Fish Life Cycle

Like all animals, fish need to survive and grow large enough to reproduce. Fish that make it to adulthood and to spawning time use a range of strategies to ensure successful reproduction. This lesson looks at the factors that go into keeping a fish species alive while examining the life cycle of Great Lakes fish.

Grade level: 4-8 grades

Performance Expectations:

• 3-LS1.1 From Molecules to Organisms: Structures and Processes. Develop models to describe that organisms have unique and diverse life cycles, but all have birth, growth, reproduction and death in common.

For alignment, see: Fish Life Cycle NGSS Summary

Lesson Objectives

• Name stages of the fish life cycle.
• Describe two general animal reproductive strategies.
• Contrast the reproductive strategies of at least two Great Lakes fish.
• Diagram progression from egg, larval fish, fry, juvenile, adult.

Background

Each fish species has a unique reproductive strategy and favors certain habitats for spawning and for early development of their newly hatched young. Many Great Lakes fish can be found in shallow water during part of their life cycle. Many species use shallow waters of lakes or rivers as spawning habitat either in the spring or fall. Some, such as northern pike, prefer wetlands with aquatic vegetation. Others such as lake whitefish prefer shallow reefs, which provide rich areas for food and rocky structure to protect the eggs and later the fry.

Fish life cycles vary among species. In general, however, fish progress through the following life cycle stages:

• Eggs: Fertilized eggs develop into fish. Most eggs do not survive to maturity even under the best conditions. Threats to eggs include changes in water temperature and oxygen levels, flooding or sedimentation, predators and disease.
• Larval: Larval fish live off a yolk sac attached to their bodies. When the yolk sac is fully absorbed, the young fish are called fry.
• Fry: Fry are ready to start eating on their own. Fry undergo several more developmental stages, which vary by species, as they mature into adults. Young fish are generally considered fry during their first few months (during their first few months to just less than one year in some species).
• Juvenile: The time fish spend developing from fry into reproductively mature adults varies among species. Most fish do not survive to become adults. Threats to survival include fluctuations in water temperature, changes in oxygen levels, competition for habitat and predators.
• Adult: When fish are able to reproduce, they are considered adults. The time it takes to reach maturity varies among species and individual fish. Fish with shorter life spans reach maturity faster. For example, female round gobies mature in approximately one year and live for two to three years. Lake sturgeon can live from 80-150 years, but females don’t reach maturity until they are approximately 25 years old.
• Spawning: Female fish release eggs into the water (either into the water column or into a nest) and male fish fertilize eggs by releasing milt. Not all eggs are fertilized. Some fish spawn each year after reaching maturity, others spawn at intervals (every four years, for example), while others spawn only once and then die.

Activity

• Diagraming the Fish Life Cycle
  Summary: Students will learn about fish life cycles and then will diagram the progression from egg to adult.
  Time: One 50-minute class period