Ecology, conservation, and restoration of oyster reefs in North Carolina

On Tuesday I went to the monthly pizza lunch at Sigma Xi, featuring a guest lecture by Dr. David B. Eggleston, Professor of Marine, Earth and Atmospheric Science at North Carolina State University and the Director of Center for Marine Sciences and Technology (CMAST). 

While Dr. Eggleston conducts research in several areas (and several geographic locations), in this talk he focused on the ecology, conservation, and restoration of oyster reefs in North Carolina.

Improvements in oyster harvesting technology a century ago almost immediately decimated the oyster populations in the estuaries of North Carolina rivers and Atlantic coast. A century of harvesting, particularly harsh during the Great Depression and WWII, led to the current record lows:

Oyster beds are important for more than just a potential source of food for humans. They serve as refuge for young fish from their predators, they break the tides and potentially slow down erosion, and the oysters themselves, as filter-feeders, clean up the water from organic materials. Thus healthy oyster beds are important components of a healthy coastal ecosystem.

While preserving existing ecosystems is always easier, cheaper and more effective than reconstructing them – it may take decades for ‘artificial’ ecosystems to start functioning fully as the the natural ones – once the ecosystem is destroyed there is not much one can do but try to rebuild it from scratch. And rebuilding from scratch can be expensive, thus it has to be done in a way that is most likely to be successful, i.e., informed by rigorous scientific research. And this is where Dr. Eggleston and his colleagues come in.

Mathematization of biology in the 1930s-40s by the likes of Fisher, Haldane and Wright was not primarily concerned with conservation issues – those were the beginning of formalization of evolutionary theory and ecology. Yet many of the models built at the time and refined since have important roles to play in conservation decision making. Most of the models have been tested primarily in the terrestrical ecosystems, so more work is needed to establish how they apply to marine environments where movement of individuals is much easier, energy-efficient and faster than on dry land.

The most important ecological model in this case is that of a metapopulation that is composed of a number of small populations with some migration between them. The concepts of ‘sources’ – populations with large population growth from which surplus individuals tend to emigrate from – and ‘sinks’ – populations which would not be able to sustain themselves if not for individuals that immigrate from elsewhere – are important concepts to keep in mind when devising conservation programs. Analysis of a metapopulation provides the answer to the question if one large space needs to be conserved or rather a number of smaller spaces. In terrestrial ecosystems, it appears that preservation of one large space is a better solution, but studies of marine environments to date suggest this may not be the case there.

Dr. Eggleston’s research is testing the theoretical models, as well as simultaneously using the models to devise conservation strategies. With help from a gadget-happy fisherman, they mapped the entire ocean floor of the bay.

Then, they built about a dozen centers of artificial oyster beds out of B-grade rock and populated those with oysters. Then they started sampling and monitoring the beds as well as the entire bay. A collaborator mapped the direction of water flow within it, which they then tested by monitoring the movement of oyster larvae which are poor swimmers and are thus passively transported by the water currents. The data matched the model quite well.

Interestingly, oyster larvae decide where, after two weeks of passive swimming in the currents, to swim down an attach to the substrate by sensing dopamine. The source of dopamine are other oysters already there. This makes sense, as the likelihood of successful reproduction depends on close proximity (and temporal synchronization) of other oysters during spawning.

The researchers then evaluated each center for several parameters. What they found is that some centers show fast growth, other centers good survival rates, other centers broad range of dispersal of larvae, and yet other centers evolved a level of resistance to disease, and yet no single center was “good at everything”. What they found instead was that, although neither one of the centers was a net source of oysters, the system (metapopulation) as a whole can sustain itself. Thus, they conclude that conservation should not focus on just one or two ‘best’ locations, but the large area as a whole. Furthermore, the implications of the results of the study is that several more such centers need to be built for the oyster population to become fully self-sustaining as well as a potential source of oysters outside of the area (where presumably they could be farmed for food).

They still do not have the data – too early for that but they are working on it – about the ability of these artificial oyster beds to serve as refuge for young fish against predators, or about the ability of the oysters to clean up the water. But it looks promising for now.

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