Researchers use dye to help predict potential dispersal of non-native oyster larvae

(July 21, 2004) Coleman Bridge commuters may see what appears to be a "red tide" in the York River next Monday afternoon. But instead of a potential menace to the waterway, the red patch will be part of a study designed to further enhance biosecurity of non-native oyster trials in Chesapeake Bay.

"The red patch will result from the release of 30 gallons of rhodamine dye into the river," says project leader Dr. Roger Mann of the Virginia Institute of Marine Science.

Mann, Dr. Kenneth Moore, and other VIMS researchers are tracking the dye to help predict where tidal currents would carry larval oysters in the unlikely event that any of the non-native oysters now deployed in York River seafood industry trials successfully reproduce.

The dye study is a requirement of the recent agreement by the Army Corps of Engineers to extend the Virginia Seafood Council’s on-going trial of the non-native oyster Crassostrea ariakensis. NOAA, EPA, and the US Fish and Wildlife Service also contributed to the decision.

The Virginia Seafood Council (VSC) trials began in September 2003 when commercial growers deployed 100,000 oysters at each of 8 sites in the Virginia waters of Chesapeake Bay. The Virginia Marine Resources Commission granted a state permit for the trial through May 2005. The Army Corps of Engineers granted a federal permit through June 2004. Both dates were based on a planned starting date of August 2003. But a two-month delay in the initial deployment of the oysters led the growers to request an extension of the federal permit until April 2005 so that their animals can grow to market size.

The main concern with the VSC request is that the extension will allow the non-native oysters to remain in the water during their summer spawning season. The deployed oysters are the offspring of adult oysters that were genetically manipulated to produce progeny with three sets of chromosomes, rather than the pair of chromosomes carried by normal "diploid" oysters. These "triploid" offspring are effectively sterile. However, this process is not foolproof, and the original VSC permit allowed a maximum of 1 diploid oyster per 1,000 non-native oysters deployed.

The federal agreement to extend the VSC permit contains two key provisions to minimize the risk that any of the deployed diploid oysters might successfully spawn and produce larvae that could establish a feral population.

First, the extension permit requires that the growers further thin out their oysters, either by distributing them amongst a larger number of containment bags, or selling those of market size. The growers must also move the structures that contain the bags farther apart. Oysters are "broadcast spawners" that release eggs and sperm freely into the water. Increasing the distance between spawning oysters decreases the already low risk that water currents will bring an egg and sperm cell together for fertilization.

The second provision requires the completion of the upcoming dye study, to better understand where tidal currents are likely to carry any larvae that might result from successful reproduction between two diploid non-native oysters. If tidal currents scatter the larvae so that they settle over a broad area, they will be too far apart to reproduce as adults.

VIMS Assistant Professor Jian Shen will feed the data from the dye study into a high-resolution computer model of lower Chesapeake Bay. The model will help Mann and Moore track and sample the dye patch in the week following its release (after which it becomes too dilute to measure). In turn, their field data will help the model predict the movement of the patch for another two weeks, and can be used to calibrate the model so that it can more accurately predict larval dispersal at other sites and under different conditions.

Shen estimates that the patch will be around 2 miles long and will be visible for less than an hour. He adds "since the dye will be released from the bottom, we might not be able to see it from the surface at all."

Researchers have used rhodamine dye in hundreds of studies since the 1960s to trace water movement through lakes, rivers, and estuaries. They use it because it dissolves readily in water, is harmless in low concentrations, and is strongly fluorescent and thus easy to detect. Mann and Moore will trace the dye patch using a boat-mounted “Dataflow” sensor that can measure dye concentrations continuously while the vessel travels at speeds up to 20 knots.

The researchers will initially release dye near Crossroads Aquafarms & Chessie Seafood and Aquafarms, on the York River at Yorktown. Additional dye studies will take place in the coming months at three other sites selected to provide a broad range of tidal conditions and salinity. These are Seafarms Inc., at Milford Haven in Mathews County, Shores & Ruark Seafood Inc., on the Rappahannock River near Urbanna, and either Kinsale in the Yeocomico River or Accomac Aqua Farms on Foley Creek.

"Our overall objective is to demonstrate and quantify dispersal of a larval surrogate--fluorescent dye--at four of the trial sites to provide quantitative input to the risk analysis," says Mann. "Release of fluorescent dye provides a tractable method to simulate passive dispersal of larval forms on a site-specific basis in real time. Site-specific data are required to provide critical values for risk assessment in the ongoing permit process."

Funding for the study comes from EPA and NOAA through the Chesapeake Bay Program, and from the Virginia Marine Resources Commission.