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Water fleas: The ideal live food for freshwater fry
by Junelyn de la Rosa
April-June 2003
Volume 5 No. 2

Water fleas (Moina spp.) are a good if not a better substitute for the popular live food, Artemia nauplii for freshwater fry or larvae. Water fleas are easy to culture, cheap, and as nutritious as A. nauplii.

In a related study, a team of scientists from BIOTECH-UPLB and the Institute of Biological Sciences, Selangor, Malaysia recommended Moina spp. after testing various kinds of live feed organisms as live food for river catfish larvae (Mystus nemurus).

Moina spp. are small freshwater crustaceans commonly called "water fleas" so named because of their short, jerky hopping movements in water.

Young Moina (less than 400 µm), are smaller than newly-hatched brine shrimp and are of the same size or slightly larger than adult rotifers. Hence, Moina are ideal for freshwater fry. Young freshwater fry like river catfish can easily ingest young Moina as their initial food.

Water fleas reproduce readily. At only 4-7 days of age, they can produce broods of 4-22 per female producing 2-6 broods during their lifetime. Broods are produced every 1.5-2.0 days, with most females producing asexually.

Moina has an average protein content of 50% of their dry weight. Adults normally have a higher fat content than juveniles. The total amount of fat per dry weight is 20-27% for adult females and 4-6% for juveniles.

Moina culture
Moina spp. is easy to culture. Simply obtain a starter culture and a container. Containers can be aquaria, tanks, vats, or ponds. Initially, you need to disinfect the tank with 30% muriatic acid to prevent predators and algae later on.

In large containers, a half meter water depth is recommended to allow more oxygen and light to pass through the surface for photosynthesis by phytoplankton. Outdoor cultures should be protected from rain.

Clean water is a must in the culture of Moina spp. since they are very sensitive to toxins such as pesticides and detergents. Ideal water temperature is 75-88° F (24-31° C). While Moina can thrive at temperatures in excess of 90° F (32° C) for short periods, low temperatures reduce production.

Feeding or fertilizing
There are different mixtures of fertilizer materials that are good for Moina culture. These are:

yeast: 0.3-0.5 ounces (8.5-14.2 g) of baker's yeast
yeast and mineral fertilizer: 0.3-0.5 ounces (8.5-14.2 g) of yeast, and 0.5 ounces (14.2 g) of ammonium nitrate
alfalfa, bran and yeast: 1.5 ounces (42.5 g) of alfalfa pellets or meal, 1.5 ounces (42.5 g) of wheat or rice bran, and 0.3 ounces (8.5 g) of yeast
cow manure or sewage sludge, bran and yeast: 5 ounces (142 g) of dried manure or sewage sludge, 1.5 ounces (42.5 g) of wheat or rice bran, and 0.3 ounces (8.5 g) of yeast
cow manure or sewage sludge, cotton seed meal and yeast: Use 5 ounces (142 g) of dried manure or sewage sludge, 1.5 ounces (42.5 g) of cotton seed meal and 0.3 ounces (8.5 g) of yeast
horse or cow manure or sewage sludge: 20 ounces (567 g) of dried manure or sewage sludge.
chicken or hog manure: 6 ounces (170 g) of dried manure.
yeast and spirulina powder: 0.2 ounces (6 g) bakers yeast, 0.1 ounces (3 g) spirulina powder. Add this amount for the first two days, and then every other day until culture is harvested. Add warm water to yeast and spirulina powder and let sit for 30 minutes. Using a brine shrimp net, pour the contents into the Moina spp. culture.

Each fertilizer mixture should be added initially for every 100 gallons (379 liters) of water. Additional feed or fertilizer, approximately 50-100% of the initial amount, should be added 5 days later.

Scientists recommend organic fertilizers over mineral fertilizers because organic fertilizers meet the nutritional needs of Moina spp. Organic fertilizers are rich in bacterial and fungal cells, detritus and phytoplankton- all of which are excellent food for the Moina. Fresh manures are also excellent choices since they have lots of organic matter and bacteria.

Another fertilizer mixture consists of yeast, alfalfa and bran. This mixture smells better than the other mixtures and is easy to prepare. Activated yeast (baker's yeast) and bran and alfalfa meal or pellets can be bought from livestock feed stores.

A very important feeding rule is to add small amounts of feed or fertilizer at frequent intervals and slowly increase the amount as you go along. Overfeeding can be a hassle as it can cause fungi to grow and affect the growth of the Moina.

Water pH should be maintained at 7-8 as high pH (greater than 9.5), due to a heavy algae bloom and un-ionized ammonia inhibits the production of Moina. The pH of the culture can be adjusted to 7-8 with vinegar.

Harvesting
To harvest the young Moina spp., use a brine shrimp net or a filter to scoop out the culture as they concentrate in "clouds" at the surface. For semi-continuous culture, do not harvest more than a fourth of the population each day, unless you are restarting the culture. Harvest only small quantities at a time and transfer the Moina to containers with fresh water to keep them alive. Do not forget to stir the mixture occasionally to re-suspend food particles.

Maintaining the culture
To save harvested Moina for several days, keep them in clean water in a refrigerator. They will resume normal activity when they are again warmed. To ensure the nutritional quality of refrigerated Moina, fertilize them with algae and yeast before feeding them to fish.

If you want to store Moina for long period, freeze them in low salinity water (7 ppt, 1.0046 density) or freeze-dry them. Both methods kill the Moina, so adequate circulation is required to keep them in suspension after thawing so they will be available to the fish fry. Frozen and freeze-dried Moina are not as nutritious as live animals and they are not as readily accepted by fish fry.

This means that local fish breeders can culture microworms at home instead of buying expensive fish feed or supplements and be sure that their grouper fry are getting the nutrients they need.

Indeed, fish breeders should try water fleas as a better alternative for freshwater fry. It is inexpensive, readily available and easy to raise and most importantly little fry love them.

Sources:
1) Evaluation of different live food organisms on growth and survival of river catfish, Mystus nemurus (C&V) larvae
2) Culture Techniques of Moina: The Ideal Daphnia for Feeding Freshwater Fish Fry by R.W. Rottmann, J. Scott Graves, Craig Watson and Roy P.E. Yanong; This document is Circular 1054, one of a series from the Department of Fisheries and Aquatic Sciences, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: May 1992. Revised: February 2003. http://edis.ifas.ufl.edu.