The simplest is to coarsely filter 10 to 25? A second method involves filtering natural water in the same way and then fertilizing it to stimulate algae growth and reproduction. After a significant amount of algae is produced, it is fed to the oysters.
Both of these methods have worked for hatcheries but the results can vary considerably; and, the water can be contaminated by unwanted zooplankton or the wrong kinds of algae.
A third method is to separately culture several species of algae from pure cultures of each desired species. Algae species that have been used to grow oyster larvae include Chaetocerus gracilis, Isochrysis galbana, Pavlova spp. Several studies have shown that a mix of algae species results in better growth. Culturing algae can be labor intensive, requiring repeated sterilization of glassware as the algae is moved through a series of larger containers.
Several continuous culture methods have been developed that can reduce labor and provide larger volumes. See the Additional Reading section for sources of more detailed information on culturing algae. A fourth method is to purchase concentrated algae from commercial producers. While often expensive, commercially produced algae may be cost effective depending on the size of the oyster hatchery. However it is obtained, algae must be added daily to the larval culture tanks at concentrations that result in the densities listed in Table 1.
Intensively cultured algae are very dense and often a diluted subsample must be counted. To do this, a drop of diluted culture water is placed on a hemacytometer a special microscope slide with finely etched squares to aid counting and the cells within several 1-mm-square areas are counted. The cell count is divided by the number of 1-mm-square areas counted and then multiplied by 10, to get the cells per ml. This number is then multiplied by the dilution factor.
The volume of culture water needed to achieve the desired density in the larval tanks is determined from the calculated density of algae. For example, if the hemacytometer count shows cells in four 1-mm-square areas, the number of cells per 1-mm-square area is Multiply by 10, to get , cells per ml.
If the sample was originally diluted by a factor of 10, multiply by 10 to get 2,, cells per ml in the original culture. The desired density of algae at the beginning of larval culture is 25, cells per ml. Suppose the larval tank is gallons L. Larvae are ready to set when they have a well developed eye spot and are ? Larvae that are ready to set are usually selected by sieving them through a ? Larvae that pass through are restocked.
The retained larvae are sieved again on a ? Those that pass through are also restocked to a separate tank. The retained larvae larger than ? This procedure is repeated every day until the desired number of eyed larvae is obtained or the number of eyed larvae dwindles to the point that it is no longer effective to continue.
The large, eyed larvae can be set on a variety of materials cultch using several methods. The choice depends on the desired use of the resulting spat. Single oysters can be obtained by setting larvae on microcultch, very smooth and slippery surfaces, or by chemical induction. Microcultch is usually made of finely ground oyster shell sieved to produce shell pieces to ? A single larva sets on each particle. After larvae have metamorphosed to spat they can be popped off the sheet.
Chemical induction involves treating larvae with chemicals such as epinephrine or norepinephrine at very low concentrations to induce metamorphosis without the need for a substrate. Cluster oysters are the result of setting larvae on large cultch, usually whole oyster shell.
This results in a product similar to what occurs in nature—a shell with many spat. Over time, natural attrition results in two to four adult oysters per shell. Almost any non-metallic, hard surface could function as cultch for cluster oysters. Single oysters are produced by introducing eyed larvae larvae per square inch or per square cm of bottom surface area into containers with fine mesh bottoms l to ? The containers, called wellers, are immersed in shallow tanks containing treated seawater Fig.
Containers are configured so that water can flow up through upwelling or down through downwellng the mesh bottoms by routing water through an opening near the top of the container. At stocking, the container is set for downwelling and the entire tank covered with black plastic to encourage more even setting. Algae is added to feed late-setting larvae and early metamorphosed spat. Setting and metamorphosis usually takes 48 hours. After metamorphosis is confirmed, the containers are converted to upwellers using coarsely filtered ?
The bottom mesh on the upwellers should be cleaned daily. As the spat grow they are graded on different mesh size sieves and the larger spat moved to upwellers with larger mesh bottom screens to enhance water flow and growth.
Growth is highly dependent on the density of spat, water flow, and the abundance of natural food in the water supply. Food can be supplemented with algae as in larval feeding. The major drawback to single oyster production is the amount of labor required to maintain oysters until they reach the desired size.
The initial upweller culture continues until oysters are large enough to be placed in the smallest mesh size nursery bag. These bags can be kept in the hatchery under high water flow or placed in natural waters off-bottom. The small mesh size of the bags makes regular cleaning necessary. As the oysters grow they are sieved through appropriate size meshes slightly smaller than the mesh size of the bag they are being put into and the retained oysters moved to larger mesh bags.
The larger mesh bags also require periodic cleaning and inspection for predators in the bags. An example of mesh sizes and stocking densities is given in Table 2.
Given the labor costs of growing single oysters, it is very tempting to release single oysters onto suitable bottoms at a small size. However, several studies have shown that the mortality of small, single oysters is quite high when they are not protected in bags or some kind of container.
Setting on whole shell or other large cultch can be done by placing the cultch in large mesh bags and the bags into tanks with treated seawater. Eyed larvae are introduced at the rate of per shell, with a goal of obtaining 10 to 30 early spat per shell.
When the waters of the San Francisco Bay estuary warm in the summer, male Olympia oysters release sperm balls and on contact with seawater, the sperm balls disintegrate, releasing the sperm. Other male oysters sense this and soon follow suit, releasing their sperm balls into the water.
A male oyster releases hundreds of thousands of sperm balls, each containing approximately 2, sperm. Females then bring the sperm into their shells through respiratory action and fertilize their eggs internally. The fertilized eggs develop inside the female for about 10 days before being released into the environment. The larvae then become part of the planktonic community, floating with the currents and tides.
After approximately three to four weeks, the larvae metamorphose to their juvenile form and are ready to settle on and attach to a hard substrate, such as oyster reef balls, rip rap along the shoreline, or scientific monitoring devices.
If the oysters are able to find a hard substrate which can be difficult and thus the reason for our native oyster restoration efforts , they attach themselves and will hopefully remain there to live out their lives.
Wild oyster growing cycles typically follow the oyster life cycle and should technically have consistent numbers. Unfortunately, it's hard for people to leave wild oysters undisturbed. It's tempting to harvest anything market size because of the sales potential, and with climbing demand, oyster beds are being picked over in areas like Wellfleet.
Most of the pressures on wild supply, though, are due to regulations. Regulations obviously limit harvest amounts and implement sizing restrictions, but more recently, the development of vibrio regulations have had even greater impacts.
As an example, the new vibrio regulations from Connecticut this summer have greatly limited the ability for fishermen to harvest Blue Point Oysters. The lack of New England oysters are definitely more noticeable now without the support of wild supplies. So, as we wait for our oysters to grow, we're getting the farm ready for our new seedlings that arrived in May.
Last week, we finished our first grading of 2 million seed from our upweller nursery, and soon, they will get ready to grow in their aqua purses out on the tide. We have new help on the farm and a lot to do. Even though we can't sell these oysters yet, they will be 3 inches by next fall before you even know it. We're looking forward to the warm weather and the wonderful summer winds in Duxbury Bay. A new crop for a new season -- to and beyond! Oysters Clams Mussels Crab.
Resources The relationship between the oyster growing cycle and supply By. The oyster is our world.
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