This guest post was written by Ben Maxie. Ben is an undergraduate researcher at Old Dominion University who studies zooxanthellae genetics with Dan Barshis. Aside from marine biology, he is interested in beer brewing, car modification, and hiking.
In a small refuge in the Elizabeth River near the Chesapeake Bay, my colleagues and I trudged through the mud, frigid water up near the tops of our waders. We pulled behind us two cages full of juvenile oysters, some rebar, and a mallet. The tide was low, and most of the riverbed was barren mud. There were, however, some small oysters scattered among the rocks lining the way. As we got out into deeper water, I saw more oysters hidden away in the cracks, most competing with the barnacles and covered over by algae.
A few hundred years ago, we would have been crushing oyster shell underfoot. Only around 1% or less of the original oyster biomass remains in the bay from before European settlers arrived. It’s estimated that the precolonial oyster population could filter the bay in as little as a week. Captain John Smith said that oysters “lay thick as stones” in 1608.
Now, projects are underway to bring the eastern oyster (Crassostrea virginica) back from the brink of extinction. In the Harris Creek in Maryland, for example, 315 acres of oyster reef has been artificially constructed. We were working with the Chesapeake Bay Foundation (CBF), a nonprofit working on, among other things, restoring the Chesapeake Bay’s oysters. Through programs such as Spat Catchers, “we go out and harvest wild oysters from the tributary that we’re targeting, which go to a hatchery and spawn, then replant with the native oysters,” says Jackie Shannon, the Virginia Oyster Restoration Manager for the CBF.
Not everyone is so optimistic. Ian Bartol, a marine biologist at Old Dominion University, says that although some populations of oysters are doing well, wild oysters are overall declining. He suggests that although planted oysters may survive shortly after planting, they usually succumb to environmental factors, mostly disease, within just a few years.
What got us here?
Oysters were a favored fishing industry in the 1800’s, and like buffalo or carrier pigeons, their huge numbers lead to massive overharvesting. Whole towns were built on mountains of oyster shell after the canning industry took off after the Civil War. The first signs of trouble were in the late 1800’s when catch numbers started slowing down, but this didn’t deter the industry. Oysters were still harvested in massive numbers until the early 1900’s, when reseeding efforts began.
A major obstacle to oyster recovery is that the larvae need to settle on hard substrates, which are usually in short supply except for living oysters or dead oyster shell. Overharvesting oysters led to a loss of shell from the environment, essentially removing their natural habitat. This problem is exacerbated by oyster-shell-consuming organisms and predators, such as comb jellyfish, crabs, and oyster drills, and made even worse by the removal of their predators, mostly sharks.
Researchers are currently working on possible shell alternatives. Real oyster shell is usually too expensive for any kind of large scale replanting operation, but several other substrates are being tested for oyster reefs. The most promising of these seems to be concrete.
“Our field and lab experiments suggest that concrete is a viable alternative to oyster shell, marl, or other calcium carbonate-based substrates,” says Dave Eggleston, a marine biologist at NC State University. At least for the first year to year and a half, concrete holds up better to boring sponge, and settlement and subsequent survival is similar between concrete and shell.
Kicking them while they’re down
Disappearing shell is only a small part of the problem. In 1957, Japanese oysters were introduced to the Delaware Bay in New Jersey to test their growth. The oysters did not fare well, but they carried a disease known as MSX, which ferociously swept through the eastern oyster population. MSX was first seen in the Chesapeake Bay in 1959, and reached up to 90% mortality in high salinity water. Not long after, another disease called dermo became established. Dermo attacks mostly older and larger oysters. This is problematic because, like clownfish, oysters are sequential hermaphrodites. Juvenile oysters begin life as males, then some become females after they reach a certain age and size. Dermo effectively goes after females.
The disease problem is compounded by the fact that both diseases are more deadly at higher salinity, and oysters need saltier water to reproduce and recruit new juvenile oysters. Although oysters are safer in fresher water, they do not grow or reproduce as well there.
In the decades since these diseases have showed up, wild oysters have begun to show signs of resistance to the disease through natural selection. In 1985-86, MSX killed off all susceptible oysters in the Delaware Bay, leaving only resistant strains. These events are not uncommon, and there are communities of resistant oysters in scattered parts of the Chesapeake Bay.
According to Bartol, there are areas of hardy oysters, especially around the James River, but such resistance has not spread to most areas of the Chesapeake Bay. This means that until these oysters adapt to specific other areas, this resistance is unlikely to spread in the immediate future.
For resistant areas, “we don’t really see disease affecting wild oysters so much anymore,” says Jackie Shannon from the CBF. For restoration, “we’re kind of charging forth with the wild oyster, they’re demonstrating resistance naturally and evolving in a bay that’s also changing.”
There are also selective breeding programs for disease resistance in several aquaculture facilities, with generally positive results. These oysters are generally only used in oyster farming and aquaculture, however. This is mostly because of concerns that raising disease resistance might affect other oyster traits, such as reproduction or growth rates. Some altered oysters grown for harvest are also made sterile, notes Shannon.
Is restoration working?
As Roger Mann and Eric Powell noted in their 2007 review, the eastern oyster is ancient, around 60 million years old, while the Chesapeake Bay is relatively young, beginning only around 10,000 years ago. Oysters have spread across new features as they arose, and have likely faced local extinction several times before. “The eastern oyster probably won’t go extinct anytime soon,” remarks Bartol, noting disease-resistant communities across the East Coast.
It is generally agreed that restoration efforts should really focus on making sure that planted reefs survive and can replenish themselves, or at least be replenished by surrounding areas, in the long term.
The Chesapeake Bay Foundation is keeping track of natural oyster recruitment through programs such as Spat Catchers, with generally positive results. “It’s really given us a good idea of where oysters are recruiting, how many are recruiting, and whether it’s gone up over the years. We see a lot of great natural recruitment, especially around the mouth of the Lafayette River,” says Shannon.
High recruitment does not mean that a population will do well for several years, notes Bartol. More research on MSX and dermo resistance is needed to help non-resistant areas still hard-hit by disease. Established reefs need to be monitored closely to make sure that they won’t disappear as soon as human assistance ceases.