I. Objectives:

  1. Learn the mark-recapture method for estimation of population size.
  2. Describe the conditions under which the mark-recapture method provides the best population estimates.
  3. Learn the assumptions of the mark recapture technique.
  4. Using computer simulations, determine how variation in sampling parameters affects estimates.

II. Introduction:

Ecologists are often interested in finding the total number of individuals in a population and whether that number may be changing over time. If a population is sedentary, we can estimate density by quadrat or point-quarter sampling. However, populations of mobile organisms cannot always be sampled using these techniques.

A frequently used method for sampling such populations is the mark-recapture technique. In its simplest form, this method consists of capturing some individuals, marking them in some way, releasing them back into the population, and, after an appropriate time interval, resampling the population. The number of individuals in the original population can be found using the following formula:

N1=(M1n2)/M12 (Equation 1)

Where N1 is the original population size, M1 is the number of marked individuals released into the population from the first capture, n2 is the number of individuals taken in the second sample, and M12 is the number of individuals in the second sample that were marked during the first sample. However, the accuracy of this technique depends upon some rather strict assumptions:

  1. An individual caught and tagged in the first capture must be neither more nor less likely than other members of the population to be caught on the second capture. This means that tagged individuals must not have a higher mortality rate, tagged individuals mingle freely with other members of the population when released, sufficient time has elapsed between captures to allow dispersal of tagged individuals, and tagged individuals must not become "trap-happy" or "trap-shy".
  2. Tags do not become lost or unrecognizable.
  3. There is no emigration or death of tagged individuals. Emigration and death rates for unmarked members of the population must be equal to immigration and birth rates.

Sometimes, with triple rather than double catch methods, it is possible to estimate changes in population size. In this case, what we want is DP, the change in population size per individual (proportional change in population size). To do this we need a minimum of three captures to collect the following information:

N1 = the estimate of population size at the first census

N2 = the estimate of population size at the second census

M1 = the number of animals marked and released after the first capture

n2 = the number of individuals taken during the second census

M12 = the number of individuals marked during the first census and recaptured

the second census

M2 = the number of individuals marked and released during the second census

(equals n2 if all individuals caught are marked)

n3 = the number of individuals captured in the third census

M23 = the number of individuals marked during the second census that are

recaptured during the third census

Thus, DP = change in population size = N2-N1

Initial population size N1

Where, from equation 1, N1 = (M1n2)/M12 and N2 = (M2n3)/M23

The above calculations assume equal time intervals between first, second, and third censuses. The exercise described below is designed to illustrate some properties of mark-recapture method.

III. Methodology:

A. Initial Marking and recapture

The laboratory is equipped with three pools with snails. The pools are marked A, B, and C. Your group will be assigned to a given pond. You will elect a snail sampler for your group who will do all sampling for this exercise. The sampler will be blindfolded and then asked to capture all the snails he/she can within a 45 second period.

After the snails have been captured, they will be marked with the fingernail polish provided to your group. Carefully swab the RIGHT side of the snail with a Q-tip that has been dipped in acetone (do not spill any acetone on the snail’s foot - they do not like it). The right side of a snail is defined as the snail’s right when the snail has been orientated with the opening down and the apex (pointed end) towards you (you and the snail are now pointing in the same direction). Blow on the swabbed area until the acetone dries. Make sure it is COMPLETELY dry before going to the next step.

Next dip a toothpick into the fingernail polish and paint a small spot of color in the dry area. Let the fingernail polish dry (blowing on it periodically) and then return the marked snails to the pool when your instructor indicates. Swirl the snails immediately after adding them back to the pool. Record the number of marked individuals released into the pool on the data sheet provided.

B. Second capture-mark-release

After the snails have been allowed to mingle freely and disperse within the pools (1/2 hour after releasing them), capture a second sample in the same manner as the first. Record the total number of snails captured (n2) and the number of marked snails recaptured (M12). Next, mark all the snails taken in this second sample with a dab of fingernail polish on the LEFT side of the snail’s shell. In marking, ignore any marks on the right side of the shell (from the first sample) or marks from previous classes. Use the same marking techniques as outlined above. After the marks have dried, give your snails to the instructor. The instructor will return the snails to the pool, possibly adding or deleting some individuals from your pool’s population.

C. Third capture

Capture your snails as before, recording the total number of snails in the third capture (n3) and the number of snails marked in the second census and then recaptured in the third sample (M23; all snails with marks on the left side).

D. Computer Simulation

Using EcoBeaker, run the computer simulation of mark-recapture on birds.

IV. Laboratory Assignment:

  1. Calculate the snail densities in each of the 3 pools for both the second census data and the third census data. How well do these estimates agree with the actual numbers of snails per pool given to you by your laboratory instructor.
  2. Calculate the rate of population change (DP) for each pool. How well do these estimated rates of change compare to actual rates of change?
  3. What does the computer simulation indicate about the effects of varying sample size or varying the time between release and recapture samples.
  4. From you laboratory and computer exercises, what are some of the strengths and weaknesses of mark-recapture techniques?




parameter Pool A Pool B Pool C


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