Daphnia are a popular food for fish if they can be found in sufficient numbers. Supply in the wild is not reliable. One day a pond can be teeming with them, and the next they are gone.
Daphnia are a very good food for fry from about two weeks onwards. The adults are too big for the fry to eat, but the larvae of the D. Pulex variety are an ideal size.
To ensure enough are available when you need them, the best solution is to prepare your own pond or ponds and have the numbers peak when your fry are two weeks old.
I work out when I need the ponds for the fry assuming the spawning will occur near enough to on time. The daphnia will take up to four weeks to populate an 80 gallon pond in sufficient numbers to satisfy 300-400 fry for two weeks when their next cull is due.
Don’t be too concerned if your timing is a bit out and the ponds aren’t ready because of cold weather or the fry aren’t ready to be put in the ponds yet.
If the ponds still need a little more time to get the numbers up, but the fry are ready to cull, go ahead with your cull and return them to their aquarium to wait for a week.
You will be reducing the fry numbers down which gives them a bit more room for a short period. I would also do 25% water changes daily to reduce the build up of ammonia and any other toxins that stunt fry growth.
If the pond is producing vast numbers of daphnia, and the spawning was late, the culture will quickly use up all the available food. Evidence of this will be the water starting to go clear.
To extend the life of the culture, I feed the culture any green water I have available or a mix of activated yeast and soy flour. Read more on feeding cultures here...
Watch the water temperature doesn’t go above 85oF. I usually put some 50% shade cloth over the culture to cool it down.
Many writers advise not to top up the water in your culture with tap water because Daphnia don’t tolerate any chlorine or chloramine.
I have found that a top up of as little as 25% will kill a large number of daphnia. I never top up my cultures even during summer. I let rain refill the ponds or I use aged water from another pond
If your water supply straight from the tap doesn't harm your fish, then test with a few daphnia overnight.
Treat your culture the same way you would your fish regarding temperature and pH differences.
After the first cull at two weeks I transfer the fry that have been selected to 80 gallon fiberglass pools. If I get my timing right, the pools are teeming with daphnia. If there are enough present, I don’t have to worry about feeding the fry for the next two weeks.
The preparation of the pools entails creating green water. In this green water protozoans and other phytoplankton will teem. This is the daphnia’s natural food.
In early spring I wash and sterilize the pools. I fill them up with water and put a few cupfuls of well rotted vegetable matter into each. (Well rotted cow manure is better).
The pools get full sunlight.
After a few weeks the water has gone green and is ready for seeding. You will need to find a source of daphnia to seed the pool. The more adults you start with the sooner the culture will be ready for the fry.
Be very careful that you don’t introduce unwanted pests if you get your daphnia from a natural water source. Hydra are one of the worst as they multiply rapidly and kill small daphnia and goldfish fry under two weeks old.
Daphnia feed on green water, but if the green water is so thick that they can't swim in it, they will die.
Test a few daphnia in some culture water first. If they can't move through it easily, the green alga is too thick.
Change 50% of the water and try again. Keep making 50% water changes until the daphnia are swimming around happily. Test overnight to be sure.
Depending on water temperature and species, daphnia take 7-14 days to become adults, and produce 1 to 100 larvae every 4-5 days.
You will need to test the reproductive speed of the daphnia you have available to get an idea of how long it will take the culture to be ready for fry. With some of the slower reproducing species you will need to start the culture much earlier, or seed it with a lot of daphnia.
The ideal temperature for them is between 72 to 85oF.
If the water isn’t clear or very light green, the culture hasn’t peaked yet. Don’t introduce fry until you can see thousands of daphnia milling around and the water is starting to clear.
To get an idea of what thousands of daphnia look like in a culture ready for fry view this Youtube video.
If you want to keep a culture growing all year round, go to the Daphnia Cultures page to get details on what to feed them, how much and how often.