Hatchery manual for the pacific oyster

The eyed larvae can move only very small distances, once they settle, in order to find a suitable spot.

Once settled, they attach and transform into small oysters called spat. Spat soon begin feeding on algae by filtering water through their gills and a special structure labial palps located just in front of the mouth.

These are the basic procedures for spawning oysters, raising and setting larvae, and growing spat. No two hatcheries operate identically or in the same environment and there is no substitute for experience. Good water quality is essential to successful hatchery production see Site Selection but even high-quality water must be treated to remove unwanted organisms.

Water used for spawning, mixing eggs and sperm, and growing larvae is typically filtered mechanically and treated with ultraviolet radiation. Mechanical filtration is usually done with a pressurized sand filter, cartridge filters for smaller volumes, or fine-mesh bags.

Because of the rich array of organisms in Gulf of Mexico waters, mechanical filtration down to 1? Spawning oysters is the first step in the production of spat. As a rule, ten average females produce about million eggs. Under good conditions million eggs can result in million or more early-stage larvae, which require 2, gallons 10, L of treated water. Natural mortality and the need to thin out the larvae to proper densities should leave about 25 million eyed larvae ready for setting.

Approximately 10 million spat can be expected from the 25 million eyed larvae. The process begins with selecting broodstock, which may be oysters taken from the wild or oysters bred and maintained under controlled conditions for selective breeding.

In either case, oysters larger than 3 inches 76 mm are selected, although males may be smaller, and then a sample is examined for reproductive readiness. Ripening gonads have many branching tubules or prominent genital canals.

Gonads may also be cut and scraped and the material examined under the microscope for sperm and eggs. Mature eggs are pear shaped, 55 to 75? A significant amount of eggs and sperm can be produced by just a few oysters, but not every oyster may spawn and it is a good practice to have the eggs and sperm from several oysters. Therefore, 20 to 30 large oysters are thoroughly cleaned and scrubbed, then placed in a shallow, black tank containing 4 to 6 inches to mm of filtered, treated seawater at the same salinity as the broodstock holding tank.

Cycling ambient and warm water a few times may also stimulate spawning. If this fails to produce results in a reasonable time about an hour , sperm stripped from a male can be delivered by pipette to the shell opening of several oysters to stimulate spawning. To prevent unwanted fertilization, sperm used in this procedure can be placed in a microwave oven for 20 to 40 seconds to eliminate viability. These sperm should be checked under a microscope to be sure there is no motility.

When oysters begin releasing gametes, the whitish sperm and eggs can be easily seen against the black background of the tank. Males release a near constant stream of sperm and females release eggs during periodic shell closures. As oysters begin to spawn, males and females should be placed in separate containers 1 gallon or 2. When females appear to have finished spawning, they should be removed from the containers and the containers aerated. Within 45 minutes of spawning all the eggs can be sieved on a ?

Eggs are then combined in one or several aerated containers and fertilized with a small volume of sperm 20 to 50 ml combined from three or more males. After 15 to 20 minutes, eggs should be examined under a microscope to confirm fertilization. If no more than 10 percent of the eggs have a polar body Fig.

When eggs appear adequately fertilized, the egg container is brought up to a standard volume such as 2. Note: It is simpler to work in the metric system and the metric system is followed for small measures, while the English system is referred to where possible. The eggs are gently mixed and a 1-ml sample is taken with a pipette. The sample is placed on a Sedgwick—Rafter cell a special microscope slide that holds 1 ml of liquid and the number of fertilized eggs counted.

The sample count is then multiplied by the volume of the container, in this case 10, ml there are 10, ml in 10 L , to get the total number of fertilized eggs per container. This figure is used to determine the volume of water containing eggs that is needed to stock the larval tanks. Fertilized eggs are stocked into larval-rearing tanks usually gallons L or larger at the rate of 40, to 80, per gallon or about 10 to 20 eggs per ml.

The following formula can be used to determine the volume ml from the container of eggs needed to stock the larval rearing tanks:. Example: If the egg count in the previous step revealed 10, eggs per ml and the stocking rate for a gallon L tank is 10 million eggs, then:.

Algae can be added the same day to provide food for fast developing trochophore and veliger larvae. Tanks are cleaned, disinfected with sodium hypochlorite bleach , and filled with treated sea water before they are stocked with fertilized eggs.

Tanks should be gently aerated so that eggs and subsequent larvae are mixed throughout the tank. From this point until larvae are ready to set, larval care consists of feeding algae, draining tanks every 2 days daily as larvae near setting , sieving and counting larvae, cleaning and refilling tanks, and restocking larvae at the appropriate density.

Table 1 outlines a schedule for draining, the suggested mesh size for sieving, the larval density, and the food density. Most fertilized eggs develop into trochophore larvae within 12 to 20 hours.

These become veliger larvae also called straight-hinge or D-shaped larvae within 20 to 48 hours. The first draining and sieving Fig. Water is drained slowly through the appropriate size sieve Table 1 and the retained larvae are placed in a known volume of treated seawater e. Several 1-ml samples are taken, the larvae are counted in a Sedgewick-Rafter cell, and the average number is used to calculate the total number of larvae, as in the egg count.

Larvae are restocked in a cleaned and disinfected tank filled with treated seawater at the recommended density, five per ml or about 20, per gallon. This process is repeated every 2 days daily as larvae near setting with appropriate reductions in larval density Table 1 until larvae are ready to set. Oyster larvae feed by filtering small, single-cell algae from the water. They must be supplied with the right size food at a density that makes the food easy to encounter.

There are several methods for supplying algae to larvae. 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. Your Bibliography: Breese, W. Hatchery manual for the Pacific oyster.

Corvallis, Or. Your Bibliography: Calabrese, A. Survival and growth of bivalve larvae under heavy-metal stress. Your Bibliography: Clark, J. Oyster seed hatcheries on the U. Your Bibliography: Drinkwaard, A. Introductions and developments of oysters in the North Sea area: a review. Your Bibliography: Ginger, K. Your Bibliography: Helm, M. Hatchery culture of bivalves.

In-text: Herbert, Roberts, Humphreys and Fletcher, Your Bibliography: Herbert, R. Shellfish Association of Great Britain. Your Bibliography: Honkoop, P. Stocking density and growth of the Pacific oyster Crassostrea gigas and the Sydney rock oyster Saccostrea glomerata in Port Stephens, Australia.

Aquaculture , , pp. Your Bibliography: Matthiessen, G. Possible methods of improving the shellfish industry of Martha's Vineyard, Duke's County, Massachusetts. Edgartown, Mass.