Anatomy of a cod



During this interactive experience, students dissect two cod fish (one spawning, one not spawning) and record their data onto the data sheet provided. They are then presented with the data of 18 cod fish — from both phases of reproduction. Students then calculate and graph the gonadosomatic index (GSI) to determine when fish spawn and the hepatosomatic index (HSI) to determine the energy reserves and body condition (physical health) of the fish. This analysis gives them enough information to learn how the time of year relates to a cod’s reproductive phase and condition, and to make suggestions about how we might improve the sustainability of cod fishing.



The Atlantic cod is a groundfish (a fish that lives at the bottom of a body of water). It is brown and green in colour and has spots on its back. It also has a distinct pale line that runs along its side and a chin barbel (a whisker-like organ).


Atlantic cod feed on a variety of fish and shellfish including capelin, herring, flounder, mussel and crab. Northern cod around Newfoundland spend the summer feeding in coastal areas trying to increase their energy reserves. They migrate out offshore to the edge of the continental shelf where they spend the winter. In the winter, they eat less, and their bodies get ready to spawn and reproduce in the spring. They migrate back inshore to replenish their energy in the summer. Getting ready to spawn takes a toll on cod body condition. Generally, cod are in poor condition after they’ve spawned, because they have used a lot of their energy to make eggs and sperm.


Atlantic cod sexually mature between the ages of 3 and 7 years when they are between 45 and 55 cm long. When a fish reaches this stage they begin developing sperm or eggs. Typically spawning occurs in late winter and early spring — March, April, May. When it is time for spawning, the fish releases their eggs and sperm in the water for fertilization. The fertilized eggs and newly hatched cod drift in the water and become part of the zooplankton until they are large enough.

How are scientists studying the best time of year to fish cod?

Scientists use the hepatosomatic index (HSI) to estimate a fish’s energy reserve by looking at the weight of the liver as a ratio of their body weight. In a poor growth environment, the liver of a fish will be smaller and thus their HSI will be lower. The formula to calculate the HSI is:

HSI = [liver weight / body weight] × 100

A fish with a large energy reserve will have an HSI of 3 or greater, whereas a fish with a low energy reserve will have an HSI below 2.

Scientists use the gonadosomatic index (GSI) to estimate the spawning maturity of fish based on the development of the gonads (ovaries in females or testes in males). The GSI is the ratio between the weight of the gonads and its total body weight. The formula to calculate the GSI is:

GSI = [gonad weight / body weight] × 100.

The GSI can range from less than 1 to over 35! A fish with a GSI larger than 8 is considered sexually mature, and ready to spawn. When a fish is ready to spawn, its eggs will be visible and its GSI will be greater than 8.

Fishing practices

The demand for fish, along with advances in technology, has led to fishing methods that are depleting fish and shellfish populations around the world. Scientists fear that continuing to fish at this rate may soon result in a collapse of the world’s fisheries. See the video Net Gains for more information.

The cod fishing season in Newfoundland is from June to October, when the fish are not spawning. Scientists and policymakers are researching how they can best use the information about cod condition to encourage fishing when the cod are the fattest. Catching cod when they are in good condition can mean that fishers can get the same amount of cod (in terms of weight) by catching fewer fish overall. Scientists and policymakers hope that this measure will help to ensure that there are more sexually mature fish left to sustain the cod stock, while allowing fishers to be able to fish their quotas.

Recovery and sustainability

Economists, policy makers, scientists and conservationists work to foster cod recovery by:

  • monitoring cod populations for changes in their numbers, distribution, condition (i.e., health) and composition of their ecosystem over time;

  • making management decisions to help ensure the sustainability of the ecosystem and cod recovery; and

  • Working with communities to innovate ways to reduce impact of fishing while still maintaining people's livelihoods.

Learning outcomes

Use the scientific method to explore the relationship between time of year and the cod’s reproduction and condition, and use data to propose a way for fishers to ensure the sustainability of the cod stock.


Critical thinker, Collaborator, Communicator

Focus questions

    • How can we better support sustainable food?

    • What actions contribute the most to marine recovery?

Learning objectives

    • Measure and record observations

    • Identify and describe key terms

    • Explain the difference between spawning and not-spawning, and when these phases occur

    • Predict and explain each season’s impact on a cod’s health

    • Conclude the best time of year for sustainable cod fishing


The students’ activity guide for Anatomy of a Cod prompts them to:

    1. Familiarize themselves with key vocabulary.

    2. Calculate the GSI to determine when fish spawn.

    3. Calculate the HSI to determine the cod energy reserves.

    4. Graph the GSI and HSI to see the relationship between the time of year and cod’s spawning seasons and physical conditions.

    5. Analyze the data to make predictions as to the best time of year to fish cod.

    6. Draw a conclusion as to the best time of year to fish cod using data to support their reasoning.

Skill builder!

Depending on your students’ comfort level with Google Sheets, you may wish to demonstrate some skills, such as how to make a formula for a graph.

Discussion points

    • What did you notice about the data and calculations?

    • What are you still wondering about?

    • How did you determine the season when cod spawn?

    • How did you determine the season when cod have the largest energy reserve?

    • How did you determine the best time of year for fishing cod? What’s your rationale?

    • How did you determine the best time of year to fish cod so that the stock is sustained?

Preview the student activity

Anatomy of a cod: Instructions

Preview the student data sheet

Anatomy of a cod: Data collection sheet

Anatomy of a cod - Web app guide

For more information on how to use the app, read the Anatomy of a cod - Web-app guide

Anatomy of a cod - Internal organ guide

For more information about the species, read the Anatomy of a cod - Internal organ guide