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Something's Fishy Lab Mark-Recapture with crackers for IBDP Biology & ESS

It was lab time in my class once again, in case you haven't figured it out yet I love labs! Today's lab happened during my Grade 11 IBDP Environmental Systems & Societies (ESS) course, we are finishing up Subtopic 2.5 Investigating Ecosystems and one of the skills students need to master is the use of the mark-recapture method to estimate population size. I have used this fish cracker lab back when I taught AP Environmental Science as well as in Option C of the old IBDP Biology syllabus...however, the lab did need some updating for the ESS-specific criteria.

No matter which course I am teaching when I use this lab the premise remains the same. Each group of students is given a baggie with a letter or number on it and and a specific number of fish crackers. The students then reach into the bag and randomly pull out a sample of the fish population which they then mark with a drop of food colouring. It is important that students take enough time to let the fish crackers dry completely before they put them back in the baggie or release them to the environment, otherwise the food colouring may mark some other fish by accident. After the marked fish are mixed in with the rest of the population the student then averts their eyes and reaches in the baggie to randomly sample the population once more. They then count the number of marked and unmarked fish in the population. At this point the lab format changes a bit depending on which course you are teaching at the time. Please see below for more information.

Something's Fishy lab for IBDP ESS:

In the IBDP ESS subject guide this concept is found in Subtopic 2.5 Investigating Ecosystems on page 39. You can see a screenshot of the relevant part of the Subject Guide below. In ESS only three variables are necessary to estimate the population size and you use the formula as described below.

In my previous version of this lab I had students do many trials and calculate the ratio of marked to unmarked fish in the population. However, since this is just one point in a very long list in Topic 2 for ESS I removed the extra trials and edited my lab sheet to use the same variable symbols as those in the ESS guide. You can download the sheet I made here for free.

ESS 2.5 Lab- Something’s Fishy Mark & Recapture Simulation
Download DOCX • 43KB

Something's Fishy lab for IBDP Biology Option C (May 2016 syllabus):

In the old IBDP Biology course (last exam May 2024) this lab was only used if you or your students chose to complete Option C: Ecology & Conservation, and even then it was only for HL students. The following information can be found on page 132 of the Subject Guide.

Since there are relatively few things for students to learn in Subtopic C.5 Population Ecology and I always need more assessment data in the last semester of Grade 12 (which is when I almost always end up doing the Options) I have made the version of this lab longer and more complex than the ESS version, although it does follow the same principles. You can find the Word document version of the lab sheet just below for free.

Option C HL Lab- Something’s Fishy Mark & Recapture Simulation
Download DOCX • 53KB

Something's Fishy lab for IBDP Biology Theme C Interactions & Interdependence (May 2025 syllabus):

Since the new IBDP Biology syllabus removed the Options, incorporating key learnings from each Option into the core syllabus this lab is now for everyone. It can be found in Theme C: Interactions & Interdependence at the Ecosystem level. You can find the full details on page 81 of the new guide, or in the screenshot below.

This version of the lab is more similar to the ESS version, with only one trial per group, but I have updated it to use the formula and symbols in the IBDP Biology guide and added an analysis question about the assumptions of the method. You can find this version below.

IBDP Biology C4 Lab- Something’s Fishy Mark & Recapture Simulation
Download DOCX • 33KB

In conclusion, here are a few tips to help you prepare and run this lab smoothly no matter how you choose to use it.

  • Begin with a bunch of fish-shaped crackers, I used two big jars for my class of 12 students this week, in my experience you need over 100 fish per group to have it work well

  • Divide the whole crackers evenly among the groups (we used 6 groups) and count as you go

  • To make sure that each group has a slightly different population size remove crackers from one bag and add them to another, be sure to keep track of any changes on a scrap piece of paper (see mine to the left)

  • If you want your students to be able to eat the crackers at the end be sure to use only food-safe materials and containers, use cups rather than beakers and new eye droppers for the food colouring

  • Food colouring stains desks quicker than you think, so use paper towel or paper plates on the desks to avoid staining

  • Have fun with it! I offered a prize to the group who had the closest population size estimate to the real population number

I hope that helps & that you and your students have fun with this lab!

Thanks for reading, teachers, travelers & curious souls of all kinds.

The Roaming Scientist



Hi, thanks for stopping by!

I'm the kind of teacher who is always trying something new, new labs, new Apps, new scaffolds and even new countries to live and teach in. I'm looking forward to share what I learn with you all through my weekly blog posts. 

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