Follow the Path of an Oyster
The hatchery season begins in January when adult oysters are moved into the hatchery from the over-wintering floats to initiate the conditioning process. The University of Maryland Center for Environmental Science Horn Point Laboratory Oyster Hatchery has systems capable of heating up and cooling down ambient water from the Choptank River. The conditioning process is started by gradually warming up the water temperature on the oysters until a temperature of 20oC (74oF) is reached. The adult oysters will begin to feed on naturally occurring phytoplankton and put all of their energy into reproduction. It takes approximately two months to get oysters that are ripe enough to spawn in the conditioning system.
The hatchery is able to get a jump start on mother-nature with the conditioning system, which allows the spawning window of the oysters to be extended. By beginning the conditioning process in January, the oysters can be ripe and ready to spawn by March. In the Bay, oysters will not start to ripen until March with spawning to follow beginning as early as May. The normal spawning season of a wild oyster is June through August, sometimes into September depending on the ambient conditions. Once ambient river water warms up to 20oC, the conditioning system is switched from heating mode into cooling mode. In the cooling mode, chilled ambient water is used to maintain 20oC even though the temperature of the river is much warmer. This technology allows mature oysters to be conditioned and maintained for an extended period of time, so controlled spawning events can occur in the hatchery all summer long.
The mass spawning process begins by selecting a group of oysters and removing them from the conditioning system. Selected oysters are placed on the spawning table and water is turned on to the oysters at the same temperature, 20oC (74oF), they were taken from. This is to lessen any stressors the oysters encountered when moved. After an hour, the water temperature is increased on the oysters. The water temperature continues to be gradually increased over a few hours until a temperature of 30oC (86oF) is attained. If the oysters have not spawned naturally at this point, oyster gonad will be introduced into the water column to stimulate the oysters. Once the oysters filter and detect the gonad in the water, they will start to release their own gonad and kick off the spawning frenzy. This process is called mass stimulation.
Oysters are sexed by how they spawn, and what they release. Males will emit sperm out from the dorsal side of their shell. Sperm is white in color and is released in a cloudy or milky stream. Males will release sperm in a continuous flow for an extended period of time. Females clap their valves together to release eggs in a puff from the front of their shells. Eggs are white in color and are approximately 40 microns in size. A cloud of eggs looks similar to very small sugar granules suspended in a glass of water. Females can clap every 30 seconds up to an hour, producing anywhere from 2 to 70 million eggs per spawning event. The fecundity, or amount of eggs a female can produces, depends on the female’s size, age, health, condition, and if it’s the first or last spawn of the season.
Once the sex of the oyster is determined, the spawning oyster is removed from the spawning table and placed into a smaller Rubbermaid dish pan filled with water at the same temperature of the larger table. All females are placed in one container and all of the males into another. This is done to collect the eggs and the sperm, get a diverse genetic gene pool, and to prevent polyspermy, the occurrence when too many sperm fertilize one egg. Polyspermy results in over fertilization and the eggs will not be viable. One spawning event can last up to one hour, with the total time of the acclimation of the oysters at three to four hours.
Once the oysters are done spawning, the adults are carefully removed from the spawning tubs and measured. The egg mixture is transferred into a large 30 Liter bucket and diluted with filtered ambient water at the correct salinity and temperature. The eggs are counted using a microscope to tally the number of eggs found in one milliliter of the diluted egg mixture. Once the estimated number of eggs is determined, the proper amount of sperm is added to the buckets. The eggs will be allowed to fertilize for ten minutes and will be checked to see how they progress under the microscope. If the eggs are not fertilized more sperm will be added, but if 90-95% of the eggs are starting to undergo cell division, the eggs will be transferred into large larval tanks for grow-out.
The larvae remain in larval tanks filled with filtered ambient water for three days during which they are fed a cultured source of micro-algae. The eggs develop into D-hinge larvae and begin to swim around and eat. A complete water change is performed on the larval tanks twice a week to exchange out old water and waste products, uneaten algae, and to check on the health of the larvae. Large sieves covered in nitex fabric are placed under the drain valve of the tank, and the tank is slowly drained through the sieves. Larvae are retained on the sieve while water passes through. A 10,000 gallon tank can be drained through a sieve in about 3 hours. Once the tank is drained, a dilution of the larvae will be made and counted under a microscope noting size and gut contents. The tank is scrubbed clean and refilled. Larvae are placed back into a tank where they will continue to grow. This process is repeated for 14-20 days with the larvae growing in size from 40 microns to over 250 microns.
Larvae nearing the 14 day mark develop an eye-spot and a footŁ. When larvae reach this pediveliger stage they are carefully graded and sorted according to size. Larger larvae will have eye-spots and an active foot and are ready to set. These larvae are pulled off from the others and introduced into a setting tank. The smaller larvae that have not reached the pediveliger stage are put back into the larval tanks and allowed to mature for a few more days.
Setting tanks are large outdoor tanks that hold containers of shell where the eyed larvae are introduced to become spat. The Oyster Recovery Partnership is responsible for cleaning and filling the stainless steel containers with shell, and transporting them into the setting tanks. Loaded tanks are filled with ambient river water and the salinity and temperature of the water is adjusted to proper setting requirements. Air blowers provide even distribution of salt and temperature throughout the tank and provide oxygen for the larvae. Eyed larvae are carefully introduced into the tank by gently washing them out of a bucket on top of the shell containers. The top of the containers are washed down to ensure that all of the larvae have been rinsed into the water of the tank.
After 48 hours, shells in the setting tank are inspected under a dissecting scope to see if the eyed larvae have successfully attached to become spat. If the tank is determined to have successfully set, ambient water will be turned onto the tank and the spat will begin to feed on naturally occurring algae in the river water. If the tank has not set well, more eyed larvae can be introduced or the tank can be given more time to set.
Once the setting tank is on ambient water, the spat will harden by eating, growing and putting more strength into their shell. The spat are then ready to be moved out of the setting tank into the nursery or planted directly onto a prepared site. If the tank is being planted directly from the setting tank, the containers of shell will be dumped onto the deck of the Oyster Recovery Partnership’s planting vessel, the Robert Lee. Shell samples are collected and taken back to the hatchery for counting. Once all of the tanks are unloaded onto the boat, the boat will sail to the prepared oyster site where the spat will be blown overboard with a fire hose. Tanks going into the nursery will be removed from the setting tank and placed directly into the Choptank River in the shallow nursery area. The nursery grounds allow the spat on shell to grow to a larger size in a protected area while freeing up the setting tank for another round. The stainless steel containers will remain in the nursery until the spat reach a pre-determined size. Once the spat reach the correct size, the containers are removed from the nursery waters and dumped onto the deck of the boat. Random samples are taken throughout this process so we can determine the number of spat being planted from each tank. After the boat is loaded, the boat will steam to the designated planting site and plant the spat overboard.
Once the spat are planted on a site, the monitoring process can begin. The Paynter Lab at the University of Maryland College Park is responsible for monitoring all spat planted by the Horn Point Oyster Hatchery. For more information visit the Paynter Lab website.