Lab 2: Mark-Release-Recapture Simulation

Introduction

In this lab, you will study mark-release-recapture techniques and equations by means of simulating these types of studies. You will begin with a simple situation, and then work your way through progressively more complex scenarios.

The simulations

  1. A closed population: Fish in a lake. NOTE: Do not do an actual count of your fish until the end of Part I.
  2. Trial 1: All assumptions for a closed population hold true
  3. Capture a netful of fish from your “lake” and mark them all, keeping track of how many you marked.
  4. Replace them and thoroughly “shake your lake”.
  5. Recapture

a)Calculate your population

  1. Trial 2 (a continuation): Mark any unmarked fish that were captured in this run, keeping track of how many additional new fish you mark.
  2. Replace them and thoroughly “shake your lake”.
  3. Recapture

a)Calculate your population, being sure to use the appropriate number for “total marked population” in the equation. [This version of the equation is slightly different than the simplest form given in class, where only the first and second captures are considered.]

  1. Trial 3: A presumed closed population. All assumptions for a closed population are trueexcept that 25% of the fish that are tagged die due to the stress incurred during tagging.
  2. Capture a netful of fish from your lake and mark them all. Disregard the original tags (mark in a different color or with a different symbol). Keep track of how many you marked.
  3. Remove 25% of those you just marked. These are the ones that die after they are tagged.
  4. Replace the remaining marked individuals and thoroughly “shake your lake”.
  5. Recapture, and then

a)Calculate with the Peterson equation as if you do not know there has been selective mortality of tagged fish.

b)If you didn’t know about such selective mortality, would you tend to underestimate or overestimate your population? Explain your answer.

c)BONUS POINT: How would you adjust the Peterson equation if you knew that there was 25% mortality of the tagged individuals?

  1. Trial 4: A presumed closed population: All assumptions for a closed population are true, except that fish do not disperse throughout the population.
  2. Capture a netful of fish from your lake and mark them all. Disregard any other tags (mark in a different color or with a different symbol). Keep track of how many you marked.
  3. Replace the marked individuals by dropping them straight down into the bag. Do not “shake your lake”.
  4. Recapture as though you were fishing in the same spot as they were dropped.

a)Calculate as usual with the Peterson equation.

b)Do you think lack of dispersal would tend to underestimate or overestimate the population? Would it depend on where you fished for your second sample (recapture)? Explain your answer.

  1. Count all the fish in your “lake”. How close were your various estimates to the actual population size? Express as a percentage of actual population size (may be >100%). Which estimate was closest? Was it the one you expected to be closest? Explain.
  1. An open population: Fish in a coastal estuary. Prepare a new bag with an unknown number of fish for this simulation. Assume all assumptions for an open population hold true.
  2. Conduct a mark-release-recapture experiment with six time periods.
  3. Capture a netful of fish and mark all of them individually.

a)Note on the attached spreadsheet titled “Marked fish” that these particular fish were marked in Time Period 1 by indicating the fish number and placing an “M” in the Time Period 1 column.

b)Note all other relevant data on the “Method B” table (similar to the table in Appendix II).

c)Replace the marked fish and thoroughly “shake your estuary”.

d)Scoop out some fish and place them in a baggy marked: “mortality and emigration between Time 1 and Time 2”. Do not count anything, but instead, set this bag aside.

e)Take some new fish from the new fish supply bag (not from your current fish population). It should be a reasonable proportion, but less than a third, of the volume you have in the bag. Have one person in your group count these out and write down the number as “natality and immigration between Time 1 and Time 2—actual”.

  1. Recapture. This is the beginning of Time Period 2.

a)Note all the marked individuals that were recaptured by placing a check markon the “Marked Fish” chart in the Time Period 2 column.

b)Enter all additional data on the “Method B” table for Time Period 2.

Note that the top part of the Method B table can be completed based on the information included in the “Marked Fish” chart.

Note that “Total marked” on the method B table refers to the total marked fish that were caught, regardless of when they were marked. (This will be become more important for the later time periods.)

c)Mark all individuals in the sample that have previously not been marked, and enter their data on the “Marked Fish” chart as described for Time Period 1 above.

d)Replace all the (now) marked fish and “shake your estuary”.

e)Scoop out some fish and place them in a baggy marked: “mortality and emigration between Time 2 and Time 3”. Do not count anything, but instead, set this bag aside.

f)Take some new fish from the new fish supply bag (not from your current fish population). It should be a reasonable proportion, but less than a third, of the volume you have in the bag. Have one person in your group count these out and write down the number as “natality and immigration between Time 2 and Time 3—actual”.

  1. Repeat the process in step 2 for Time Periods 3 through 6.
  1. Calculate the following for your population
  2. Estimated size of the marked population for time periods 2-5.
  3. Estimated size of the total population for time periods 2-5.

a)Remember that this is easy to do once you have estimated the size of the marked population…

  1. Estimated mortality+emigration between periods 2 and 3, and 3 and 4.
  2. Estimated natality+immigration between periods 2 and 3, and 3 and 4.
  1. Determine the actual number of fish at each of these 4 time periods (2.3.4. and 5). You will first need to count the current number in your population, and then work backward, based on the numbers of fish you added and removed each time.
  2. Compare your estimated values for total population, mortality+emigration, and natality+immigration with the actual numbers. How close were you? Express as a percentage of the actual number.