- Culture Tetraselmis chuii in 1.8-ton (1,800-liter or 475- gallon) circular, fiberglass tanks. The elevated fiberglass tanks should be equipped with drains that gravity feed to rotifer tanks, or the algae can be pumped to rotifer tanks. Gravity feed is preferred; it helps control contamination of algae tanks with rotifers.
Rotifer tanks are usually the same size (1,800 liters) as the algae tanks. Rotifer tanks must also have drains and harvest baskets or nitex screen socks (48- to 60-micrometer mesh) to capture the rotifers. Both rotifer and algae tanks should have aeration and illumination.
- A few days after inoculations the Tetrasalmis cultures will turn a darker green and cell densities will be about 132,000 cells per ml. Then gravity flow or pump algae to the rotifer tank and replace the algae volume with clean, sterilized seawater and nutrients.
In the rotifer tank, place a stock culture of rotifers in the algae (at least 1 rotifer per ml). Note: The larger the stock culture, the faster the desired numbers of rotifers will be reached.
- After several days the algae numbers should be obviously decreasing (water looks clearer) and the rotifer numbers increasing. Start drainharvesting the rotifer tank into the mesh sock or harvest screen until approximately 30 to 50 percent of the culture tank is drained.
Replace the drained volume with algae culture. Initially, the collected rotifers can be placed back into the culture container. However, once the desired density is reached (about 100 to 150 per ml) about half of the rotifers will have to be harvested each day.
- Continue harvesting or discarding rotifers and refilling the rotifer culture tank with new algae culture daily. Volumes harvested from the rotifer tank may vary according to demands of the hatchery; or standard volumes may be harvested routinely by dropping to the 50 percent level.
Even if the rotifers are not needed in the hatchery, the volume in the tank should be reduced, and rotifers discarded.
- Drain-harvest rotifers for 1 month unless a problem occurs such as a “crash” or die-off. If this occurs, drain, clean, disinfect and restart the tank. Restart the cultures in clean tanks monthly.
Starter cultures of rotifers should be maintained at low densities and in a separate facility. Densities of rotifers at harvest will vary, but the ranges to expect using this technique are 100 to 150 per ml.
The health of the rotifers will primarily be determined by the availability of an adequate food supply. Hence, algae should be supplied in slight excess. In general, the cultures should not be cleared of algae in less than 24 hours (i.e., after replacing algae; the culture should have a rich color that will clear to a lighter color in not less than 24 hours).
Rotifer numbers and health should be checked daily. Using a dissecting microscope, a sample of the rotifers should be observed for swimming speed (fast is good, slow is bad), gut fill (well packed gut that is easy to see indicates good feeding; little or no food indicates poor food densities, an undesirable species of algae, or contamination), percentage of rotifers with eggs (the more eggs the better the culture), and number of egg sacks carried (one indicates an adequate culture, two or more a very healthy culture).
Most problems with rotifer cultures are caused by an inadequate supply of algae because of poor algae culture techniques, undersizing of algae production, or an inability of the culturist to match the rotifer populations with the algae supply. The latter is generally a result of not discarding excess rotifers.
Batch/semi-continuous culture using mixed feeds:
Even though zooplankton are generally considered good food sources, they can be deficient in several essential nutrients, especially the n-3 highly unsaturated fatty acids (n-3 HUFAs) required for good growth and development of marine fish larvae.
This is one of the primary disadvantages of rotifers, especially if they are grown on a food source that is not rich in HUFAs. Because a variety of factors influence the nutritional quality of the rotifer, most production systems now use several food sources to enhance the nutritional content of live feeds.
Follow these steps to culture rotifers on algae, baker’s yeast and oil emulsion:
- Follow the steps above for growing rotifers with algae. Hold a rotifer starter/backup culture at lower densities (100 per ml) in green water and use it to initiate the cultures as previously described.
- After algae is depleted for the first time in the rotifer tank, stop feeding algae. Instead, add the following two products daily: baker’s yeast at 0.5 g/10 liters and oil emulsion (see makeup below) at 1 to 2 ml/10 liters.
The remaining volume can be replaced with clean seawater or de-chlorinated tap water. Lowering the salinity to 16 to 18 ppt in the rotifer tank can be beneficial and may improve growth after the rotifers are no longer being fed algae. Discard water when the rotifers are harvested so they can adjust rapidly to the higher salinity in the fish larvae environment.
- Once rotifer population density reaches 100 per ml, increase this daily yeast and oil emulsion level to 0.7 to 1.0 g yeast per million rotifers and 2 to 3 ml oil emulsion per million rotifers.
- Once rotifer densities are 200 per ml, drain-harvest by draining 30 to 50 percent of the tank volume daily and capturing the rotifers in a 48- to 60-micrometer mesh net. Repeat until rotifer density drops. This culture method should maintain rotifer densities at 150 to 200/ml for about 30 days.
Emulsified oil is a mixture of seawater, fish oil, and egg yolk at a ratio of 100 ml: 5 ml: 1g, with the addition of vitamin mix at 0.5% weight/volume of oil mixture. Vitamin E is also added at 0.1% weight/volume of oil mixture.
This mixture usually is a cod liver or menhaden oil, raw chicken egg yolk, vitamin E (Tocopherol), and a vitamin mix (AIN Vitamin Mixture 76® ). The mixture is blended for 2 minutes in a blender and then stored in a refrigerator up to 1 week.
The oil adds essential fatty acids and vitamins not found in yeast. The eggs can be purchased at a grocery store. The oil, vitamin E and vitamin mixture can be purchased from ICN Nutritional Biochemicals, Cleveland, Ohio. The menhaden oil is produced by Zapata-Haynie Corp., Reedville, Virginia.
Dry baker’s yeast can be obtained from wholesale grocery companies or most grocery stores. Some researchers and commercial producers choose not to mix their own oils, but prefer to purchase commercial enrichment products.
There are also a variety of prepared rotifer feeds that can be used as a replacement for the yeast. Commercial enrichments and rotifer feeds are available from companies such as Aquafauna Biomarine Inc. ( California), Sander’s Brine shrimp Co. (Utah) and Inve Aquaculture, Inc. (Utah).
Algae pastes or concentrates are also available (Reed Mariculture Inc., California). The algae is grown under controlled conditions, concentrated using a creamseparator, then preserved and packaged. These products can be refrigerated for 1 month or frozen for more than a year.
Although the concentrated algae products seem to work well in a wide range of densities, prepared feeds generally work best in super-intensive batch production systems (more than 100 rotifers per ml), which are harder to manage over long periods of time. Because they are not live products, they do not stay suspended without considerable aeration.
It should be noted that using batch cultures and intensive feeding regimes, an initial starter culture of 100 rotifers per ml can reach densities of 1,300 per ml in 6 to 7 days. Although such densities are desirable, the cultures are much harder to manage and require careful attention to water quality and feeding regimes.
Authors:
Granvil D. Treece and D. Allen Davis
