Author: Toovey Leia
Institution: Physical Geography
Date: September 2005
The Disappearing Oyster
After an hour of hard work, in the sun and breeze off the Virginia coast, you might dig up one small glistening oyster from the mud of Chesapeake Bay. Seventy years ago, the same effort would have brought you 100 oysters. The oysters of Chesapeake Bay are dying out.
Those who delight in dining on these delicious mollusks are not the only ones concerned with the oyster's disappearance. The true crisis is in the critical role these oysters play in the Bay's ecosystem, eating algae and filtering up to five liters of water every hour. Historically, the booming oyster community off the shores of Chesapeake could filter the entire estuary in just three or four days. Now, they need over a year.
Figure 1. Oyster Aquaculture Pens.
For several years, the Chesapeake region has agonized over whether to replace ailing native oysters with introduced Asian Suminoe oysters. The bay's native oysters were once the largest source in the US, but over-harvest and habitat destruction decimated them to the point where two diseases in recent years have rendered them commercially useless. Many watermen want to bring in the hardier, disease-resistant Asian Suminoe oysters, and Virginia has already put supposedly sterile oysters into confined areas experimentally. They are thriving.
But many Marylanders, environmentalists and the National Academy of Science say that more study is needed to determine whether introducing an exotic species is a wise alternative to trying to rebuild native oyster populations. They fear Asian oysters might crowd out the native oysters, spread beyond the bay, and possibly become a nuisance. Sport fishermen are concerned that they might upset the food chain that supports striped bass and other popular fish.
Where did they go?
The oysters did not pack their bags and leave for a better place. But, with the way that humans have been treating them, if they could have, they certainly would have. Their populations have dwindled due to continual abuse from humans. Years of over-harvesting, in which high volumes of shells were violently scraped from the bay floor, wrecked havoc on the fragile reef habitat for future oyster populations. Further destruction came by environmental pollution and algal blooms resulting from an overabundance of nutrients such as nitrogen and phosphorus from nearby agricultural processes, leading to a severe depletion of oxygen which in turn hindered the development of oyster larvae. Pollutants such as toxic chemicals and heavy metals pumped into the system off of the nearby land threatened the development of juvenile oysters. To make matters worse, the suffering population has been bombarded with two parasites: MSX caused by a protozoan (Haplosporidium nelsoni) that thrives in low salinity conditions brought on by the drier climatic years, and Dermocystidium, a protozoan (Perkinsus marinus) that thrives in higher salinities. All of these activities have accumulated to plummet the oyster stocks to such a low level that it is doubtful that natural reproduction is even occurring in the bay.
Anthropogenic Introductions, why play with Nature's balance?
Invasions by plants, animals, and pathogens in non-native environments pose one of the most significant, least-addressed international threats to biodiversity, both within natural ecosystems and agricultural settings. For agriculture, one recent study estimated that 40% of all insect damage to crops in the US is attributable to non-indigenous species.
Figure 2. Harvesting Oysters.
For every known ecosystem the number of established non-native species is increasing. Some of these species are intentionally introduced and highly valued by humans. Most others are transported and released accidentally-- a number of these cause ecological and economic damage. Such harmful species are referred to as invasive species, and should be the targets of prevention and control efforts.
When a new species thrives in its environment to the point where it causes economic, biological, and human harm it is termed an Invasive Species. When introducing a non-indigenous species, it is difficult to predict what will be the long-term effect on the ecosystem. Sometimes the introduced species thrives in the absence of natural population checks such as predators and disease, present in the natural range, but absent in the introduced range. The introduced species may thrive to the point that it displaces native species and disrupts the entire balance of the ecosystem.
So, the question is, do the benefits outweigh the risks? Is it worth having a thriving oyster population in the chance that the entire ecosystem may be destroyed? Nature is a complex web of interactions that has taken millennia to develop. As humans, why do we keep interfering?
What Else Can Be Done?
Introduction of non-native Asian Suminoe oysters into the Chesapeake Bay ecosystem is not the only solution scientists seek. Maryland and Virginia have been managing their oyster resources since the 1950's. Over the years, the Bay Program Partners have worked together to develop regional management plans in efforts to restore oyster populations. Currently, representatives from the Bay states, the federal government, academia, environmental organizations and the oyster industry are working together to develop an Oyster Management Plan (OMP). The primary goals of the OMP are to rebuild native oyster populations and improve oyster management.
Figure 3. Wild Oysters.
Several factors limit the restoration of native oyster populations in the Bay including disease, habitat degradation, and the current low population levels of native oysters. The OMP includes a number of key strategies to help rebuild oyster populations. These strategies are: the evaluation of the use of sanctuaries and harvest reserves to obtain optimum ecological and economic benefits, the reconstruction of habitat, increasing hatchery production, breeding disease-resistant oysters, evaluation of impediments to aquaculture, management of harvest, improving coordination among the oyster partners, development of a database to track oyster restoration projects, and the monitoring of results.
A programmatic Environmental Impact Statement (EIS) is currently underway to evaluate alternative approaches to increasing oysters in the Chesapeake Bay. The alternative approaches include continuing current management practices, expanding the native oyster restoration program, implementing a harvest moratorium, establishing and/or expanding aquaculture operations, and introducing and propagating a non-native oyster species. Upon completion of the EIS, the OMP will be reviewed to determine whether any management actions need to be added or amended.
What does the Future Hold?
Proposals to offset the dramatic decline of native oysters in the Chesapeake Bay by introducing a reproductive population of oysters from Asia are being delayed until more is known about the potential environmental risks. In the meantime, there is hope that the natural management processes will be successful at restoring the health of the bay's oysters and ecosystem.