University of Maryland Center for Environmental ScienceHorn Point Oyster Hatchery

Broodstock Conditioning System

Oysters in the Chesapeake Bay go through a yearly cycle of gonadal development. During winter, oysters exhibit no gonad. As bay water temperatures begin to rise in spring, gonadal development commences and continues until the animals are “ripe” and ready to spawn. In this region, spawning normally occurs around the end of June or the beginning of July. Oysters in Maryland may remain in spawning condition throughout the summer months and, rarely, into the fall. As water temperatures begin to cool off in autumn, oysters stop producing gonadal material and begin to produce glycogen, a food reserve they use to survive the cold winter months. As spring approaches, the cycle is repeated.

In the hatchery, oysters are brought into the broodstock conditioning lab and cleaned of fouling organisms like barnacles and mussels, and scrubbed to remove any dirt or mud, then placed into isolation compartments in specially constructed tanks designed for holding adult oysters for use as broodstock. Gonadal development is accelerated by the addition of heated water (during winter and spring) or delayed by adding chilled water (during summer).

Broodstock are cleaned of fouling organisms like barnacles and mussels

Broodstock conditioning tanks allow us to “trick” oysters into sensing that it is springtime by artificially raising the water temperature thereby initiating gonadal development. The system is able to accomplish this through the use of a sophisticated seawater system that allows us to both heat and cool water pumped from the Choptank River. Heated or cooled water is then automatically mixed with ambient Choptank River water in the mixing boxes located above the broodstock conditioning tables to insure the proper conditioning temperature.

oyster hatchery conditioning system

A critical component of efficient hatchery operation is to have a supply of ripe brood oysters when needed throughout the entire spawning season. This can be a tricky and delicate process, especially as more and more of the populations become ripe. It is sometimes difficult to prevent an uncontrolled spawn while keeping oysters on the verge of spawning. Accurate temperature manipulation is one key to the success of this system. Personnel from HPL have developed a unique procedure that allows for an expanded spawning season.

Broodstock tables are separated into compartments, each containing a single population of oysters. It allows us to easily separate oyster broodstocks which have been collected from many different locations. Currently, there are oysters originating from several different states along the Atlantic and Gulf coasts. These oysters have been used to produce selected strains of seed for use in several research projects including large scale outplantings designed to evaluate and test for disease resistance. These compartments also help prevent uncontrolled spawnings. By keeping each population in isolation, the risk of a mass spawning, involving oysters from all the populations on the table, is minimized.

Spawning Table

The majority of all oyster spawns in the AREL facility are mass spawns. Mass spawns are spawns where many broodstock are placed in a single spawning container and manipulated until spawning occurs. In our facility, groups of ripe oysters are placed on specially designed spawning tables where heated Choptank River water is allowed to flow over them. Typically rapidly raising the temperature of the water where the oysters are being held usually initiates spawning. Frequently, in order to minimize the amount of time required for spawning, one or more broodstock will be sacrificed to determine the stage of gonadal ripeness. This is usually done before the oysters are placed on the spawning table. This accomplishes two things. First, it allows us to determine the stage of “ripeness” of the gonads, by microscopic observation if necessary as it makes no sense to attempt to spawn un-ripe broodstock and second, it provides some gonadal matter that can be used as a stimulus (if needed) to encourage a mass spawning to begin. If needed, a suspension of eggs or sperm collected from the sacrificed individuals is made. This suspension can then be “fed” to broodstock on the spawning table. Introduction of the gonadal suspension enhances the spawning response of the broodstock.

mass spawn of oyster broodstock

Oysters are “broadcast” spawners, that is they release their eggs or sperm directly into the water column. The eggs are then fertilized external to the brood oyster. For fertilization to occur, it is important that many oysters spawn at the same time. One oyster spawning individually would not result in any egg fertilization, therefore no young oysters would be produced. Oysters have developed the ability to sense when spawning is occurring. In nature, on a healthy oyster bar, spawning could begin with a single individual whose gonadal material (either eggs or sperm) would be released and carried by the currents to neighboring oysters. Through normal pumping and filtering activity, neighboring oysters would detect the first oyster’s spawning activity which would subsequently trigger them to spawn. If conditions are right, a chain reaction would occur where a large percentage of the oysters on that bar would spawn in synchrony. In this way, the chances of egg fertilization are greatly enhanced.

In the hatchery, as oysters begin to spawn, they are sexed and separated into plastic tubs (one for males and one for females) where they are allowed to spawn out. Sexing broodstock when they begin to spawn is done in two ways. First and easiest is by observing their spawn behavior. Males spawn by expelling sperm in a continuous stream about midway along the side of the shell margin. Once a male begins to spawn, spawning may last up to an hour, usually in periods of about five minutes. The presence of spawning females may greatly enhance this activity. Females usually spawn very differently from males in that they gently open their valves and expel eggs in a “pulse” from the front of the shell. Once spawning commences in females, it too can last for an extended period of time perhaps up to an hour. As with males, the presence of gonadal material from other spawning oysters may enhance a female’s spawning activity. The second is to collect a concentrated sample of what the oyster is expelling into the water column and examining it under a microscope to see if it is eggs or sperm.

After most spawning activity has ceased, broodstock are removed from the spawning tubs and the eggs are counted. During counting, evidence of fertilization is noted and the proper amount of additional sperm is then added to insure that maximum fertilization has occurred. Spawning tubs are used to prohibit too much sperm from coming into contact with the newly spawned eggs which can result in a condition called polyspermy. Polyspermy occurs when more than one sperm attempts to fertilize the same egg, resulting in eggs that either do not hatch or in oyster larvae that are deformed. Larvae produced in this manner usually do not live more than a few days.

Oyster Larvae Rearing Tank

Once eggs have been properly fertilized and counted, they are distributed to large culture tanks labeled mass oyster larval/algae culture tanks where the eggs hatch and the larvae are allowed to grow and develop. This process takes two to three weeks, during which time water in the tanks is changed about every two days.

tank water is drained through large fine meshed sieves where the larvae collect

Tank water is drained through large fine meshed sieves where the larvae collect. At each water change, larvae are examined and may be graded to size. The tanks are cleaned and refilled with filtered water from the Choptank River that has been tempered to ensure optimum growth. Cultured algae is added, and the proper amount of larvae are replaced.

The number of oyster larvae varies according to tank size and the size and stage of the larvae themselves. The largest tank in this hatchery can hold up to 1 billion eggs. Over the two-week larval period, this would gradually be reduced to around 75 to 150 million setting stage larvae. Smaller larval tanks are used for replicated experiments that test a wide range of conditions effecting oyster larval growth and survival.

Production Setting System

Setting tanks are used to induce larval settlement. Oysters undergo a metamorphosis at the end of the larval period when they change from a free swimming larvae to one that is attached to some form of substrate. Once attached, they are referred to as spat and they lose their ability to swim or to move away from conditions unsuitable for their growth and survival. This is a stressful period on the delicate larvae, and typically they undergo a major mortality during setting. It is not uncommon for over 90% of the larvae to perish during this process.

oyster setting tanks

Large fiberglass tanks, located on the concrete tank pad outside the hatchery building and down near the pier, are used to produce oyster spat on cultch (shell). Cultch is a term referring to any hard substrate used by oyster larvae to attach during settlement. In nature, this is usually other oyster shell, but almost any hard clean substrate can be used by setting oysters for cultch. These production setting tanks are each capable of producing approximately one million oyster spat per week. They are loaded with mesh bags of cleaned oyster shell and filled with heated water from the river before oyster larvae and algae are added. Electric immersion heaters are used to maintain a constant temperature of 75 – 86F during the setting process. The number of larvae added to these tanks varies with particular research project objectives. Larval oysters are distributed throughout the tank by their swimming action and by gentle aeration. During the first two days of the setting process, no water is allowed to drain from the tank and no water is added. However, algal food cultured here at the hatchery is added as needed to provide the oysters with nutrition during this critical stage. Two days after the introduction of oyster larvae into them, raw, un-filtered water from the Choptank River is allowed to continuously flow into the tanks. The flow continues for a few days, giving the newly settled spat a chance to feed on natural food found in the river water, and to begin the process of “hardening” their shell. Newly settled spat are very fragile and need a few days to complete metamorphosis, grow, and harden before it is safe to transport them from the tank to either a nursery area or for final deployment.

oyster spat on shell

Nursery areas are simply a site, usually located in a protected section of the river, with good tidal flow where the spat are placed and allowed to grow to a pre-determined planting size which may vary according to project requirements. During the nursery process, shells with the newly-settled oyster spat are allowed to remain in their containers or bags for protection and to allow for easy re-deployment. In the mid-Chesapeake area, oysters can remain in the nursery areas for a period of a month or two until they have reached planting size and can be moved to the final grow-out site. Not all spat on shell produced at the HPL hatchery are required to go through this nursery phase. Many are deployed directly to the final grow-out site directly from the setting tanks. Studies are underway to determine the effectiveness of this practice.

Downweller Setting System

Downweller setting tanks allow us to provide suitable clean substrate to which oysters may attach in small experimental batches. This is accomplished in the wooden frames positioned in larger fiberglass tanks. A different batch of larvae can be introduced into each frame where, if conditions are favorable, setting will occur. Fine mesh nylon material called NITEX is used to retain the larvae within the frames and allow algae rich river water to be slowly circulated down through the finely crushed shell (hence the name “downweller”). Setting usually occurs within 48 hours of the introduction of eyed oyster larvae into the downweller. We commonly use the downweller setting system for controlled experiments on a small scale. We do not use this method in our restoration efforts.

Upweller System

Once settled in the downweller, oysters are no longer in larval form, and are referred to as cultchless spat. Since they can no longer move about on their own, they are much easier to handle. Newly settled cultchless spat are moved into upweller tanks for initial growout. Upwellers get their name from the way the river water flows through the oysters. Water is introduced into the bottom of the tray holding the upweller trays and passes through a screen on the bottom of the individual tray, past the oysters being held on the screen, up and out the drain. Upwellers utilize ambient Choptank River water (water that is unfiltered and unheated or unchilled) for oyster spat growth. Upweller screens are of differing mesh sizes to accommodate different size spat.

During the time the spat are in the upwellers, they must be cleaned and graded as they grow. Under optimum conditions, spat growth can be very rapid in this type system, with oysters reaching one inch in length in only a month or two. Growth will vary depending on many factors, including temperature, salinity, and food availability. Once at the correct size, individual cultchless oysters can be used in a variety of experiments, and are not used in our larger restoration endeavors.