Over the past two years researchers from the University of Florida, the Department of Agriculture and Consumer scientists, and local watermen have teamed up to study the health and population trends of oysters reefs. From Horseshoe Bay to Corrigan`s Reef this team has been working to document not only the current health of oysters in our area, but also to examine how these oyster reefs have changed over time by examining aerial photos taken over the last 30 years. Dr. Jennifer Seavey from the University of Florida reports here on study findings regarding the future for oyster resources.
Freshwater limitation is likely cause of past decline and critical to future sustainability of oysters?
Many Cedar Key residents know that marine organisms respond differently to period of freshwater input - some can take it (blue crabs, oysters, some shrimp), and some cannot (clams). Oyster predators, such as fish and predatory mollusks, are more abundant and successful under salty conditions. When freshwater pulses create periods of low salinity, predators either move (fish) or die (drills). In addition, one major parasitic disease of oysters (caused by Perkinsus marinus, a single-celled marine organism, commonly known as "Dermo") is much less prevalent and infective at low salinities. It is no surprise that the eastern oyster thrives in areas that have seasonal periods of lower salinity, often in bays or estuaries (think Apalachicola and Chesapeake) or at the mouths of rivers like the Suwannee. Take away fresh water pulses and you get very low oyster survival
We propose that reduced freshwater in the region has dramatically increased the periods of high salinity, leading to increased oyster predation and disease. Under these conditions, living oysters on reefs would die off faster than they can be replaced, leaving the reef covered with dead oysters. These dead reefs, having lost their protective cover of living oysters that act to cement the structure together, become vulnerable to erosion and break up by wave action. Since most oyster reefs in our study area are built on riverine sediments, the breakup of oyster shell would likely trigger a spreading and sinking of the loose underlying sediment. The former oyster reef has now become a sand bar in a few short years. You can see these vast sandbars now out at the outmost sections of Corrigan`s Reef, Lone Cabbage and Great Suwannee reefs where only twenty years ago, there were healthy reefs.
Once this chain of events occurs, the oyster reef cannot easily re-establish because the substrate is now sand, where young oysters (`spat") cannot survive. In 2010, we did observe spat settling in large numbers on offshore reefs, but we also saw that they do not survive in the shifting sediments, which are poor adhesion sites and also offer no protection from predators. We also saw drives of crabs and small oyster drills systematically mowing these young oysters down. Normally, spat would have plenty of crevices and cracks in the live oyster reef structure in which to survive. On sand bars, they have little hope for survival.
What does the future hold for oysters in this area?
Can this process be reversed or are we witnessing the end of oysters in the area? To be frank, we have little control over the amount of freshwater coming down the Suwannee River (that is a great topic for another report). This is a long-term, regional battle into which we should invest. However; in the short-term and at a local level, we need to focus on trying to maintain some oyster reefs under this new regime of decreased freshwater flow. The offshore reefs - being farthest from the freshwater source - may never come back to the way they were. But, what about those closer to shore? We believe that under low flow events, many of those reefs close to shore will die. However, there is probably enough freshwater that they can exist for long periods between droughts. The questions is, could these reefs come back if they had stable substrate for spat to attach and survive on?
We are currently embarking on a new project to test the idea that reefs in our study area are limited by a lack of structure on which to rebuild. Stable structure could be established by using various kinds of cheap, durable materials that can withstand storms and tides. In our area, clam bags that have been abandoned and colonized by oysters turn out to make great instant oyster reef building blocks - just look at the created reef next to the dock on Atsena Otie. So there may be uniquely local solutions to keeping good oyster settlement sites around.
As I alluded to above, the creation of good settlement sites are only one part of the equation. The key to future viability of oyster resources in the Big Bend region ultimately resides in effectively balancing the freshwater needs of humans with the needs of seafood producing estuaries like ours. Global climate change and increased development will only increase the intensity and frequency of drought events, as well as raise sea-levels and increase storm intensity. Restoration of oyster habitat in the Gulf must include balancing freshwater resources to meet the needs of people and estuary systems upon which we rely. And here`s the thing - this is not just about oysters - over 90% of our sport and harvested finfish and crabs need the low salinity estuaries to reproduce and rear young. If we want a healthy and rich marine environment in Cedar Keys future, we need to ensure that our estuary gets its fair share of fresh water.