By inference, increasing predator abundance does not appear to be a primary factor producing the long-term decline in hard clam populations. Predation pressure per recruit may still have increased because of declining hard clam population abundance and the concomitant decline in recruitment. [ABSTRACT FROM AUTHOR]

Northern quahogs Mercenaria mercenaria were grown in trays suspended in the York River, Virginia from November 1954 (4 months old) to December 1970 (16 years old). Measurements of shell length (mm) were made at least once a year from 1954 to 1970 and measurements of shell width (mm) were made in November 1962 and August 1965 and then once per year from 1967 through 1970. Quahog densities within the trays ranged from 1500 m-2 (November 1954 to October 1955) to 269 m-2 (November 1955 to December 1970). Quahog shell length (mm) increased with age (yr) and was described with a Von Bertalanffy growth model (coefficient of determination = 0.93). Most of the growth in shell length occurred in the first 6 years with clams reaching shell lengths of 58 mm by November 1960 and maximum shell lengths of 77-84 mm by 1963 (age 9). Shell length:shell width relationships were similar for tray held and wild quahogs collected from the York River during 1967 to 1970. The observed annual shell length growth increment decreased exponentially with quahog age. Standardized shell length growth index (SGI) values for 11 of the 15 years for which data are available describe better than expected quahog growth trends although there was no clear relationship between SGI and average annual surface temperature or average growth period (water temperature >7°C, typically March to November) surface temperature in the York River between 1955 and 1967. [ABSTRACT FROM AUTHOR]

Because the accumulation of inbreeding within hatchery-propagated stocks of the hard clam, Mercenaria mercenaria (Linnaeus, 1758), could result in reduced growth and survival, we studied the potential for improving performance through out-crossing among existing hatchery strains. We produced all 10 possible out-crossed combinations among 5 strains of clams as well as all 5 pure parental strains simultaneously in the hatchery and measured their size at the time of metamorphosis (the spat stage) and at the end of a nursery period in mesh bags at a single field site (the seed stage). We then planted replicate plots of all fifteen strains at five field sites in Virginia, USA encompassing the range of salinity conditions used by commercial growers, and monitored growth and survival for two growing seasons. We found significant phenotypic differences among strains at the spat and seed stage, but different strains performed best at each stage. In the field we found significant site effects, strain effects and strain-by-site interactions, but there was no evidence of negative correlations in performance among sites indicating strong trade-offs that would be problematic for selective breeding. Three different linear contrasts designed to compare out-crossed and pure strains for each parental stock, test for nonadditive genetic effects within each pairing of different parental strains, and estimate the general combining ability of parental strains reveal a complex pattern. We found both inbreeding and out-breeding depression depending on the developmental stage of the clams and the parental strain examined. Within strain crosses generally produced larger spat but smaller seed. Out-crossed progeny were generally smaller at the spat stage than the average of their parental lines but larger at the seed stage. The two best performing parental strains had significant, positive, general combining abilities, whereas this measure was negative for the two worst parental strains. In the field, inbreeding depression was restricted to lines that showed poor pure strain performance, and these strains also showed poor general combining ability, whereas strains with good pure line performance showed out-breeding depression and good general combining ability. Only the poorest performing pure parental lines showed non-additive effects when we compared each out-crossed strain to the mean of its parental strains, suggesting that heterotic effects are unlikely to be useful for selective breeding. Finally, there were significant correlations between seed measurements and field performance indicating that it may be possible, in the context of selective breeding programs, to weed out inferior strains or families early in the life cycle. [ABSTRACT FROM AUTHOR]

The Town of Islip, NY has collected a long-term data set (1977-2004) on northern quahog (hard clam), Mercenaria mercenaria, abundance. The data comprise approximately 350 duplicate 1 m² samples each year taken with a clam shell bucket. All samples were sieved through a 6.4 mm sieve and the hard clams enumerated by size. In addition, clam landings data for the town waters are available for the same time period. Clam populations have declined from their peaks in the 1970s to very low levels in the 1990s and 00's. These dramatic shifts in population abundance have made the exploration of spawner/recruit relationships possible. A number of alternate models were attempted, but based on knowledge of the biology of the species and other factors, all but two did not appear to be plausible. The two models (2nd order polynomial and Log) yielded high r² values and intercepted the 0 axis between 0.5 and 0.75 adult clams m² indicating a density dependent effect on recruitment. The polynomial model also suggested a carrying capacity level of about 5 adult clams m² and a density dependent upper level of density. This is the first time a spawner recruit relationship has become apparent for hard clams. [ABSTRACT FROM AUTHOR]

Recent (2001-2002) surveys of hard clam Mercenaria Mercenaria density and distribution, using patent tongs in a stratified random design (n = 7,358 stations) in lower Chesapeake Bay are not consistent with historic descriptions of clam habitats and densities. The highest average densities observed, up to 3.1 clams m-2, were in the lower James River. The highest modem average density observed is half that of clam densities commonly observed in these same habitats during the early l970s. Current distribution is significantly affected by water depth and substrate composition. Hard clam density in Chesapeake Bay is positively associated with increasing sediment grain size; 78% of all clams collected were found in shell or sand habitats. However, 44% of sand habitats and 54% of shell habitats were unoccupied suggesting that even habitat types that typically support higher clam densities may currently be underused. [ABSTRACT FROM AUTHOR]

Research that identifies potential bottlenecks in survival and growth penalties during the different phases of clam grow-out is necessary to maximize the profitability of clam aquaculture and reduce pressure on already threatened wild stocks along the Atlantic coast of the Eastern United States. In this study, initial planting density (489, 729, and 972 clams m[sup-2]) did not affect survival (64.8-77.5%) during the first year of clam grow-out. Clams planted at the lowest density outgrew (greater final shell length, SL, and individual clam volume) those planted at higher densities; therefore, clam growth was density dependent during the first year of grow-out. In the second experiment, size of clams (26.2, 32.5, 37.7, and 42.0 mm SL) planted after year one dial not affect survivorship (92.3-96.6%) or growth (36.2, 41.7, 45.1, and 49.2 mm SL, respectively). Evaluations of the economic feasibility of clam culture demonstrated that clams planted at intermediate densities would result in the greatest return on the initial investment. To increase the robustness of our economic feasibility analysis to interannual variations in clam survivorship and growth during the initial year of grow-out, we performed an identical analysis with data from an earlier study. Taken together, these studies bracket a realistic range of survivorship and growth during the initial year of clam grow-out: low survivorship and growth (this study) and high survivorship and growth (earlier study). Based on this range, the estimated expected profitability ranged from $4893 to $7717 per 100,000 seed clams. In contrast to aquaculture of other bivalve species (e.g., oysters), our analysis demonstrates that the profitability of clam aquaculture is fairly robust to substantial variations in market prices primarily as a result of the development of methods over the last decade that enable relatively high survivorship with moderate growth penalties. [ABSTRACT FROM AUTHOR]