Animal Population Survey: Tag and Recapture

 

Introduction:

 

Scientists often need to know about the conditions of an area. In order to accomplish this task they survey and map the land, test the water, investigate the soil and rocks, and survey and count the local populations of plants and animals.  Because many animals are constantly moving around and hiding from humans, it is difficult to count them.

 

How do biologists determine the population of a species in a particular area? There are a variety of ways that it can be done, however the most common method involves tagging.  With tag and recapture, instead of counting every animal, you capture a group of animals and tag them and then release them.  Then, after some time has passed for the animals to return to their environment and to "redistribute" themselves, you capture another group of the same kind of animals and see how many of them were previously captured and tagged.  We would expect that these sample percentages should vary "around" the true population percentage.  Using this assumption, you can calculate the approximate population size. 

 

Biological Sampling:  Tag & Recapture

 

Materials:         goldfish crackers            markers            plastic cups (2 oz.)        plastic sandwich bags

 

Procedure:

 

Each of the teams in class has a population of cheddar "goldfish" in front of them. Your goal is to approximate the size of your population using the same tagging and sampling methods that biologists use. The directions are as follows:

 

1. Each group should have a bag with approximately I measuring cup (8 oz) or large drink cup (8‑16 oz.) of fish, a small cup, a marker, and the data table.

 

2. "Capture" a small cup full of fish. This will be your tagged group. Count and mark these fish. Record the captured amount on the data page as "total number tagged".

 

3. Return the captured group to the bag and shake the fish up.

 

4. Capture a small cup full of fish. This is a recapture; try to make it the same size as your original capture. Count how many fish you captured this time (tagged and untagged) and how many of them were tagged. Record this information in you data table.

 

5. Repeat step #4 nine more times, recording the numbers caught in the data table.

 

6. Find the sum (add up) of the tagged column and the sum of the captured column.

 

7. Find the average of the tagged and captured column by dividing the sum by the number of samples.

 

8. Use the proportion formula to find the total number in the population.

 

9. Count the number of "goldfish" that were actually in the bag and compare that to your estimate.

 

How do we predict our population size? Well, we expect that the sample percent tagged should vary fairly closely around the true population proportion of originally tagged fish. Using ratios from Algebra, we can do the following:

 

# originally tagged

= mean sample percent tagged  ( X  )

total population (p)

 

Solving the equation above, one gets the following results:

 

# tagged                                                               # tagged

                    =   X                     therefore     p =

      p                                                                            X

 

 

Using your information, find the predicted population size.  Now, count your entire population and determine how close your estimate was:

 

Actual Population: __________________________

 

% Error: __________________________________

 

Problems and Discussion:

I am sure that you didn't predict your actual population exactly. Consider the following questions as you think about your results.

a)       What could cause your results to be off from the actual population?

 

b)       How would sample size and population size affect these results?

 

c)       How would the number of samples affect these results?

 

d)       If you were predicting a large population (as in a real pond) would the number you were off really have been that bad relatively speaking?

 

e)   What concerns should a biologist have about a specie’s habits before he/she uses this method to approximate the size of a population? Even with these concerns, does this mean that tagging should not be used by biologists? Are there other uses of tagging