John N. Kraeuter

Long-term Trends in Oyster Population Dynamics in Delaware Bay: Regime Shifts and Response to Disease

Abstract We evaluate a 54-y survey time series for the Delaware Bay oyster beds in New Jersey waters to identify the characteristics of regime shifts in oyster populations and the influence of MSX and Dermo diseases on population stability. Oyster abundance was high during the 1970s through 1985. Oyster abundance was low at the inception of the time series in 1953, remained low through 1969, and has been low since 1985 and very low since 2000. Natural mortality was low in most years prior to the appearance of MSX in 1957. From 1957 through 1966, natural mortality generally remained above 10% annually and twice exceeded 20%. Natural mortality remained well below 15% during the 1970s and into the early 1980s when oyster abundance was continuously high. The largest mortality event in the time series, an MSX epizootic that resulted in the death of 47% of the stock, occurred in 1985. Mortality rose again with the incursion of Dermo in 1990 and has remained above 15% for most years since that time and frequently has exceeded 20%. The primary impact of MSX and Dermo diseases has been to raise natural mortality and ultimately to cause a dispersed stock to retreat into its habitat of refuge in the moderately low salinity reach of the bay. The time series of oyster abundance on the New Jersey oyster beds of Delaware Bay is dominated by two regime shifts, the 1970 abundance increase that was maintained for about 15 y thereafter, and the 1985 abundance decrease that continues through today. These two regime shifts ushered in long-term periods of apparent constancy in population dynamics. The 1985 regime shift was induced by the largest MSX epizootic on record that produced high mortalities throughout a population distributed broadly throughout its habitat range after 15 y of high abundance. A putative new regime commenced circa 2000 as a consequence of a series of Dermo epizootics. Mortalities routinely exceeded 20% of the population annually during this period, with the consequence of a greater degree of stock consolidation than any previous time in the 54-y record. Extreme consolidation of the stock would appear to be a characteristic of the populations response to Dermo disease. The 1970–1984 and post-1985 regimes each were ushered in by a confluence of events unique in the 54-y time series. Each was characterized by a period of relative stability in population abundance. However, the stability in total population abundance belies a more dynamic process of stock redistribution during both time intervals, demonstrating that the appearance of constancy in stock abundance is not necessarily a result of invariant stock dynamics. Rather, the Delaware Bay oyster time series suggests that regime shifts delimit periods during which differential, often offsetting, local trends impart similar abundance levels, and thus constancy at the level of the stock masks substantive changes in local population dynamics potentially fostering future catastrophic changes in population-level attributes. Understanding such regime shifts will likely determine the success of decadal management goals more so than measures designed to influence population abundance.

OYSTER GROWTH ANALYSIS: A COMPARISON OF METHODS

Abstract The increase in disease related mortality has made managing oyster resources increasingly difficult. In Delaware Bay sustaining harvest requires specific information on growth rates of adult oysters on the different beds. We reviewed the literature and found such information, particularly that depicting oyster growth directly on subtidal oyster beds, lacking. We used three methods to determine growth of oysters on Delaware Bay seed beds: measuring the new growth at the lip of oysters retrieved from the bottom, tethering individually identifiable oysters on specially designed frames, and size-at-age information based on ages developed from check marks in growth lines in the oyster hinge. Measuring new growth on the lip of the oyster was not accurate. Use of the frames was exact but very labor intensive and only provided data for the one season. Use of the check marks in the hinge to estimate age provided a growth history, and was relatively accurate. Intensive harvesting of oysters appeared to significantly reduce the accuracy of all techniques. Based on literature values, it is difficult to determine latitudinal gradients in oyster growth, but these data indicate more rapid growth in the northern Gulf of Mexico than in other locals. In general, oysters in higher salinity portions of the Delaware Bay seed beds grew faster than those in the lower salinity regions.

Aquaculture-associated factors in QPX disease of hard clams: density and seed source

Abstract Quahog Parasite Unknown (QPX), a recently discovered pathogen of hard clams, Mercenaria mercenaria , appears to be most prevalent in clams held in hatchery or nursery tanks or grown to market in culture parks. The persistent link with cultured clams indicated that culture practices might increase the susceptibility of clams to an opportunistic or facultative parasite. We investigated two hypotheses: (1) that increased density would accelerate the development of QPX infections and (2) that seed originating from nonlocal (i.e. another state) sources would be more susceptible than seed from local sources. There was a significant trend toward higher QPX levels at higher planting densities, but considerable variability in the data made it difficult to determine the effect of density with a high degree of confidence. During 1995–1998, 3-year classes of clams imported as seed from a South Carolina hatchery and grown at several sites in New Jersey acquired heavy QPX infections and suffered mortalities of 26% to 92%. Clams of the same age from New Jersey hatcheries that had been planted in adjacent plots for the same length of time acquired few or no detectable infections and experienced no unusual deaths. QPX seems to be widespread in clam growing waters from Canada to at least Virginia, but causes disease and mortality only in certain groups that may be disadvantaged in some way, perhaps from an unfavorable genotype–environment interaction. The fact that no mortalities have been reported in New Jersey since the purchase of South Carolina seed was curtailed indicates that the problem was localized to these clams and has neither spread nor persisted to a noticeable extent. Nevertheless, the results underscore the potential dangers of using nonlocal stocks in molluscan aquaculture.

A POPULATION DYNAMICS MODEL OF THE HARD CLAM, MERCENARIA MERCENARIA: DEVELOPMENT OF THE AGE- AND LENGTH-FREQUENCY STRUCTURE OF THE POPULATION

Abstract An individual-based model was developed to simulate growth of the hard clam, Mercenaria mercenaria, in response to temperature, salinity and food supply conditions. Unique characteristics of the model are that: (1) length and tissue weight are related only by condition index, so that weight, up to a point, can vary independently of length, and (2) age is decoupled from length. Tissue weight changes result from the difference in assimilation and respiration. Changes in hard clam condition are determined from a standard length-weight relationship for average hard clam growth. Changes in hard clam length (growth) occur only when condition index is greater than zero, which happens when excess weight for a given length is attained. No change in length occurs if condition index is zero (mean case) or negative (less weight than expected at a given length). This model structure resolves limitations that accompany models used to simulate the growth and development of shellfish populations. The length-frequency distribution for a cohort was developed from the individual-based model through simulation of a suite of genotypes with varying physiological capabilities. Hard clam populations were then formed by the yearly concatenation of cohorts with partially independent trajectories that are produced by cohort- and population-based processes. Development and verification of the hard clam model was done using long-term data sets from Great South Bay, New York that have been collected by the Town of Islip, New York. The ability to separately track length and age in the simulations allowed derivation of a general mathematical relationship for describing age-length relationships in hard clam populations. The mathematical relationship, which is based on a twisted bivariate Gaussian distribution, reproduces the features of age-length distributions observed for hard clam populations. The parameters obtained from fitting the twisted bivariate Gaussian to simulated hard clam length-frequency distributions obtained for varying conditions yield insight into the growth and mortality processes and population-dependent processes, compensatory and otherwise, that structured the population. This in turn provides a basis for development of theoretical models of population age-length compositions. The twisted bivariate Gaussian also offers the possibility of rapidly and inexpensively developing age-length keys, used to convert length-based data to age-based data, by permitting a relatively few known age-length pairs to be expanded into the full age- and length-frequency structure of the population.

Vertical Disturbance of Sediments by Horseshoe Crabs (Limulus polyphemus) During Their Spawning Season

Biological and physical reworking of sediments can profoundly affect the structure and functioning of benthic communities. The depth of the disturbance is an important factor that controls the types of organisms that can exist within the sediments. Large numbers of horseshoe crabs,Limulus polyphemus, spawn each spring on the sandy shores of Delaware Bay beaches. We have used this abundance peak to provide an estimate of the depth of sediment disruption caused by this species on an intertidal flat adjoining a major spawning beach. Vertical columns of marked sediment were placed in three locations of an intertidal flat. Some columns were protected with cages while others remained unprotected. Analysis of variance of the depth of disruption of the marked sediment indicated that different areas of the intertidal zone were disrupted to different depths. Caged marked sediment was disturbed to an average depth of 3.2 cm while unprotected sediment was disturbed to a mean depth of 11.1 cm. Deepest mixing occurred in a trough between sandwave crests and averaged 17.7 cm deep. These mixing depths are greater than the 1.2 cm deep disturbance produced by nonstorm wave action in the study area.

A NOTE ON A SPAWNER—RECRUIT RELATIONSHIP FOR A HEAVILY EXPLOITED BIVALVE: THE CASE OF NORTHERN QUAHOGS (HARD CLAMS), MERCENARIA MERCENARIA IN GREAT SOUTH BAY NEW YORK

Abstract 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 00s. 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 m−2 indicating a density dependent effect on recruitment. The polynomial model also suggested a carrying capacity level of about 5 adult clams m−2 and a density dependent upper level of density. This is the first time a spawner recruit relationship has become apparent for hard clams.

INFLUENCE OF HOST GENETIC ORIGIN AND GEOGRAPHIC LOCATION ON QPX DISEASE IN NORTHERN QUAHOGS (=HARD CLAMS), MERCENARIA MERCENARIA

Abstract QPX (Quahog Parasite Unknown) a protistan pathogen of northern quahogs (=hard clams), Mercenaria mercenaria, has caused disease outbreaks in maritime Canada, and in Massachusetts, New York, New Jersey, and Virginia, USA. Although epizootics have occurred in wild hard clam populations, the parasite has most seriously affected cultured hard clams, suggesting that aquaculture practices may promote or predispose clams to the disease. In this investigation the influence of clam genetic origin and the geographic location at where they are grown on QPX disease susceptibility was examined in a common garden experiment. Aquaculture stocks were acquired from hatcheries in Massachusetts, New Jersey, Virginia, South Carolina, and Florida and spawned at a single hatchery in Virginia. All stocks were originally, although not exclusively, derived from wild hard clam populations from each state. The seed clams were deployed at two sites, New Jersey and Virginia, and evaluated during the subsequent 2.5 y for growth, survival, and QPX disease. At both sites, South Carolina- and Florida-derived clam stocks exhibited significantly higher QPX prevalence and lower survival than New Jersey and Massachusetts clam stocks. Levels in the Virginia stock were intermediate. In Virginia, mortality at the termination of the experiment was 78%, 52%, 36%, 33%, and 20% in the Florida, South Carolina, Virginia, Massachusetts, and New Jersey hard clam stocks, respectively. Mortality was significantly correlated with QPX prevalence. Maximum QPX prevalence in the South Carolina and Florida stocks ranged from 19% to 21% and 27% to 29%, respectively, whereas in the Virginia, New Jersey, and Massachusetts stocks prevalence was 10% or less. Similar trends were observed in New Jersey where mortality at the termination of the experiment was estimated to be 53%, 40%, 20%, 6%, and 4% in the Florida, South Carolina, Virginia, Massachusetts, and New Jersey clam stocks, respectively. QPX prevalence peaked at 18% in the Florida stock, 38% in the South Carolina, 18% in the Virginia, and 5% in the New Jersey and Massachusetts stocks. These results suggest that host genotype is an important determinant in susceptibility to QPX disease. As such, hard clam culturist should consider the genetic origin of clam seed stocks an important component of their QPX disease avoidance/management strategies.

Reevaluation of Eastern Oyster Dredge Efficiency in Survey Mode: Application in Stock Assessment

Abstract Dredge efficiency measurements were conducted in 2003 by comparing paired dredge and diver samples taken on eastern oyster Crassostrea virginica beds in the New Jersey waters of Delaware Bay. We evaluated whether ancillary data collected during the tow could be used to estimate dredge efficiency rather than periodically conducting costly field experiments. With the exception of market-size eastern oysters, dredge efficiency did not vary between the time when it was last measured (2000) and 2003. Dredge efficiencies were lower upbay in both studies. Capture efficiency was significantly greater for live eastern oysters than for boxes (dead, articulated valves) and significantly greater for boxes than for cultch (shell clumps, shells, and shell fragments without attached live eastern oysters or boxes) in both studies. In the single significant exception, dredge efficiency was significantly lower for market-size eastern oysters in 2003. Survey quantification of eastern oyster abundance by calibrating...

Minchinia mercenariae n. sp. (Haplosporidia) in the Hard Clam Mercenaria mercenaria: Implications of a Rare Parasite in a Commercially Important Host

ABSTRACT. During routine histopathology of 180 juvenile hard clams, Mercenaria mercenaria, from a site in Virginia, USA, in 2007, we discovered a single individual heavily infected with a parasite resembling a haplosporidian, some members of which cause lethal bivalve diseases. Scanning electron microscopy of spores and sequencing of small subunit ribosomal DNA confirmed a new species: Minchinia mercenariae n. sp. Further sampling of clams at the site found prevalences up to 38% using polymerase chain reaction (PCR). No parasites were found in routine histological screening of the same individuals, but re‐examination of clams judged positive by in situ hybridization (ISH) revealed very faintly staining plasmodia. No unusual mortalities have occurred among the sampled groups. Analysis of clams from Massachusetts to Florida by PCR failed to detect the parasite, but a haplosporidian found in a clam from New Jersey in 2001 was subsequently identified by ISH as M. mercenariae. No other haplosporidians have been reported in thousands of hard clams from the US east coast examined histologically since the mid‐1980s. The discovery underscores critical questions about how to assess the risks associated with parasites in groups known to be lethal, but that themselves are not considered a problem.

EFFECTS OF THE FISHERY ON THE NORTHERN QUAHOG (¼HARD CLAM, MERCENARIA MERCENARIA L.) POPULATION IN GREAT SOUTH BAY, NEW YORK: A MODELING STUDY

Abstract A numerical bioenergetics simulation model 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.