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)