Estimating the number of fish that return to spawn using capture-recapture methods.

Instructions on using the sampling bowl to simulate a capture-recapture experiment

Purpose

To investigate what happens when a sampling procedure is repeated under identical conditions.

Equipment

Sampling bowl
Sampling paddle
Bag of 4000 green beads
Bag of 400 white beads
Calculator
Data collection sheet (print off a copy)

Protocol

This experiment will represent what happens when DFO tries to estimate the number of fish that return to spawn using capture-recapture methods.

The green beads represent the population of returning fish. The goal is to estimate the number of green beads (of course you know the true answer is 4000).

Place the green beads into the sampling bowl.
Select and remove 400 of the green beads from the sampling bowl. Replace them with 400 white beads. This represents the tagging process, so n₁=400.
Mix the beads in the sampling bowl. This represent the mixing of the tagged and untagged fish as they migrate back to the spawning grounds.
Select 100 beads from the bowl using the sampling paddle. This represents examining 100 carcasses on the spawning grounds, so n₂=100
Count the number of white beads in your sample. This represents the number of carcasses that have tags. The count is the value of m₂.
Enter the summary statistics (n₁, n₂, m₂) into the data sheet.
Replace the beads from the sampling paddle in the bowl.

With only a single estimate, nothing much can be said about the process. But unlike real life experiments, the simulation can be repeated. Repeat steps 3-7 until 30 sets of summary statistics are obtained (you may wish to work together with other groups for this part).

Complete the data sheet by obtaining the estimates of the population size for each trial. Here is a sample data sheet as an example.

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