Quarterdeck 2.1

Oysters Revisited: Monitoring Mollusc Health in Galveston Bay

By Eric Powell

The oyster supports a multifaceted research program at Texas A&M University and, indeed, throughout the United States, because oysters play a central role in environmental monitoring, environmental impact assessment, and fisheries management. The oyster's importance stems from its varied characteristics as the keystone species of an estuarine community offering home to a variety of commercially and recreationally important species. Oysters proliferate in a wide variety of habitats where they build geologically significant features, reefs, that modify current flow and affect navigation, and thus they enable comparative studies over broad spatial and temporal scales. As a large filter feeder, the oyster can regulate or modify water clarity and phytoplankton abundance over entire bay systems. Over half of the U.S. commercial oyster fishery currently resides in the bays of the Gulf of Mexico, and the recreational taking of oysters and other shellfish resources ranks Texas third among U.S. states. The oyster's physiological and ecological success responds to both anthropogenic impacts and climate variation which modify population health, production and reproductive capacity. The oyster remains sessile throughout its juvenile and adult life and is affected by the many environmental factors present in the habitat as they change over time. As it filter feeds, the oyster accumulates contaminants of many biological and chemical types, and so provides an integrative measure of environmental contamination over time spans of weeks to months. The oyster is a sensitive ecological indicator because of its predilection towards population-level catastrophic mortalities. In the Gulf of Mexico the sensitivity is produced by an important disease, Dermo, caused by a small endoparasitic protozoan, Perkinsus marinus. Dermo generally causes 50% or more of the yearly mortality in oyster populations and, during epidemics (called epizootics), can produce nearly 100% mortality in infected populations.

The oyster is the monitoring organism used by the National Oceanic and Atmospheric Association (NOAA)'s Status and Trends Program (NS&T) in the Gulf of Mexico. This monitoring program, run by the Geochemical and Environmental Research Group (GERG) and the Department of Oceanography at Texas A&M University, is designed to define the spatial distribution and temporal trends in environmental contamination by heavy metals, pesticides, and petroleum hydrocarbons in Gulf of Mexico bays. Each winter, the NS&T program samples oysters from a minimum of 50 sites from Brownsville, Texas to the Florida Everglades. Besides the contaminant analyses, a series of ancillary biological measurements have been conducted. These record long-term changes in Dermo disease intensity and other measures of population health.

One of the key results of the NS&T program, now in its 9th year, has been to document the response of oyster populations to climatic cycles, particularly the El Nino cycle, which affects salinity in Gulf bays by modifying rainfall. Dermo disease responds to temperature and salinity, intensifying during periods of high temperature and high salinity. Mortalities generally occur in the summer months. Oysters have only a limited ability to control this disease through their own immune system, so they depend on climatic conditions unfavorable for the parasite to control the disease. During periods of low rainfall, catastrophic mortalities can occur as Dermo intensifies with increasing salinity. In the Gulf of Mexico, these cycles of intensification are determined by climatic cycles that raise salinity.

The Galveston Bay National Estuary Program (GBNEP) recently completed an assessment of biological resources in Galveston Bay. One of these resources is the system of oyster reefs that collectively account for over 10,000 hectares of bay substrate. Over the years, concerns have been raised about whether these reefs are declining due to environmental degradation and the activities of the commercial fishery. GBNEP commissioned a mapping study in 1990 to compare the current areal extent of these reefs with that present in 1970 when the Texas Parks and Wildlife Department completed a survey of the commercially important reefs. Due to the large size of the Galveston Bay system, a new method for mapping oyster reefs based on acoustic profiling was developed. The acoustic signal distinguished a variety of bottom types, including oyster reefs and clam beds. The survey nearly doubled the known reef area in the Galveston Bay system. Significantly, the survey identified over 1900 hectares of man-made reef, much of which was distributed along the spoil banks that parallel the major ship channels of the bay. Furthermore, the survey determined that the reef system was expanding in many areas of the bay, rather than declining. Reef expansion was particularly apparent along the Houston Ship Channel and in the area of the bay between San Leon and Smith Point where most of the commercial fishery is located. Thus, no evidence was found that might indicate that the fishery adversely effects the aerial extent of oyster reefs. [43K]

The Army Corps of Engineers and the Houston Port Authority have proposed to widen and deepen the Houston Ship Channel that extends from the inlet, Bolivar Roads, to the Port of Houston. This channel bisects the primary oyster producing area of Galveston Bay. The Army Corps of Engineers commissioned a modeling study to determine whether a larger channel would significantly impact the oyster populations in Galveston Bay. On the one hand, the larger channel might increase bay salinity and, therefore, Dermo disease, eventually resulting in population decline. On the other hand, rapid reef accretion along the present channel suggests that the present channel has been beneficial. Benefits include increased water flow that provides increased food supply and protection of adjacent reefs from killing floods by increasing salinity locally during periods of high freshwater inflow.

The project involved the development of a hydrodynamic model by the Waterways Experiment Station in Vicksburg, Mississippi and the development of a population dynamics model by Oceanographers from Texas A&M University (TAMU) and Old Dominion University (ODU). These two models have been coupled together to permit full-scale simulations of the oyster populations in Galveston Bay. Important issues being addressed, besides the impact of a larger ship channel, are the impact of proposed changes in freshwater inflow by the diversion of water from the Trinity River watershed, the quantity of food resources (phytoplankton) required to support the bay's oyster populations, and the triggering mechanisms for Dermo epizootics that might produce population collapses throughout the bay. The latter question arises from the recent collapse of the Chesapeake Bay oyster fishery produced by a widespread epizootic that resulted in the nearly complete demise of Chesapeake Bay oyster populations over a period of a few years. Simulations with the TAMU/ODU population dynamics model show that small changes in climate may suffice to trigger widespread epizootics. These epizootics rarely terminate without the near extinction of the population. Accordingly, environmental managers must be particularly careful in identifying factors that might trigger epizootics. One of the most important environmental parameters besides salinity turns out to be food supply. Small oysters grow as fast as the parasitic protozoan when food supply is high, and faster growth limits intensification of the disease.

A number of other research efforts have been carried out recently or are in progress. The Texas Sea Grant College Program supports fundamental research into oyster reproduction, oyster disease, and the effect of current flow on oyster food supply. This research effort aims to develop better tools to measure the rate of egg reproduction, the rate of disease intensification and the use of microcurrent meters to measure current flow through oyster populations. Houston Power and Light, with the help of a number of other sponsors, has developed a new method for creating artificial substrate for oysters to promote reef growth in areas where substrate is not presently adequate. The Center for Energy and Minerals Resources at TAMU supports research on the accumulation of contaminants in oyster gametes. Oyster egg and sperm accumulate hydrocarbons and pesticides which may affect their variability. Because the oyster plays such a central role in the Texas bays, we can expect the oyster to continue to support extensive research projects at Texas A&M University for the foreseeable future.


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Updated July 20, 1995