How would I go about estimating the population of my daphnia culture?

[Sack Boy]

The culture is in a standard 10 gallon tank (20"x10"x12"). I started with about 200 daphnia (<1mm to 3mm). I would like to know where I am at right now.

 

[jim mcnamara]

Estimate by sampling:

If you can safely assume:
daphnia are dispersed thru the 2400 $$in^-3$$ volume more or less randomly - ie. 90% of them are not parked on the walls or the bottom of the tank then

This is the poor man's version:
find a small clear container that has a water tight top. A 10ml test tube is great. Measure the volume of water it holds. Empty the water out.

Cover the container and open it underwater. Count the daphnia. Repeat this several
times - 10 - 15 times. Retreive the samples from different places in the tank. Record the results for each count.

Calculate the mean -
sum all of the numbers then divide that total by the number of samples. Let's pretend you got 8.2 as the value of mean. This is a large number but it does not matter.

Your tank has approximately 39.3 liters of volume. So if your sample container
was 10ml (that is .010L) then your population estimate is:

8.2 * ( 39.3 / .010 )
or
8.2 * 3930 = 32226 daphnia

 

[quicknote]

To estimate the population of your daphnia culture, you can use a method called "mark and recapture." This involves marking a small portion of the population, releasing them back into the tank, and then recapturing a larger sample at a later time to estimate the total population size.

Here are the steps to follow:

1. First, you will need to mark a small portion of the daphnia in your tank. This can be done by adding a small amount of food coloring or a non-toxic dye to the water. Make sure to mix it well so that the dye is evenly distributed.

2. Next, take a small sample of the marked daphnia using a pipette or a small net. Count the number of marked daphnia in the sample and record this number.

3. Release the marked daphnia back into the tank and allow them to mix with the rest of the population.

4. After a period of time, say 24 hours, take a larger sample from the tank using the same method as before. This time, count the total number of daphnia in the sample, including both marked and unmarked individuals.

5. Use the following formula to estimate the total population size: Total population size = (Number of marked daphnia in first sample x Number of daphnia in second sample) / Number of marked daphnia in second sample.

In your case, if you started with 200 daphnia and marked 10 of them, and in the second sample you counted 50 daphnia (including the 10 marked ones), the estimated population size would be (10 x 50) / 10 = 50 daphnia.

It is important to note that this method is an estimation and may not give an exact population size. It is also recommended to repeat the process multiple times and take the average to get a more accurate estimate.

In addition, factors such as mortality, growth, and reproduction rates of the daphnia should also be taken into account when estimating the population size. Regular monitoring and recording of the population size can also help in tracking the growth and health of your daphnia culture.