The Virginia Oyster:Part I
Author: Aaron Rowland, Virginia Certified Ecotour Guide
Only one species of oyster, Crassostrea virginica, is native to the eastern seaboard. This oyster is sometimes called the Eastern oyster or the Virginia oyster. Whatever it is called, it runs from the east coast of Canada down to Florida, and then from Florida through the Gulf. If you have ever heard people refer to Blue Points, Chesapeake’s, or Gulf oysters, they are all Virginicas.
Oysters are amazing little creatures. Despite being relatively small and completely sedentary, they provide several critical ecological benefits. One of their most important functions is cleaning the water. Oysters are filter feeders, and an adult can filter between 20 and 50 gallons of water a day. This filtration helps to clarify the water by removing sediment and phytoplankton, both of which can cloud the water. Cloudy water is bad for a lot of things, but one of the worst impacts is the damage to underwater vegetation. Cloudy water blocks sunlight, and just like any other plant, underwater vegetation needs light to survive.
But oysters do more than clear the water. They also filter out and absorb excess nutrients like nitrogen and phosphorus. These pollutants enter the water by way of runoff and have a negative impact on the Bay. Excess nutrients will cause large algae blooms, which both cloud the water and cause oxygen starved areas, called dead zones, when the blooms settle and decompose. These dead zones are so depleted of oxygen that animals can’t survive in them, meaning they either leave the area or end up dying. Oysters help prevent this by removing excess nutrients that can lead to oversized algae blooms.
Oyster reefs also provide habitat for lots of marine animals. Reefs can act as a nursery for juvenile animals, but also a refuge for smaller fish; this, in turn, attracts larger predators and makes oyster reefs very active areas, not unlike coral reefs in tropical climates. This makes oyster reefs important ecological areas, but also great recreational fishing spots! One of the Chesapeake Bay‘s most popular sportfish actually gets its name from its association with reefs. In the region, striped bass are called rockfish. They are called this because they hang around oyster “rocks,” which is a regional term for oyster reefs.
On top of creating habitat, reefs can also prevent erosion. Shallow water reefs can absorb wave energy as waves approach shore, lessening the impact of the wave on the shore. This not only helps the shoreline directly, but also slows down the water and movement of sediment enough to help grass beds form. Those beds in turn also reduce wave energy and slow the movement of sediment, further reducing erosion. For this reason, oysters are often used in shoreline restoration projects to help create ‘living shorelines,’ which are natural alternatives to using stone or concrete barriers. These projects can actually create new shoreline by catching sediment, and allowing new shoreline grasses to take root.
The Virginia Institute of Marine Science established an oyster breeding program in 1997. This program began breeding sterile triploid oysters as well as selectively breeding for disease resistance in both diploid and triploid oysters. A diploid is a regular oyster. A triploid oyster is the same oyster, but through tricks of breeding it has three chromosomes instead of two, making it effectively sterile. This is the same basic process used to make seedless fruit. The result of this program has been both a decrease in mortality in wild stocks, and the establishment of an oyster aquaculture industry in the Bay.
Triploids put all of their energy into growth. This means that they both grow faster than diploids, and do not spawn. A faster growing oyster means a shorter time from planting to harvesting. This reduces the probability that the crop could suffer loss from disease, weather, or predation. But more importantly, an oyster that doesn’t spawn is an oyster that you can sell all year as a consistent, quality product. When an oyster spawns, it puts all of its energy into sperm or egg production, and commits a lot of its body mass to this process. After it spawns, there just isn’t much oyster left in that shell. For this reason, most oyster farms grow few, if any, diploids.
Oyster aquaculture, or oyster farming, refers to raising oysters in some sort of containerized system. Most farms in the Bay utilize gear that sits on the bottom, with some working in intertidal zones. Float or suspended farms that occupy the water column are becoming more popular, but face a more rigorous permitting process.
Farms do not use wild oysters, but source their oysters from hatcheries. Hatcheries produce baby oysters, called seed or spat, that farmers buy and place on their farms. The hatcheries do this by either raising or acquiring brood stock and inducing a spawn. When water temperatures are around 70, oysters naturally spawn on their own, so inducing a spawn is basically just giving oysters a warm bath.
Hatcheries capture the spawn in tanks and allow larvae to form and set in a controlled environment, producing single seed oysters. This means that no oyster is attached to another, unlike in the wild where new oysters attach to existing oysters to form reefs. This helps farmers produce the single oyster half shell product you get in restaurants. A major benefit of aquaculture for the environment is that hatcheries are producing their own oysters, not taking any from the environment. Because of this, aquaculture serves to protect natural reefs by reducing harvest pressures. Buying a farmed oyster has no impact on the wild oyster population.
Oyster farms perform many of the ecological functions performed by natural beds and have been shown to have a net-positive impact on their local environment.
They provide a similar nursery and habitat function and so also attract larger predator animals. Farm gear actually provides a surface for macro-algae, which produces oxygen. And of course, the oysters filter the water.
Instead of just selling ‘Chesapeake oysters,’ farms highlight the differences in their oysters based on where they are grown. Oysters will take on different flavors from their environment, with things like the sediment over which they are grown, the species of algae they eat, the salinity of the water, and the mineral composition of the water all playing a part in the flavor profile of the oyster. These different flavor profiles and the ability of farmers to produce a consistent half-shell product, has led to an increased demand for farmed oysters, which has led to an increase in the number of oysters farms are putting in the water!
Coming soon… The Virginia Oyster: Part II