CULTURING THE FRESHWATER ROTIFER

 

A major focus of our efforts has been to develop culture techniques for use with the freshwater variety of rotifer as a live food for freshwater ornamental fish larvae. This rotifer is a food source that is smaller than the ubiquitous nauplii but larger than infusoria and it does not pollute the water.

Rotifers can reproduce in a variety of mediums, the most common of which is a phytoplankton culture. Experiments have been conducted to assess the suitability of a "green water" phytoplankton culture that is produced from four 15-gallon aquaria stocked with guppies. The advantage of this method of producing phytoplankton is its simplicity.

Adult guppies (approximately 20 individuals) are stocked into 15-gallon aquaria filled with tap water and feel a diet of flake food ad libitum. When the water becomes so green that the guppies can barely be seen swimming, it is a suitable medium for culturing freshwater rotifers, which is in about four days. Four gallons of "green water" from one of the four aquaria (which are tapped on a rotating schedule) are strained through a 20 micron (R) mesh netting into a 5-gallon bucket and stocked with rotifers at 20-30 individuals/ml.

The aquarium is then refilled to the top so that it can green up again over a 2or 3-day period. The rotifers are then allowed to multiply in the "green water" for 48-72 hours. At a high density (> 100 rotifers/ml), the rotifers can clear the water in less than 24 hours and may require a new "green water" feeding nearly every day. This is done by straining the medium containing the rotifers through a 55 m mesh netting and placing the collected rotifers into a bucket containing fresh "green water."

This fresh "green water" is taken from a second aquarium, allowing the phytoplankton from the first aquarium to regain the necessary density of phytoplankton. Over the course of 2 or 3 days, the rotifer density (number of rotifers/ml) has been observed to rise and can exceed 100 rotifers/ml. A summary of three different rotifer culture trials presented in Figure 2, shows that the rates of growth are similar, but not the same. This is considered due to the varying types and concentrations of phytoplankton that make up "green water” in a particular aquarium. The best "green water" is one that is relatively fresh, at least a week old and no more than one month old. A density of rotifers at 100 rotifers/ml in the 4-gallon (or 15-1) culture would signify approximately 1.5 million rotifers available as food for fish larvae. Actually, only half (0.75 million) to a third (0.5 million) of the rotifers are harvested to use as food, using the remainder as a "starter batch" to multiply for the next harvest.

The key to the culture process is producing sufficient “green water” to maintain the rotifer culture process, which could go on indefinitely. If 0.5 million rotifers are harvested daily, that would mean that a total water volume of approximately 50 liters (or 12 gal.) of larval culture can be fed a density of 10 rotifers/ml. This would mean that between 500 and 1,000 larvae could be supported using this simple method of rotifer production.

Although the initial results appear to be encouraging, the variability in the observed growth of rotifers using 11 ”green water” as a source of food requires additional investigation. Likewise, "green water” cultures can become a major constraint in areas where there is limited sunlight and/or space in the event larger numbers of larvae are to be produced. For example, commercial hatcheries routinely produce larvae in the order of tens of thousands per week.           

(excerpts from article by Clyde S. Tamaru, Sea Grant Extension Service/State Aquaculture Development Program, Tel: (808) 956-2869, E-mail: ctamaru@hawaii.edu, and Harry Ako, UH Molecular Biosciences and Biosystems Engineering, in Makai, Sea Grant College Program, Vol. 22, No. 12, December 2000)


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