Something's Fishy
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. In this method,
biologists first capture and tag a sample of organisms. Then, after some time has passed the animals
are returned and allowed to "redistribute" themselves. The scientists then take repeated random
samples and estimate the total population size.
Statistics of repeated "large" samples of a population will vary in a regular and predictable pattern
from the real population parameter that we are trying to measure. The mean of our simple statistics
should approximately eaual the population parameter we are trying to measure if we use proper
scientific methods and are careful to take many random samples.
Each team has a population of cheddar goldfish in front of them. Do not eat the sample population!
Your task is to estimate the size of your population using the same tagging and sampling methods that
biologists use.
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Remove a sample (a large handful of fish, approximately 40).
Replace this sample with an equivalent number of tagged fish (pretzel goldfish).
Record how many you tagged in the data table.
Mix the population thoroughly to get the tagged fish redistributed among the population.
Without looking (to prevent personal bias) remove a sample of fish from the "pond".
Count the # of tagged and total # of fish in your sample and record them in the table below.
Return the sample to the "pond", mix thoroughly and repeat the process for a total of 20
samples. The sample sizes do not have to remain the same but you do want to get large
handfuls of fish each time.
How do we predict the population size? If a specific number of individuals are captured, tagged and
released into the wild population, it is possible to estimate the total population size using the following
ratio:
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Population size/# Originally tagged = Total sample size/# Tagged fish in sample
o Therefore
Population size = Total sample size/# Tagged fish in sample x # Originally tagged
Use your data and the above formulas to estimate the size of your population of goldfish.
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Mean estimated population size = __________
Now count your entire population and determine how close your estimate was.
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Actual population size = __________
Estimated population size = __________
% Error = __________
Conclusions:
You probably did not predict your actual population size exactly. Even if you did, consider the following
questions as you as you think about your results:
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What could cause your results to be different from the actual population size?
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How would sample size and population size affect these results?
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How would the number of samples affect the results?
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If you were predicting a large population (as in a real pond), would your % error really be that
bad relatively speaking?
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What concerns should a biologist have about a species' habits before he/she uses this method
to estimate the size of a population?
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Are there other uses of tagging?
# Tagged (M):
Sample
# Tagged Fish in
Sample (m)
Actual Population =
Sample Size
(n)
Population
error
estimate
Actual – Estimate
N=(Mn)/m
Estimate
m/n
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Something’s fishy extension
1) Graph sample size vs. | error | (absolute value = make negatives positive)
2) Draw a line of best fit
3) Given the data in the graphs, what conclusions can you draw regarding
population estimation reliability?
Something’s fishy extension
1) Graph (marks /sample) vs. | error |
2) Draw a line of best fit
3) Given the data in the graphs, what conclusions can you draw regarding
population estimation reliability?