Hard Clam Studies
A modeling study of the Growth, Survival and Recruitment of hard clam (Mercenaria mercenaria) larval and post-settlement populations

Effects of Aureococcus anophageffrerens brown tides in coastal lagoonal systems: Coupling numerical simulation modeling with field and laboratory studies to determine population effects on the hard clam, Mercenaria mercenaria.

Clam Disarticulation Rates



A modeling study of the Growth, Survival and Recruitment of hard clam (Mercenaria mercenaria) larval and post-settlement populations

E.E. Hoffman and J.M. Klinck, Center for Coastal Physical Oceaography, Old Dominion University;
V.M Bricelj, Institute for Marine Biosciences, National Research Council, Canada
J.N. Kraeuter, E.N. Powell, Haskin Shellfish Research Laboratory, Rutgers University
S. Buckner, Town of Islip, Environmental Control.

Hard clam (Mercenaria mercenaria) populations in the south shore bays of Long Island, NY, have shown a downward trend in numbers since the 1970s when peak abundance was recorded. The causes of this decline have not been categorically identified, but it has been attributed to intensive harvesting, the occurrence of brown tides, caused by the picoplanktonic alga Aureococcus anophagefferens, which affect filtration rate of adult and juvenile hard clams and have resulted in recruitment failure of bay scallops and mussels, changes in food supply via changes in phytoplankton species composition, increased predation by blue crabs (Callinectes sapidus), and environmental change such as increased winter water temperatures. The many potential factors contributing to the decline in hard clams resulted in the issuance of a Request for Proposals (RFP) by New York Sea Grant in 2000 with the objective of studying, identifying and quantifying processes that control the growth, recruitment, and survival of hard clams in the south shore estuaries of Great South Bay, NY. Results from previous studies, one of which was the development of a model to simulate growth and development of hard clams, showed that the factors that contribute to hard clam growth and survival are complex. Simulations run with the hard clam model for the period of the 1970s to 1980s show that the decline in hard clams was likely precipitated by overfishing. One factor not explicitly included in the previous research efforts, but one that has been identified by simulation and experimental research as being of importance, is survivorship and subsequent recruitment of hard clam larvae. Simulations for the period of the 1990s show that issues related to recruitment are the likely causes for the failure of hard clam populations to recover once intensive harvesting ceased. It is the larval component of the hard clam life cycle that is the focus of this proposal.

This proposed research is designed to: 1) provide a numerical model that can simulate the growth and development of hard clam larvae; 2) develop broodstock-recruitment relationships for hard calm populations in Great South Bay; and 3) implement the larval model and broodstock-recruitment relationships with an existing model for the growth and development of post-settlement hard clam. The processes modeled in the basic individual and population portions of this model are generally applicable to either wild stocks or the grow-out of aquaculture stocks. In addition, the same information could be utilized in restoration programs to evaluate spawner-sanctuaries or seeding via aquaculture.



Figure 1. Flow chart showing the processes and interactions that are included in the hard clam growth model.



Figure 2. Schematic showing the approach used to extend the simulation results from the individual-based hard clam model to a cohort and population level. Cohorts are produced by varying the simulation results obtained for an individual over a range of values related to characteristics or metabolic processes that reflect phenotypic variability within a population, e.g. birth size and growth efficiency. Populations are produced by summing over the cohorts.




Effects of Aureococcus anophageffrerens brown tides in coastal lagoonal systems: Coupling numberical simulation modeling with field and laboratory studies to determine population effects on the hard clam, Mercenaria mercenaria.

J.N. Kraeuter, E.N. Powell, Haskin Shellfish Research Laboratory, Rutgers University
E.E. Hoffman, and J.M. Klinck, Center for Coastal Physical Oceaography, Old Dominion University;
V.M Bricelj, Institute for Marine Biosciences, National Research Council, Canada
J.E. Ward, Department of Marine Sciences, University of Connecticut
J. Pecchioli, NJ DEP, Division of Science and Research

Blooms of the brown tide alga Aureococcus anophagefferens have become common in shallow coastal estuaries in the mid-Atlantic US. The Barnegat Bay/Little Egg Harbor system, part of the EPA Estuary Program, has experienced brown tide blooms in 5 of the last 8 years. Typical bloom conditions often occur during the time when hard clams are spawning (Figure 1).



These blooms inhibit feeding of suspension-feeders and have caused recruitment failure and mass mortalities of commercially important bivalves, including mussels and bay scallops in the region. The hard clam, Mercenaria mercenaria, an ecologically and commercially important species, is documented as sensitive to brown tide. Population level effects of short-term bloom events on a long-lived species such M. mercenaria, are difficult to evaluate experimentally. Cumulative sublethal or lethal effects, especially on early life history stages, may not be evident at the population level for many years. In complex systems such as estuaries, it is difficult to isolate population levels effects due to environmental fluctuations and fishing mortality from other factors. Numerical simulation models offer a means to evaluate the relative significance and cumulative effects of these multiple factors. Such a model has been constructed for the hard clam (Hofmann et al., 2003), and preliminary work has incorporated the effects of brown tide, as far as they are known. Initial simulations show that model predictions are sensitive to brown tide effects on early life history stages, for which there is limited information. This additional information could substantially improve the reliability of model predictions.

This project combines experimental and modeling efforts to explicitly examine the effects of brown tide on hard clam population dynamics in the Barnegat Bay system. We will experimentally determine effects of brown tide on survival, growth and metamorphic success of hard clam larvae. In addition we will evaluate the size-specific effects of varying concentrations and duration of exposure to Aureococcus on growth and survivorship of hard clam juveniles in the field and in the laboratory. Lastly we will compare the toxicity of various Aureococcus isolates and examine the effects of various concentrations of brown tide in mixed suspensions on feeding activity of adult hard clams using video-endoscopy.

The results of these experimental manipulations will be used to update and refine the existing hard clam model with a larval submodel larval component that simulates the brown tide effects on rates and processes affecting clam recruitment. The model will then be utilized to evaluate the timing and severity of blooms on the individual, year class and population levels relative to other environmental factor such as seasonal temperature variation, changes in food availability, climate change, and fishing pressure in a typical east coast lagoonal estuary.

The same set of conditions affect hard clam aquaculture through reduction in feeding and growth both in the field, but also in the nursery systems (Figure 2).



The reduction in growth once the clams are planted, or in two year grow-out process may lower yields, and if clams do not reach market size by fall of the second year, an additional growth season may be needed. This increases the risk to clam growers and reduces their profits. In addition, the presence of these brown tides also increases the variability in growth rates, thus making it difficult to establish contracts for timely delivery.



Clam disarticulation rates

J.N. Kraeuter, Haskin Shellfish Research Laboratory, Rutgers University

Assessments of standing stock of native hard clam population take place very infrequently. To assess the sustainability of the population during fishing, information is required on the size frequency distributions of standing stocks, recruitment rates and size specific mortality rates. Recruitment rate can be inferred from the size distribution, but the mortality figures are often derived from box counts (numbers of dead clams with both valves still intact). For oysters, there are a few studies of the length of time valves of various size animals remain intact following death. There is no such information for any infaunal bivalve, and thus the rate at which valves buried in the sediment separate (disarticulation) is unknown, and estimates of mortality may be under or overestimated. The current study has deployed dead clams of a variety of sizes in the intertidal of Cape May harbor and will be following them over the next few years in an effort to evaluate the disarticulation rates.